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Using a Dynamic Landscape Model for Planning the Management of Alien Plant Invasions
Ecological Applications
Abstract
Biological invasions are widespread phenomena that threaten the integrity and functioning of natural ecosystems. In this paper we develop a model that is designed to be a decision-making tool for planning and managing alien plant control operations. Most decision tools adopt a static approach; in this application we integrate a dynamic simulation model of alien plant spread with decision-making tools commonly used for reserve design. The model is a landscape-scale implementation of a fine-grained individual- based spread model. We first describe the scaling up of the fine-scaled model into a landscape extent model. Comparisons between the fine-grained local-scale and coarse-grained land- scape-scale model show that the scaling-up process did not introduce significant artifacts into the behavior of the model. The landscape model is used to explore a range of strategies and funding schedules for clearing alien plants. These strategies are evaluated in terms of the cost of the clearing operation, the time it takes to eradicate the plants, and the impact the plants have on three components of native plant diversity (all species, rare and threatened species, endemic species). Clearing strategies that prioritize low-density sites dominated by juvenile alien plants proved to be the most cost effective. Strategies that used information on the distribution of plant diversity were not much more expensive than the most cost- effective strategy, and they substantially reduced the threat to native plant diversity. De- laying the initiation of clearing operations had a strong effect on both the eventual costs of the clearing operation and the threat to native plant diversity. We conclude that the integration of dynamic modeling with decision-making tools, as illustrated here, will be useful for the management of biodiversity under global change.
... To achieve this goal, clear objectives, strategies, adaptive planning, adequate resources and funding for long-term implementation are required (Esler et al., 2010;Foxcroft and McGeoch, 2011;van Wilgen et al., 2016a). Predictive models that consider ecological drivers such as fire, invasion rate, ecological impact and factors that increase uncertainty, such as clearing efficiency, can provide estimates of expected outcomes defined by particular sets of conservation objectives and resource allocations (Le Maitre et al., 1996;Higgins et al., 2000;Krug et al., 2010;Cheney et al., 2019). Prioritisation models for IAP management that attempt to account for multiple objectives and uncertainties have been developed for a number of applications and include water catchment areas (van Wilgen et al., 2007;Forsyth et al., 2012), protected area management (Forsyth and Le Maitre, 2011;van Wilgen et al., 2016a) and maximisation of economic cost-benefit ratios (Higgins et al., 1997;de Wit et al., 2001). ...
... Fig. 1; Cheney et al., 2019). The model simulates Acacia population size, age structure and area invaded, within each of the 809 management units, based on two key drivers of Acacia persistence, namely fire dynamics and plant population (growth and seedbank) dynamics (Le Maitre et al., 1996;Higgins et al., 2000;Krug et al., 2010). Starting population data for the model (year 0) were based on fine-scale data collected from a systematic survey in-field from 10,057 plots that uniformly covered all management units (Cheney et al., 2018). ...
... While an increasing number of studies have focused on the effectiveness of alien clearing programmes, only a few (e.g. Higgins et al., 2000;van Wilgen et al., 2016a) have attempted to model the future impact of alternate prioritisation strategies. These simulations of future invasions have focussed on funding scenarios, as opposed to efficacy of clearing in relation to Acacia ecology. ...
Conservation managers are required to make decisions in complex and uncertain contexts. To strengthen the robustness of conservation decisions, several approaches have been proposed to facilitate stakeholder engagement in the setting of conservation objectives and priority actions. While such processes have led to the formulation of several invasive alien plant management strategies to achieve specific objectives, the long-term consequences and trade-offs inherent in these strategies have not been tested. The performance of five of these strategies over 50 years was tested in the protected area context using empirical data from Table Mountain National Park, South Africa. A simulation model based on data for invasive Acacia species in a fire-driven ecosystem, focused on the interaction between strategy performance and clearing efficacy in achieving a management goal or reducing Acacia density to below 1 plant per hectare. At near perfect levels of clearing efficacy, all strategies converged towards reaching the management goal, while at lower efficacy levels the strategies diverged in their ability to achieve desired outcomes. Despite working across the largest area, strategies that focussed on clearing low density invasions, maintained the least area in a maintenance state over time. In contrast, strategies that focussed on a mix of post-fire, low density areas and high altitude areas cleared less area annually, but maintained a much larger area in a maintenance state. At higher levels of efficacy, strategies that return to previously worked areas were more successful than a post-fire strategy. Strategies that focused solely on securing water, performed poorly in maintaining low overall density of aliens. However, the influence of efficacy was significant and substantial and a much larger difference in area reaching the management goal was achieved by varying efficacy than varying strategy. As such, improving quality of work and implementation will have a far greater effect than which areas are prioritized or how this prioritization is done. While acacias are likely to persist in the long-term, improving work quality coupled with correct strategy selection will ensure continued gains in the area under maintenance and improved return on investment over time.
... Many models of weed spread incorporate discrete disturbance events such as fire (38), acute herbicide application (71), and cessation of agriculture (81). In fact, in most cases it is this disturbance that drives the model; for example, pines are assumed to colonise only following fire in the large-scale model of Higgins et al. (38). ...
... Many models of weed spread incorporate discrete disturbance events such as fire (38), acute herbicide application (71), and cessation of agriculture (81). In fact, in most cases it is this disturbance that drives the model; for example, pines are assumed to colonise only following fire in the large-scale model of Higgins et al. (38). While fire is of minor importance in determining the distribution of weeds in New Zealand landscapes, other disturbance factors might play an important role in creating suitable habitats for invasion by weeds, though tree-fall appears to have no effect on weed invasions in forests (91). ...
... Higgins et al. (38) describe a spatially-explicit model for the spread of Pinus species in the Cape Peninsula of South Africa. This model is based on scaling up one of the fine-scale individual-based models described above. ...
This review was prepared for the New Zealand Department of Conservation as the first step towards the development of landscape-scale population models for invasive pests. Such models have been identified as important tools for future management of the conservation estate.
We summarise previous studies of weed invasions in New Zealand and discuss techniques for modelling invasive populations in space and time. We then review existing projects of particular relevance and draw some conclusions about fruitful approaches to modelling pest spread across real landscapes.
Long-term permanent plot data have been used to reconstruct weed invasions in some New Zealand conservation areas. Such data will be invaluable for parameterising and testing the predictions of landscape-scale population models.
GIS-based statistical techniques are able to predict habitat suitability for particular species and therefore their potential distributions.
Existing models for invasive species tend to concentrate on rates of spread, and so highlight the importance of long-distance dispersal. However, local demography and short-distance dispersal is also important for practical
management of such species.
While spatial population modelling is well advanced, realistic modelling of populations in real (i.e. GIS-based) landscapes is in its infancy. The most advanced GIS-based models are for the spread of wildfire, which has Obvious similarities to pest invasions but tends to operate on smaller spatial scales.
Few existing weed population models have explicitly evaluated alternative management scenarios.
One study used loosely-coupled GIS and population models to evaluate weed control strategies at a regional level in the UK. This approach might be enhanced to form a useful tool for evaluating pest control strategies in the New Zealand conservation estate.
... We used limited available information to estimate plausible rates of spread for invasive alien plants in the CFR, using Pinus species, and Acacia mearnsii as indicator taxa. In the case of Pinus, Higgins et al. (2000) used successive aerial photographs to estimate the rate of spread of Pinus pinaster at four sites. They used a linear regression of the natural logarithm of the area versus time, and estimated doubling times at between 10 and 30 years, or 3-8% per year. ...
... Richardson and Brown (1986) estimated the number of squares (50 × 50 m) that were occupied at different times by invasive Pinus trees of "moderate" density, again using aerial photographs. Higgins et al. (2000) used this data to estimate an annual spread rate of 7.9%. ...
... Unfortunately, the control programmes operate in an extremely bureaucratic and rule-bound environment from which such flexibility is essentially absent (van Wilgen and Wannenburgh, 2016). Research has also clearly demonstrated that control efforts would be much more effective if they focused on areas with lower densities of young alien plants (as opposed to clearing dense invasions, see Higgins et al., 2000), but this advice is certainly not rigorously followed in the current operating environment where longerterm planning is for all practical purposes virtually absent (van Wilgen and Wannenburgh, 2016). In addition, the presence of large blocks of pine plantations immediately adjacent to protected areas provides a ready source of seeds from which cleared areas can be re-invaded (McConnachie et al., 2015), and although there are ongoing discussions about how to best address this problem, no real sustainable solutions have been found to date (van Wilgen, 2015). ...
... For example, one study has advised prioritization of large populations for control because these are principal sources of propagules in metapopulation theory . Other studies have advised to prioritize small outlying populations because from a population dynamics perspective they contribute most to range expansion due to their large edge-toarea ratio Higgins, Richardson & Cowling 2000;. ...
... Yet, even tough a management strategy may be very effective, its costs may still be tremendous. Thus, one may examine management options for different levels of costs, either estimated by real financial costs (Regan et al. 2006) or by cost proxies, such as area treated (Naidoo et al. 2006), frequency of treatment in time or number of removed individuals (Higgins, Richardson & Cowling 2000). Cost proxies are generally more intuitive to develop and understand than real financial costs. ...
... Bossenbroek, Kraft & Nekola 2001) or cellular automata (e.g. Higgins, Richardson & Cowling 2000). Using such models in sensitivity analyses has proven useful to determine the intensity and frequency of treating most vulnerable life-stages (Byers et al. 2002;Caswell, Lensink & Neubert 2003;Shea et al. 2010) and to determine the efficiency of management strategies within simple landscapes . ...
Global changes in climate, land-use and the economy rapidly alter large-scale distributions of animal and plant species. Under these varying climatic and land-use conditions, species may shift their distributions or, if they are too migration limited to do so, they can adapt to the new conditions or they may go extinct. Additionally, because of the increase in international trade, species are being introduced to new areas more frequently and, due to their ability to spread rapidly, are becoming one of the main reasons for species decline. The aim of this thesis was to investigate the factors that drive such long-term dynamics at different spatial scales by using species distribution models. The results showed that competition between species was a key factor influencing species distributions, especially under favourable climate conditions. It was further shown that migration limitation was an additional key factor shaping species distributions and was mainly caused by competition, adverse climatic conditions and habitat fragmentation. As a result, tree species that occur mainly in competitive environments may not be able to keep pace with climate and land-use change, while ruderal species may have only minor delays in the readjustment of their distributions. Another important finding was that knowledge on the variation in migration rates due to habitat heterogeneity can be used successfully for developing strategies to control invasive plants more economically. Overall, the findings of this thesis will be helpful for investigating and better predicting large-scale species distributions under global change scenarios and for developing mitigation strategies to reduce the effects of global change on species distributions.
... While there has been substantial research published on invasive species management (Epanchin-Niell and Hastings 2010), important gaps remain, particularly concerning general management strategies. Many recommendations are problem-specific and do not give general insights that could support rapid decision-making for new problems, even when these new problems are similar to those already treated in the existing literature (Higgins et al. 2000, Burnett et al. 2007, Hyder et al. 2008). ...
... For example, some models assume that growth rates and removal costs do not vary with the density of the invasive population (Hastings et al. 2006), even though densitydependent population dynamics are an essential element of population dynamics (Pearl 1927, Hixon andJohnson 2001) and removal costs are notoriously dependent on population densities (Cacho et al. 2010). Additionally, some previous studies look for cost-efficient suppression strategies over fixed time periods (Higgins et al. 2000, Baxter et al. 2008, even though managers might reasonably want to suppress the species in the shortest possible time, or conversely at a minimum cost, regardless of project length. ...
... Although it may not be possible to implement the optimal solution in all circumstances, it provides a valuable yardstick against which to compare alternate strategies. Many previous analyses identify the best option from a finite set of possible strategies (Menz et al. 1980, Higgins et al. 2000, Crespo and Sun 2002, Zhang et al. 2006, Baxter et al. 2008, Cacho and Hester 2011, or restrict the form of the control strategy (e.g., Sharov 2004, Carrasco et al. 2010a). Some restrictions represent legitimate limitations on managers' logistical or physical capabilities (e.g., budgetary constraints) and must be considered. . ...
Invasive species are a worldwide issue, both ecologically and economically. A large body of work focuses on various aspects of invasive species control, including how to allocate control efforts to eradicate an invasive population as cost effectively as possible. There are a diverse range of invasive species management problems, and past mathematical analyses generally focus on isolated examples, making it hard to identify and understand parallels between the different contexts. In this study, we use a single spatiotemporal model to tackle the problem of allocating control effort for invasive species when suppressing an island invasive species, and for long-term spatial suppression projects. Using feral cat suppression as an illustrative example, we identify the optimal resource allocation for island and mainland suppression projects. Our results demonstrate how using a single model to solve different problems reveals similar characteristics of the solutions in different scenarios. As well as illustrating the insights offered by linking problems through a spatiotemporal model, we also derive novel and practically applicable results for our case studies. For temporal suppression projects on islands, we find that lengthy projects are more cost effective and that rapid control projects are only economically cost effective when population growth rates are high or diminishing returns on control effort are low. When suppressing invasive species around conservation assets (e.g., national parks or exclusion fences), we find that the size of buffer zones should depend on the ratio of the species growth and spread rate.
... While there has been substantial research published on invasive species management (Epanchin-Niell and Hastings 2010), important gaps remain, particularly concerning general management strategies. Many recommendations are problem-specific and do not give general insights that could support rapid decision-making for new problems, even when these new problems are similar to those already treated in the existing literature (Higgins et al. 2000, Burnett et al. 2007, Hyder et al. 2008). ...
... For example, some models assume that growth rates and removal costs do not vary with the density of the invasive population (Hastings et al. 2006), even though densitydependent population dynamics are an essential element of population dynamics (Pearl 1927, Hixon andJohnson 2001) and removal costs are notoriously dependent on population densities (Cacho et al. 2010). Additionally, some previous studies look for cost-efficient suppression strategies over fixed time periods (Higgins et al. 2000, Baxter et al. 2008, even though managers might reasonably want to suppress the species in the shortest possible time, or conversely at a minimum cost, regardless of project length. ...
... Although it may not be possible to implement the optimal solution in all circumstances, it provides a valuable yardstick against which to compare alternate strategies. Many previous analyses identify the best option from a finite set of possible strategies (Menz et al. 1980, Higgins et al. 2000, Crespo and Sun 2002, Zhang et al. 2006, Baxter et al. 2008, Cacho and Hester 2011, or restrict the form of the control strategy (e.g., Sharov 2004, Carrasco et al. 2010a). Some restrictions represent legitimate limitations on managers' logistical or physical capabilities (e.g., budgetary constraints) and must be considered. . ...
Invasive species are a worldwide issue, both ecologically and economically. A large body of work focuses on various aspects of invasive species control, including how to allocate control efforts to eradicate an invasive population as cost effectively as possible: There are a diverse range of invasive species management problems, and past mathematical analyses generally focus on isolated examples, making it hard to identify and understand parallels between the different contexts. In this study, we use a single spatiotemporal model to tackle the problem of allocating control effort for invasive species when suppressing an island invasive species, and for long-term spatial suppression projects. Using feral cat suppression as an illustrative example, we identify the optimal resource allocation for island and mainland suppression projects. Our results demonstrate how using a single model to solve different problems reveals similar characteristics of the solutions in different scenarios. As well as illustrating the insights offered by linking problems through a spatiotemporal model, we also derive novel and practically applicable results for our case studies. For temporal suppression projects on islands, we find that lengthy projects are more cost effective and that rapid control projects are only economically cost effective when population growth rates are high or diminishing returns on control effort are low. When suppressing invasive species around conservation assets (e.g., national parks or exclusion fences), we find that the size of buffer zones should depend on the ratio of the species growth and spread rate.
... This alters the flow, availability and/or quality of nutrient resources within biogeochemical cycles, trophic resources within food webs, and physical resources such as light, sediment, space or water. Alien species that change the physical structure of the ecosystem in this way are sometimes called 'transformers' (Richardson et al. 2000) or 'ecosystem engineers' (Crooks 2002). ...
... Invasive alien plants have major impacts on these ecosystems, infesting river channels and using more water than comparable indigenous species (Le Maitre et al. 2000). Trees and shrubs are the most important alien invaders in the CFR (Nel et al. 2004), and have been shown to reduce water run-off and affect hydrological processes in mountain catchments (Dye & Poulter 1995;Higgins et al. 2000). Increased evapo-transpiration from them may also affect the overall water balance within a river system (Weins 2002). ...
... Addressing this question requires a spatially explicit framework that considers both clonal expansion and dispersal via seeds. A potential disadvantage of such detailed spatially explicit models, however, is that they typically provide limited means for analytical analyses, and therefore often constrain the analyses to specific questions or case studies (Higgins et al. 2000, Grevstad 2005, Baker 2017. On the other hand, empirical data may provide a means to constrain model parameters in a way that enables comprehensive analyses through simulation (Bernik et al. 2018). ...
... the number of exotic species introductions is still increasing (Tittensor et al. 2014), there is an urgent need for effective eradication and control strategies (SCBD 2014, Crowley et al. 2017. Previous studies have successfully identified optimal solutions that balance economic costs associated with the control efforts and their ecological success (Sharov and Liebhold 1998, Higgins et al. 2000, Taylor and Hastings 2004, and considered how management effort should be distributed in space (Travis and Park 2004, Hauser and McCarthy 2009, Chades et al. 2011, Baker and Bode 2016, Bonneau et al. 2017. Less attention has been paid, however, to the mediating role of invasive patch geometries and their relative position in the landscape. ...
Effective management strategies are needed to control expansion of invasive alien plant species and attenuate economic and ecological impacts. While previous theoretical studies have assessed optimal control strategies that balance economic costs and ecological benefits, less attention has been paid to the ways in which the spatial characteristics of individual patches may mediate the effectiveness of management strategies. We developed a spatially explicit cellular automaton model for invasive species spread, and compared the effectiveness of seven control strategies. These control strategies used different criteria to prioritize the removal of invasive species patches from the landscape. The different criteria were related to patch size, patch geometry, and patch position within the landscape. Effectiveness of strategies was assessed for both seed dispersing and clonally expanding plant species. We found that, for seed‐dispersing species, removal of small patches and removal of patches that are isolated within the landscape comprised relatively effective control strategies. For clonally expanding species, removal of patches based on their degree of isolation and their geometrical properties comprised relatively effective control strategies. Subsequently, we parameterized the model to mimic the observed spatial distribution of the invasive species Antigonon leptopus on St. Eustatius (northern Caribbean). This species expands clonally and also disperses via seeds, and model simulations showed that removal strategies focusing on smaller patches that are more isolated in the landscape would be most effective and could increase the effectiveness of a 10‐yr control strategy by 30–90%, as compared to random removal of patches. Our study emphasizes the potential for invasive plant species management to utilize recent advances in remote sensing, which enable mapping of invasive species at the high spatial resolution needed to quantify patch geometries. The presented results highlight how this spatial information can be used in the design of more effective invasive species control strategies.
... Therefore several competing strategies exist for managing plant invasions, reflecting different assumptions made about the processes driving tree invasions. A management strategy can target or prioritize peripheral sites (Moody and Mack 1988), sites with high density (thus decreasing propagule pressure,Von Holle and Simberloff 2005), or populations on highly suitable habitat (Higgins et al. 2000). However, most models have not integrated dispersal with spatial variation in habitat suitability and subsequent reproductive and recruitment response into a single assessment of strategies to slow or stop an invasion. ...
... Management strategies to slow the spread of tree invasions included effort allocation and the spatial selection of stands; these multi-scale strategies are both practical and in line with the dominant theory for reducing the spread of biological invaders (e.g. Moody and Mack 1988;Higgins et al. 2000). However, in most of the simulations no differences were observed among management strategies, particularly when management effort was high enough to strongly reduce the final area invaded. ...
Biological invasion by non-native tree species can transform landscapes, and as a consequence, has received growing attention from researchers and managers alike. This problem is driven primarily by the naturalisation and invasion of tree species escaping from cultivation or forestry plantations. Furthermore, these invasions can be strongly influenced by the land-use matrix of the surrounding region, specific management of the source populations, and environmental conditions that influence seed dispersal or habitat quality for the invader. A major unresolved challenge for managing tree invasions in landscapes is how management should be deployed to contain or slow the spread of invading populations from one or more sources (e.g. plantations). We develop a spatial simulation model to test: (1) how to best prioritise the control of invasive tree populations spatially to slow or contain the biological invader when habitat quality varies in the landscape, and (2) how to allocate control effort among different management units when trees spread from many source populations. We first show that to slow down spread effectively, management strategy is less important than management effort. We then identify the conditions affecting the relative performance of different management strategies. At the landscape scale, targeting peripheral stands consistently yielded the best results whereas at the regional scale, management strategies needed to account for both habitat quality and tree life-history. Overall, our findings demonstrate that knowledge of how habitat affects tree life-history stages can improve management to contain or slow tree invasions by improving the spatial match between management effort and efficacy.
... Regional or biome-scale assessments can indicate the relative susceptibility to invasion of certain larger units within a region (e.g., climatic zones) (Reichard and Hamilton, 1997). Landscape-scale assessments of the spread of alien species can also indicate when and where mechanical control could be used effectively ( Guido et al., 2016;Higgins et al., 2000). Third, the histories of alien species from their introductions to invasion, the purposes and processes of introduction, and the management processes applied should provide important information. ...
... These factors are often important in facilitating the naturalization and invasion of an alien species. Therefore, this information is useful in the management of invasive species at the landscape scale (Higgins et al. 2000(Higgins et al. , 2001, and it is also particularly helpful in planning long-term strategies at regional and national levels (Rouget et al., 2002). Different areas or countries differ in aspects, such as climate, geographical environment, human activities, and laws, and thus the biomes of invasive species are diverse. ...
A comparative analysis of invasive plants in China and the USA, the climates of which are similar, revealed both similarities and differences in taxonomic characteristics, morphological characteristics, areas of origin, invaded regions, and histories of introduction. Generally, there were more species from Asteraceae and Poaceae than from other families, and a high proportion of families were represented by only one invasive species. Most invaders had at least one reproductive mode, with sexual reproduction being the main mode, as well as the adaptation of flower, fruit, and seed characteristics for successful seed production and dispersal. Many invasive plants are able to reproduce clonally. An analysis of the invasive species in China and the USA indicated that they are mainly derived from continents with climates similar to that of the invaded area or at latitudes within the same range. Species from tropical areas invaded more easily than those from other climatic zones. Our study of the invasive histories of species introduced into China and the USA demonstrates that it takes about 100 years for an exotic species to become an invasive species.
... One approach has been to consider two spatial sites, core and periphery, where most studies taking this approach have focused on eradication (Moody & Mack, 1988;Taylor & Hastings, 2004;Wilen, 2007) and some on containment (Panetta & Cacho, 2014). In turn, other bioeconomic studies have considered continuum space models or cellular automatons (Epanchin-Niell, 2017;Higgins et al., 2000). Some of these studies have examined how to allocate treatment efforts over space and time to eradicate an invasive species (Baker, 2017(Baker, , 2018. ...
Despite efforts to prevent their establishment, many invasive species continue to spread and threaten food production, human health, and natural biodiversity. Slowing the spread of established species is often a preferred strategy; however, it is also expensive and necessitates treatment over large areas. Therefore, it is critical to examine how to distribute management efforts over space cost‐effectively. Here we consider a continuous‐space bioeconomic model and we develop a novel algorithm to find the most cost‐effective allocation of treatment efforts throughout a landscape. We show that the optimal strategy often comprises eradication in the yet‐uninvaded area, and under certain conditions, it also comprises maintaining a “suppression zone,” an area between the invaded and the uninvaded areas, where treatment reduces the invading population but without eliminating it. We examine how the optimal strategy depends on the demographic characteristics of the species and reveal general criteria for deciding when a suppression zone is cost effective.
... A step further in the prioritization process involves dividing these strategically prioritized areas into smaller management units based on site-level characteristics such as density and age of IAPs, identity of IAPs, topography and accessibility and past clearing operations ('tactical prioritization';sensu Potgieter et al., 2018). Detailed IAP data for these sites can allow for the prioritization of management units dominated by low-density alien plants -a strategy proven to be the most cost-effective for established IAP populations (e.g.Higgins et al., 2000). During this process, site assessments can be conducted whereby each site is surveyed and baseline information captured, for example, recording all alien species present, their densities (per cent cover) and categorizing them according to predefined size classes (i.e., seedlings, young, mature). ...
Rapid urbanization is placing increased pressure on natural, restored and designed ecosystems to provide services to growing human populations. The establishment and spread of invasive alien species within and around urban areas threaten biodiversity and ecosystem functioning, and the services they provide. Consequently, there is a need to protect and manage areas where invasions will have the greatest socio‐ecological impact. Limited resources call for the strategic prioritization of these areas, yet there are few widely adopted, standardized approaches for prioritizing sites vulnerable to species invasions in urban areas.
We applied multi‐criteria decision analyses in a geographic information system to develop a strategic, spatial prioritization approach for identifying those sites most sensitive to terrestrial alien plant invasions. To test this approach, we use the Toronto region as a case study, the most populous metropolitan area in Canada and one of the fastest growing urban centres in North America. Through consultation with local conservation authorities, we developed an objective, hierarchical set of 19 criteria grouped into two categories: biodiversity and ecosystem functioning, and ecosystem services. Spatial data layers were assigned to each criterion and used to map areas most important for biodiversity and ecosystem functioning and providing ecosystem services. We overlayed these priority areas with distribution data of Vincetoxicum rossicum, one of the Toronto region's most widespread and damaging invasive alien plant species (IAPs) to determine the potential threat species’ invasions pose to these important areas.
High‐priority sites identified by our prioritization model include areas of significant biodiversity conservation value such as intact forests, meadows and wetlands which are crucial for providing regulating and supporting services. Our IAPs distribution map showed that these high‐priority sites are heavily invaded (92.9% of the area occupied by V. rossicum comprises medium‐high‐priority sites) and should be prioritized for management to ensure biodiversity and ecosystem functioning and the provision of ecosystem services are maximized.
The approach applied in this study can be useful for conservation practitioners in guiding the selection of high‐priority, socio‐ecologically significant sites for management action in urban landscapes across different geographic regions and spatial scales.
... Hanging roots of water hyacinth also traps moving sediment, combine with detrital production and siltation under water hyacinth mats results in higher sedimentation loading (Nel et al., 2004). Furthermore, water hyacinth infestation management through mechanical harvesting or herbicidal treatment will cause damages to nearby desirable vegetation such as ornamental plants (Higgins et al., 2001). According to Stuckey and Les (1984) in Florida, water lettuce Pistia stratiotes is one of the invasive floating aquatic weed. ...
Macrophytes are plants that adapted to wet environment and easily be found all over the world. Macrophytes have structures that are more complex, interdependent and physically substantial; make them one of the important components of rivers, lakes and any other wetland ecosystems. Macrophytes can be categorized into four different types; emergent, floating-leaved, submerged and free-floating plants based on their structure and life form. Light, water current and wind flow are among the most important limiting factors of macrophytes occurrence in the water system. Environmental conditions such as lotic and lentic environment influence the limiting factors and would be the key for successful macrophytes distribution. Each macrophyte species could respond differently to different environmental circumstances. It also has been widely used as subject for biological indicator of ecosystem health. This paper aimed to describe the general environmental condition for macrophytes distribution, discuss their role and impact of excessive growth.
... Biological invasions have been identified as a major threat to the reservation of modern biodiversity, ecosystem functioning and ecosystem service provision (Higgins et al. 2000;Chown et al. 2009 Ndumo and these include aggressive invaders known for their conservation concern such as Lantana camara (Ghisalberti 2000) and Chromolaena odorata (Macdonald & Jarman 1985;Apori et al. 2000). The impact of these invasive alien species on the quality of herbivore browse, specifically for black rhino, in the reserve is however unknown. ...
... Contrastingly, the large (~40%) increases in P. hysterophorus density, recorded within plots which were cleared without the addition of native grass seed, are accounted for by the resulting effects of this technique. Disturbances caused by intensive alien clearing activities frequently open patches in the landscape, readily facilitating the invasion and/or densification of noxious annual species, including P. hysterophorus (DiTomaso, 2000;Higgins et al., 2000). Moreover, soil perturbance, resulting from clearing efforts, is known to enhance the germination and recruitment of P. hysterophorus (Navie et al., 2004) and is highly likely given the weed's dominance within the site's seedbank (Strathie and McConnachie, 2019;Cowie, 2020). ...
The annual herb Parthenium hysterophorus L. (Asteraceae), remains one of Southern Africa's most significant invasive weeds, commonly invading savannas, and their rangelands, causing severe losses to agriculture, livestock production and native biodiversity. Previous studies have suggested that perennial grasses may act as useful competitive species, capable of suppressing the growth and invasion of P. hysterophorus. To explore this, a total of 48 plots were established within an invaded savanna, using a randomised block design, and included treatments with and without the clearing of P. hysterophorus, as well as with and without the sowing of native perennial grass seed (Anthephora pubescens, Chloris gayana, Cynodon dactylon, Digitaria eriantha, Eragrostis curvula, Panicum maximum and Themeda triandra). Plots were assessed yearly in terms of P. hysterophorus density and growth as well as grass species composition, basal cover, and biomass over a three-year period. Clearing alone was found to exacerbate invasion, increasing P. hysterophorus density by 40%. Whereas the sowing of grass seed, in both the cleared and uncleared plots, increased the abundance of perennial grass species by 28%, subsequently reducing the size, reproductive output and density of P. hysterophorus over the three years. In addition, these sowing efforts contributed towards partial restoration of the plots, enhancing grass basal cover by ~15% and biomass production by 17%. Overall, this research suggests that sowing of native grass species, with or without clearing, may be a useful supplementary control or restoration tool towards the long-term management of P. hysterophorus invasions in managed savannas and rangelands in Southern Africa.
... Management of invasive species requires a targeted approach for maximum effect. A number of studies using spatially explicit models have demonstrated that where the effort is initially targeted can have a profound effect on outcome 5,[22][23][24][25] . Controls based on population biology have generally supported prioritization of the edges of a population as primary targets of initial control efforts 5,23,24,26 . ...
Rapid range expansions of invasive species are a major threat to ecosystems. Understanding how invasive species increase their habitat ranges and how environmental factors, including intensity of human activities, influence dispersal processes is an important issue in invasion biology, especially for invasive species management. We have investigated how spatially heterogeneous factors influence range expansion of an invasive species by focusing on long-distance dispersal, which is frequently assisted by human activities. We have developed models varying two underlying processes of a dispersal event. These events are described by source and destination functions that determine spatial variations in dispersal frequency and the probability of being a dispersal destination. Using these models, we investigated how spatially heterogeneous long-distance dispersal influences range expansion. We found that: (1) spatial variations in the destination function slow down late population dynamics, (2) spatial variations in the source function increase the stochasticity of early population dynamics, and (3) the speed of early population dynamics changes when both the source and the destination functions are spatially heterogeneous and positively correlated. These results suggest an importance of spatial heterogeneity factors in controlling long-distance dispersal when predicting the future spread of invasive species.
... Once invasive plants are reduced to less than 5% live cover (maintenance levels) managers should reevaluate the necessary treatment intervals to maintain progress. Some research suggests that spending time, money, and resources on treating smaller, younger, or more rapidly growing infestations is cost-effective at reducing infested acres, money spent per acre, and protecting biodiversity(Moody and Mack 1988;Higgins et al. 2000). All three of the focal species in this study are believed to have detrimental effects on biodiversity(Turner et al. 1998; Gordon 1998). ...
To successfully reduce overall invasive plant cover over time, an effective treatment plan must be established such that mortality exceeds new colonization and re-spouting growth rates. However, few evaluations of the effects of long-term, consistent treatment at different intervals exist. We report the effects of treatment intensity on Old World climbing fern ( Lygodium microphyllum (Cav.) R. Br.), Brazilian pepper ( Schinus terebinthifolia Raddi) and punktree ( Melaleuca quinquenervia (Cav.) S. T. Blake), as part of a large restoration project that has been underway for six years in Telegraph Swamp at Babcock Ranch Preserve, a 68,000 acre conservation area in Florida, U.S.A. We found that at the end of the six-year period, for all three species, average live cover did not exceed 5% across all transects. In addition, dead foliar cover was higher than live cover for all three invasive plants, indicating progress towards restoration goals. We also found that percent live cover of Old World climbing fern were significantly reduced only after four or more treatments were applied during the six-year period, as opposed to when three or fewer treatments were applied. Reductions in percent cover of live foliage were apparent only when the treatments were applied more often than biennially, as opposed to less often than biennially. Additionally, we found higher Old World climbing fern cover in clear-cut and replanted cypress stands than in natural stands. Based on these findings, we conclude that treatments applied four or more times, or more often than biennially, were more effective at significantly reducing advanced invasions of Old World climbing fern, Brazilian pepper, and punktree, especially where previous management activities or their effects may have increased the cover of invasive plants.
... For many ecosystems, temporal and spatial environmental variation can induce strong, but short-term demographic variation that has major implications for the success of invasive species (Tickner et al. 2001;Jongejans et al. 2011). Thus, both temporal and spatial effects are particularly important to consider for invasive species management in environmentally dynamic habitats such as on riparian floodplains (Higgins et al. 2000;Meier et al. 2014). For riparian floodplains, seasonal flood events may act as a non-additive driver that modulates the outcome of invasive species by producing conditions that favor native species (Didham et al. 2007). ...
Invasive species present one of the largest threats to the recruitment and persistence of native plant communities. Land managers generally apply two approaches to help control invasive species and restore native plant communities: (1) improve ecosystem processes that preferentially support native species (e.g. reintroduce disturbance regimes) or (2) directly treat non‐native species (e.g. herbicide use). Few utilize both. Over nearly a decade, we tracked the establishment and population growth of Lepidium latifolium (perennial pepperweed) on a hydrologically restored floodplain and adjacent grassland sites, concurrently with a controlled herbicide application experiment. We found that perennial pepperweed stem counts were lower in years with longer duration flood events (p<0.0001) and after herbicide application (p<0.0005). In floodplain areas, native species increased in the years following herbicide treatments or after wet water years, while grassland sites were re‐invaded by other non‐native species. Our results suggest that long inundation periods can effectively control perennial pepperweed over large areas when used in concert with herbicide control during dry years or in drier areas. Given the natural variability in inundation periods, our study highlights the need for longer term (>3‐5 years) studies to evaluate the effectiveness of invasive species control programs. Overall, our study emphasized the need for further integration of traditional weed control approaches with restored ecological function to effectively control invasive plants and stimulate native plant recovery.
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... Il a été estimé que sur toutes les espèces introduites, environ 10% survivaient à cette introduction et 10% de celles-ci étaient à l'origine de perturbations majeures de l'écosystème (Williamson and Fitter 1996). L'étude des colonisations, au-delà de la compréhension qu'elle apporte sur ces processus naturels, peut permettre une meilleure gestion des espèces invasives, en délimitant les processus (dispersion, croissance, parasitisme, …) les plus critiques (Higgins, Turpie et al. 1997;Higgins and Richardson 1998;Higgins, Richardson et al. 1999;Higgins, Richardson et al. 2000;Dunn and Joanne 2009;MacDougall, Gilbert et al. 2009) pour minimiser l'effet délétère de ces invasions, ou les zones où la gestion sera la plus efficace (voir 3.1 et 3.2). L'étude de la colonisation trouve aussi une résonance dans la gestion durable des ressources forestières. ...
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... While prevalent invaders are often assumed to have ecological impact, quantification of these impacts is typically lacking, in part largely due to methodological challenges , Hulme et al. 2013, Pearson et al. 2016, Tekiela and Barney 2017. Waiting until an invader becomes widespread to recognize impacts is disadvantageous given that management is most effective at earlier stages of invasion when relatively few sites are occupied (Higgins et al. 2000, Simberloff et al. 2013, Epanchin-Niell 2017. Better tools are needed to evaluate invader impacts, particularly to identify problematic invaders before they become prevalent at broad scales. ...
Methods that allow rapid and robust evaluation of plant invader impacts are needed to identify problematic species before they become too widespread to effectively manage. While observational data can be readily gathered to identify negative relationships between invading and native species, these patterns are not necessarily indicative of invader impact and may instead reflect the legacy of past conditions influencing invasibility. We augmented standard observational methods by using local‐scale time‐since‐invasion information in conjunction with rapidly gathered plant abundance data to evaluate potential impacts of common buckthorn (Ramnus cathartica), a woody exotic in the early stages of invasion in Montana, USA. This species occurred as scattered populations of limited distribution and was proposed for listing as a state noxious weed, but empirical information on ecological impacts in the region was lacking. We recorded cover of understory and overstory plants across gradients of buckthorn invasion at 12 riparian sites representing five river drainages throughout the state. Uninvaded plots were located proximal to invaded plots at each site. Time since invasion per plot was approximated by aging buckthorn plants via annual rings. We found strong negative correlations between cover of native plants and buckthorn. In addition, buckthorn overstory cover increased with time since invasion, while native overstory cover decreased with invasion time, consistent with an impact scenario wherein the progression of invasion and associated increases in invader abundance suppressed native taxa. Although environmental factors that simultaneously promoted the increase of invaders and the decline of natives over time could have produced patterns mimicking invader impact, such a mechanism would have been more likely to manifest at broad scales to affect both uninvaded and invaded plots at a site. Our approach of using local‐scale time‐since‐invasion data to examine temporal signatures strengthened inferences made from standard observational methods and provided key input to support the listing of an emerging invader as a noxious weed in Montana. These results suggest that rapid empirical assessments of plant communities that consider time since invasion could be used to more confidently evaluate invader impacts and better inform the listing process for noxious and other regulated species.
... On the other hand, optimization modeling can evaluate very large numbers of treatment alternatives and permit trade-off analyses that might be impossible with simulation Hof et al., 2002). Taylor and Hastings (2004) states that the models that capture the spread of invasive species successfully have been used quite infrequently to find optimal control strates (Higgins et al., 2000), while direct simulation of different strates is the most appropriate scheme in many cases but does not allow for mathematical or algorithmic optimization of strates. Olson (2006) reviewed the economic literature on terrestrial invasive species, control and prevention of a biological invasion in general as well as the literature on international trade and trade policy with invasive species. ...
Invasive species cause large economic losses and are difficult to control due to the high cost of locating and treating the continual emergence of new individuals. The national strategy against invasive species encompasses prevention, early detection, and rapid response. In particular, determining an optimal route for search and treatment under limited budget is critical to reducing management costs. In this paper, we present a new integrated simulation-optimization framework to effectively search and treat invasive species under a limited management budget. The simulation mimics invader growth over a landscape and 12 years, while a bio-economic optimization model finds an optimal search and treatment path to minimize its economic damage to agricultural production. Our optimization model is new in the sense that it prescribes the optimal path for searching and treating sites for controlling the invader. This study is also the first in the literature to present and combine simulation and optimization models of the complex bio-economic search-path problem, and to prescribe a pathway for search and treatment that is applicable to the real-world management of invasive species. Our case study data and parameter calibration are based on the large-scale field data on Sericea, an aggressive invasive plant threatening the Great Plains of the U.S., collected over twelve pastures in Kansas during the past two years. Solving simulation and optimization models in a consecutive fashion provides a considerable computational advantage to find an optimal solution to practical size problems in a reasonable time. Our results imply that applying yearly treatment with a slow search-and-treatment speed results in the biggest bang for the buck under most invasion scenarios.
... For species with limited dispersal capacity, control efforts should target vanguard populations to hamper rapid range expansion (Wadsworth et al. 2000). This simple rule of thumb has been especially well supported for slowing the spread of invasive trees (e.g. Watson 1985; Moody and Mack 1988;Doren and Jones 1997;Higgins et al. 2000). For species with the capacity for long-distance dispersal, particularly plant species with seeds dispersed by vectors, priority should be given to first controlling large source populations so that the primary propagule supply to long-distance dispersal is reduced, thus blocking the Fig. 3 Hierarchical partitioning of the independent contribution of each explanatory variable to the variation of spreading velocity. ...
... The colonization and extinction rates can be empirically modelled independently (Mackenzie and Royle 2005) or they can be fitted simultaneously along with the other two cases, cells remaining absent and cells remaining present (Jackson 2011), allowing estimates of spread rates. Simulations of this type allow managers to have a locally parameterized tool to test various management alternatives by simulating the effectiveness of different interventions over time and space (Caplat et al. 2014;Higgins et al. 2000). Several shortcut methods can also be useful for rapid estimation of spatio-temporal dynamics of invasive trees (Aslan et al. 2012). ...
Scientists, managers, and policy-makers need functional and effective metrics to improve our understanding and management of biological invasions. Such metrics would help to assess progress towards management goals, increase compatibility across administrative borders, and facilitate comparisons between invasions. Here we outline key characteristics of tree invasions (status, abundance, spatial extent, and impact), discuss how each of these characteristics changes with time, and examine potential metrics to describe and monitor them. We recommend quantifying tree invasions using six metrics: (a) current status in the region; (b) potential status; (c) the number of foci requiring management; (d) area of occupancy (AOO) (i.e. compressed canopy area or net infestation); (e) extent of occurrence (EOO) (i.e. range size or gross infestation); and (f) observations of current and potential impact. We discuss how each metric can be parameterised (e.g. we include a practical method for classifying the current stage of invasion for trees following Blackburn's unified framework for biological invasions); their potential management value (e.g. EOO provides an indication Electronic supplementary material The online version of this article (of the area over which management is needed); and how they can be used in concert (e.g. combining AOO and EOO can provide insights into invasion dynamics; and we use potential status and threat together to develop a simple risk analysis tool). Based on these metrics, we propose a standardized template for reporting tree invasions that we hope will facilitate cross-species and interregional comparisons. While we feel this represents a valuable step towards standardized reporting, there is an urgent need to develop more consistent metrics for impact and threat, and for many specific purposes additional metrics are still needed (e.g. detectability is required to assess the feasibility of eradication).
... Multiple ecological models have supported recommendations for managers to target small, outlier patches of invasive species, which most efficiently reduces invader spread and is more cost effective (Moody and Mack, 1988;Higgins et al., 2000;Taylor and Hastings, 2004). One Phragmites-specific model offered evidence that prioritizing small patches of Phragmites is more effective for reducing spread (Alminagorta Cabezas, 2015), explained by Phragmites' ability to rapidly expand vegetatively (Kettenring et al., 2016), and the larger edge to area ratios of small patches. ...
The natural recolonization of native plant communities following invasive species management is notoriously challenging to predict, since outcomes can be contingent on a variety of factors including management decisions, abiotic factors, and landscape setting. The spatial scale at which the treatment is applied can also impact management outcomes, potentially influencing plant assembly processes and treatment success. Understanding the relative importance of each of these factors for plant community assembly can help managers prioritize patches where specific treatments are likely to be most successful. Here, using effects size analyses, we evaluate plant community responses following four invasive Phragmites australis management treatments (1: fall glyphosate herbicide spray, 2: summer glyphosate herbicide spray, 3: summer imazapyr herbicide spray, 4: untreated control) applied at two patch scales (12,000 m² and 1,000 m²) and monitored for 5 years. Using variation partitioning, we then evaluated the independent and shared influence of patch scale, treatment type, abiotic factors, and landscape factors on plant community outcomes following herbicide treatments. We found that Phragmites reinvaded more quickly in large patches, particularly following summer herbicide treatments, while native plant cover and richness increased at a greater magnitude in small patches than large. Patch scale, in combination with abiotic and landscape factors, was the most important driver for most plant responses. Compared with the small plots, large patches commonly had deeper and more prolonged flooding, and were in areas with greater hydrologic disturbance in the landscape, factors associated with reduced native plant recruitment and greater Phragmites cover. Small patches were associated with less flooding and landscape disturbance, and more native plants in the surrounding landscape than large patches, factors which promoted higher native plant conservation values and greater native plant cover and richness. Herbicide type and timing accounted for very little of the variation in native plant recovery, emphasizing the greater importance of patch selection for better management outcomes. To maximize the success of treatment programs, practitioners should first manage Phragmites patches adjacent to native plant species and in areas with minimal hydrologic disturbance.
... We consider that juvenile plants in isolated populations should be controlled fi rst. Prioritising low-density populations will be the most effi cient at containing the invasion and reducing the threat to endemic, rare and threatened plant species without signifi cantly increasing the cost (Higgins, et al., 2000). Targeting mature and juvenile individuals in isolated populations is a recommended strategy for other invasive plants in the Pacifi c Islands and likely to be more successful than removing large stands (Meyer, et al., 2011). ...
Invasive alien ornamental plants are a global problem, especially on oceanic islands, and can have severe impacts on native biodiversity. Pinanga coronata, is an ornamental palm tree that can form mono-dominant stands in its native habitat and is widely cultivated throughout the tropics. Here we investigate the introduction, spread, impact and
management of this invasive palm in the Fiji Islands, using extensive discussions with local experts and field surveys. Pinanga coronata was introduced in the 1970s to the Colo-i-Suva area, eastern Viti Levu island, Fiji´s principal island, and has since become invasive in mahogany plantations and lowland rainforest. It has also been introduced and is becoming invasive on the western side of that island. However, the distribution of P. coronata remains geographically limited to the immediate vicinity of introduction sites but it is rapidly spreading. In each location, the species has formed mono-dominant stands in the understorey and appears to be displacing native plant species, as suggested by a negative correlation of its abundance with that of native tree ferns. This highlights the need for rapid control of P. coronata in Fiji. Local experts state management should involve manual removal of seedlings and saplings, killing of adult palms by injection of herbicide, and education and legislation to prevent the further spread of the species. Based on these recommendations and fi eld data, management actions to control P. coronata are proposed and steps to develop these into a management plan are discussed. Given P. coronata threatens native biodiversity in Fiji and has the potential to invade other rainforest ecosystems in the tropics, proposed management approaches are urgent and relevant for other tropical countries.
... The chal- lenges are compounded where urban areas adjoin or enclose regions of high conservation value, when actions required to safeguard biodiversity may conflict with those deemed appropriate for requirements of urban residents. For exam- ple, managing urban plant invasions often requires deviation from the standard approach of prioritising less dense areas over dense areas (which is most cost-effective and beneficial in preventing impacts; Higgins et al. 2000) -dense stands of IAPs pose immediate risks in terms of fire, and safety and security and must often be prioritized. Managers frequently also need to deviate from strategic plans based on cost- effectiveness and biodiversity considerations to accom- modate prioritisation for socio-political reasons (e.g., public complaints). ...
The original version of the article unfortunately contained an error with the figure captions. The appropriate captions for Fig. 3-6 are published accordingly. The original article has been corrected.
... The challenges are compounded where urban areas adjoin or enclose regions of high conservation value, when actions required to safeguard biodiversity may conflict with those deemed appropriate for requirements of urban residents. For example, managing urban plant invasions often requires deviation from the standard approach of prioritising less dense areas over dense areas (which is most cost-effective and beneficial in preventing impacts; Higgins et al. 2000) -dense stands of IAPs pose immediate risks in terms of fire, and safety and security and must often be prioritized. Managers frequently also need to deviate from strategic plans based on costeffectiveness and biodiversity considerations to accommodate prioritisation for socio-political reasons (e.g., public complaints). ...
Alien plant invasions in urban areas can have considerable impact on biodiversity and ecosystem services (ES). Managing urban plant invasions is particularly challenging given the complex interactions between ecological, economic and social elements that exist in the urban milieu. Strategic landscape-scale insights are crucial for guiding management, as are tactical site-scale perspectives to plan and coordinate control efforts on the ground. Integrating these requirements to enhance management efficiency is a major challenge. Decision-support models have considerable potential for guiding and informing management strategies when problems are complex. This study uses multi-criteria decision tools to develop a prioritization framework for managing invasive alien plants (IAPs) in urban areas at landscape and local scales. We used the Analytic Hierarchy Process (AHP; a multi-criteria decision support model) to develop and rank criteria for prioritising IAP management in the City of Cape Town (CoCT), South Africa. Located within a global biodiversity hotspot, Cape Town has a long history of alien plant introductions and a complex socio-political make-up, creating a useful system to explore the challenges associated with managing urban plant invasions. To guide the prioritization of areas for IAP management across the CoCT, a stakeholder workshop was held to identify a goal and criteria for consideration, and to assess the relative importance given to each criterion in IAP management. Workshop attendees were drawn from multiple disciplines involved with different aspects of IAP research and management: government departments, scientists and researchers, and managers with a diverse set of skills and interests. We selected spatial datasets and applied our multi-criteria decision analysis in a Geographic Information System (GIS) to develop a landscape-scale prioritization map. To address issues relevant in an urban setting, we also modified an existing IAP management framework to develop a tactical (site-level) prioritization scheme for guiding on-the-ground control operations. High-priority sites for IAP management were identified at landscape-and local scales across the study area. Factors related to safety and security emerged as pivotal features for setting spatially-explicit priorities for management. The approach applied in this study can be useful for managers in all urban settings to guide the selection and prioritization of areas for IAP management.
... We propose that the species treated is, at a basic level, the proportion of species that require treatment that are treated (with the requirement defined by regulations or ideally some assessment of risk). At a more advanced level, species treated can be measured as the proportion of populations treated for each species, with an assessment of the quality of the interventions (Higgins et al., 2000, Taylor and Hastings, 2004, McConnachie et al., 2016. For sites treated, we similarly propose that the indicator would be measured as the total area subjected to control measures that needed control at a basic level, and at a more advanced level, assessed in terms of the quality of the interventions. ...
A major challenge for the management of biological invasions is to ensure that data and information from basic inventories and ecological research are used alongside data from the monitoring and evaluation of interventions to trigger and improve policy and management responses. To address this issue, South Africa has committed to report on the status of biological invasions and their management every 3 years.
We propose a framework of indicators for reporting on biological invasions at a country level; assess the feasibility of the indicators using South Africa as a case study; and outline steps needed for indicator development.
We argue that a national status report on biological invasions should explicitly consider indicators for pathways, species, and sites, and should report on interventions in terms of inputs, outputs, and outcomes.
We propose 20 indicators based on data currently available, as well as existing international policy initiatives. For each indicator, we have developed a factsheet that includes different hierarchical metrics (considering data availability) and provide suggestions on assigning confidence levels. We also combine these indicators into four high‐level indicators to facilitate broader reporting and describe how forecasted indicators based on the concept of invasion debt could assist with scenario planning.
We found that many of the data required for these indicators are already available in South Africa, but they have been poorly collated to date. However, data for the indicators of most direct value to policy and planning—those dealing with the impact of biological invasions and the outcome of interventions—are scarce.
Policy implications . The framework of indicators developed here, for what we believe is the first ever national‐level report on the status of biological invasions and their management, will facilitate the inclusion of biological invasions in environmental reporting at national and international levels. By identifying knowledge gaps, a status report will also focus efforts on determining the size of a country's invasion debt and what can be done to reduce it.
... The challenges are compounded where urban areas adjoin or enclose regions of high conservation value, when actions required to safeguard biodiversity may conflict with those deemed appropriate for requirements of urban residents. For example, managing urban plant invasions often requires deviation from the standard approach of prioritising less dense areas over dense areas (which is most cost-effective and beneficial in preventing impacts; Higgins et al. 2000) -dense stands of IAPs pose immediate risks in terms of fire, and safety and security and must often be prioritized. Managers frequently also need to deviate from strategic plans based on costeffectiveness and biodiversity considerations to accommodate prioritisation for socio-political reasons (e.g., public complaints). ...
Many alien plant species are introduced to urban areas to create, augment or restore ecosystem services (ES). However, many of these species spread beyond original plantings, sometimes causing negative effects on existing ES or creating novel ecosystem disservices (EDS). An understanding of the perceptions of urban residents regarding invasive alien plants (IAPs) and the ES and EDS they provide is needed for the effective prioritisation of IAP management efforts in cities. Using the city of Cape Town, South Africa as a case study, we conducted questionnaire-based surveys (online and face-to-face) to determine the perceptions of urban residents regarding IAPs and their capacity to provide ES and EDS. Most urban residents perceive IAPs negatively (i.e. agreeing that they create EDS), but many recognise their importance in providing ES. Although most residents are not opposed to the management of IAPs, such actions are not perceived as a high priority relative to other environmental problems. Socio-demographic variables such as age, education, environmental awareness, and ethnicity shape urban residents' perceptions of IAPs. Older, more educated respondents were more likely to perceive IAPs negatively, while respondents with greater environmental awareness were aware of the benefits provided by IAPs. This study highlights the need to integrate public perceptions into the planning and management of IAPs and emphasises the importance of including ES assessments into the decision-making process, particularly in urban areas.
... Therefore the estimated costbenefit ratio favours early intervention in outlying regions (at nascent foci) as opposed to long-term management of core areas (Higgins et al., 2000;Moody and Mack, 1988). Through this mechanism, a relatively small investment of resources can protect large areas of suitable habitat from potential invasion. ...
The report examines the development, evolution and application of national management zones for strategic management of Weeds of National Significance in Australia. In particular, the definitions, principles and parameters are examined for determining eradication, containment and outlier management and core zones. Additionally, the report outlines a decision making system and a suggested workshop planning guide.
... Invasive alien plant species are recognized as a major component of global change (Higgins and Richardson 1996;Higgins et al. 2000) with effects on biodiversity, ecosystem services, disturbance regimes, biogeochemical cycles, and human societies (Van Wilgen et al. 2001;Pauchard and Alaback 2004;Sharma et al. 2005;Thuiller et al. 2007;Gassó et al. 2009;Rödder et al. 2009;Pejchar and Mooney 2009). Species Distribution Models (SDMs) have been used for identifying suitable areas for species to invade and thus prioritizing IAPS surveys and management actions. ...
Alien plants invasion has negative impacts on the structure and functionality of ecosystems. Understanding the determinants of this process is fundamental for addressing environmental issues, such as the water availability in South Africa’s catchments. Both environmental and anthropogenic factors determine the invasion of alien species; however, their relative importance has to be quantified. The aim of this paper was to estimate the importance of 32 explanatory variables in predicting the distribution of the major invasive alien plant species (IAPS) of South Africa, through the use of Species Distribution Models. We used data from the National Invasive Alien Plants Survey, delineated at a quaternary catchment level, coupled with climatic, land cover, edaphic, and anthropogenic variables. Using two-part generalized linear models, we compared the accuracy of two different sets of variables in predicting the spatial distribution of IAPS; the first included environmental correlates alone, and the second included both environmental and anthropogenic variables. Using Random Forest, we explored the relative importance of the variables in producing a map of potential distribution of IAPS. Results showed that the inclusion of anthropogenic variables did not significantly improve model predictions. The most important variables influencing the distribution of IAPS appeared to be the climatic ones. The modeled potential distribution was analyzed in relation to provinces, biomes, and species’ minimum residence time.
... For species with limited dispersal capacity, control efforts should target vanguard populations to hamper rapid range expansion (Wadsworth et al. 2000). This simple rule of thumb has been especially well supported for slowing the spread of invasive trees (e.g. Watson 1985;Moody and Mack 1988;Doren and Jones 1997;Higgins et al. 2000). For species with the capacity for long-distance dispersal, particularly plant species with seeds dispersed by vectors, priority should be given to first controlling large source populations so that the primary propagule supply to long-distance dispersal is reduced, thus blocking the Fig. 3 Hierarchical partitioning of the independent contribution of each explanatory variable to the variation of spreading velocity. ...
Urban landscapes are highly fragmented (leading to the extinction of native species) as well as transformed and disturbed (creating novel environments). Such conditions provide non-native species with opportunities to establish and spread through “urban green areas” (UGAs). UGAs can serve as stepping stones for many alien species to recruit and may become sources of propagules to launch invasions in adjoining natural ecosystems. There is great diversity in the spatial structures of UGAs worldwide; these are determined by the city’s level of development, human density, urban planning policy, and history. We explore the invasion risks of, and the potential of invasive spread in, UGAs in the world’s 100 most populous cities (in 40 countries). Based on maps of enhanced vegetation index at 250 m resolution over the extent of 25 by 25 km for each city centre, we simulate the invasion and spread of a reference species (a virtual ruderal invasive species) from the city centre into surrounding urban or rural areas. Doing so allowed us to provide an objective baseline for comparing urban susceptibility to such invasions across diverse cultures, histories and societies. We derive the global ranking of invasive spread potential for each city based on the rate of spread of the reference species, and the ranking of 40 countries, based on the average rate of spread in their cities. We explore correlates of spread rates after 100 time steps (years) by examining the roles of climate (mean annual temperature and rainfall), human demography (city population size and growth rate), and socio-economic indicators [human footprint, human development index and gross domestic product (GDP) per capita]. Small city population size and high GDP per capita are the only significant predictors of high potential for invasive spread. Among the G20 countries, Canada, South Korea, South Africa, France, USA and Brazil all feature in the top-10 countries, and Atlanta, Washington, D.C. and Dallas in the USA, Chittagong in Bangladesh, Toronto in Canada and Brasilia in Brazil are listed among the top 10 cities overall. Our results can serve as a global baseline assessment of invasive spread risks through UGAs, and call for improved protocols for monitoring, planning and management of UGAs.
... Control programmes should be initiated as soon as possible, with priority being given to lightly invaded areas, isolated populations or the edges of invading populations. Higgins, Richardson and Cowling (2000) demonstrated that clearing strategies that prioritised low-density sites dominated by juvenile alien plants proved to be significantly more cost-effective than strategies that targeted densely invaded areas. These authors also found that delaying the initiation of clearing operations considerably increased the eventual costs of control and the risks to native biodiversity. ...
This article provides a preliminary list of alien plant species in the Serengeti-Mara ecosystem in East Africa. The list is based on broad-scale roadside surveys in the area and is supplemented by more detailed surveys of tourist facilities in the Masai-Mara National Reserve and adjoining conservancies. We encountered 245 alien plant species; significantly more than previous studies, of which 62 (25%) were considered to have established self-perpetuating populations in areas away from human habitation. These included species which had either been intentionally or accidentally introduced. Of the 245 alien plants, 212 (including four species considered to be native to the region) were intentionally introduced into gardens in the National Reserve and 51 (24%) had established naturalised populations within the boundaries of these tourism facilities. Of the 51 naturalised species, 23 (11% of the 212 alien species) were recorded as being invasive within the ecosystem, outside of lodges and away from other human habitation. Currently, the Serengeti-Mara ecosystem is relatively free of widespread and abundant invasive alien plants, with a few exceptions, but there are extensive populations outside of the ecosystem, particularly to the west, from where they could spread. We address the potential impacts of six species that we consider to pose the highest risks (Parthenium hysterophorus, Opuntia stricta, Tithonia diversifolia, Lantana camara, Chromolaena odorata and Prosopis juliflora). Although invasive alien plants pose substantial threats to the integrity of the ecosystem, this has not yet been widely recognised. We predict that in the absence of efforts to contain, or reverse the spread of invasive alien plants, the condition of rangelands will deteriorate, with severe negative impacts on migrating large mammals, especially wildebeest, zebra and gazelles. This will, in turn, have a substantial negative impact on tourism, which is a major economic activity in the area.
Conservation implications: Invasive alien plants pose significant threats to the integrity of the Serengeti-Mara ecosystem and steps will need to be taken to prevent these impacts. The most important of these would be the removal of alien species from tourist facilities, especially those which are known to be naturalised or invasive, the introduction of control programmes aimed at eliminating outlier invasive plant populations to slow down the spread, and the widespread use of biological control wherever possible.
... Areas identified as asset protection zones; . Areas of low-density invasions, where the returns on investment would be much higher (Higgins et al., 2000); . Areas that could provide access for security reasons, or to initial clearing sites; . ...
Alien plant invasions are a major threat to biodiversity and ecosystem functioning in South Africa’s Cape Floristic Region (CFR). Large-scale government-funded management initiatives are underway to reduce the extent of invasions in this region. Among the many challenges are the huge spatial extent of the invasions and difficulties in coordinating management efforts across large areas of invaded land in private ownership. Very little information is available on the success of privately-funded alien plant control initiatives. This study investigated the effectiveness of one large project in reducing alien plant cover, the challenges faced and the costs associated with long-term clearing operations on privately-owned land in the CFR. Results for the study area (Vergelegen) show that the cover of dense invasive plant stands declined by 70% over 10 years since management operations began, but that operations cost 3.6 times more than was originally estimated (ZAR 43.6 vs 12.19 million, respectively). The challenges associated with managing invasive alien plants (IAPs) on private land are very similar to those faced on state-owned land, with the efficiency of management being constrained by multiple interacting environmental and socio-economic factors. However some success in managing IAPs can be achieved by adhering to basic principles, including careful planning with clear achievable goals in mind, a commitment to stable long-term funding and regular monitoring. Most private land owners cannot afford the substantial investment of resources that would be required to clear large stands of IAPs and to ensure that cleared areas are maintained to prevent re-invasion, and finding ways to fund this remains a challenge.
... Moody and Mack (1988) showed, using a simple model, that clearing small, outlying stands first was more efficient than clearing large source stands. Mechanistic models of alien spread also show that clearing the sparsest stands first is most cost effective, and also best for mitigating threats to native biodiversity (Fig. 13, Higgins et al. 2000). give some examples of how spatially-explict models can be used to explore the implications of applying different treatments. ...
... Because seeds are spread by water along drainage lines, it is best to initiate clearing operations at the highest point in the catchment, and to work downstream. Research has also examined the relative merits of clearing alien plant invasions of differing densities on the effectiveness of control operations (Higgins et al., 2000). This research generally indicates that the most cost-effective approach is to prioritize clearing efforts on scattered rather than dense invasions (van ). ...
... • Hill et al. 2000. • Ress & Hill 2001 • Wittenberg & Cock 2001. ...
... • Hill et al. 2000. • Ress & Hill 2001 • Wittenberg & Cock 2001. ...
... • Hill et al. 2000. • Ress & Hill 2001 • Wittenberg & Cock 2001. ...
... Harris and Timmins (2009) demonstrated that if weed management is postponed until a species has become widespread, on average it is 40-times more expensive to remove and attempts are less likely to be successful than if managed earlier. Thus, one logical focus of weed management agencies should be on identifying and removing small populations of newly naturalised alien plant species that have a high likelihood of expanding their geographic ranges and becoming invasive weeds (Higgins et al. 2000). However, the assessment of climate suitability for New Zealand risk assessments is based only on current climate suitability to the region or conservancy of interest. ...
Climate change may exacerbate the impacts of plant invasions by providing opportunities for new
naturalisations and for alien species to expand into regions where previously they could not survive and reproduce. Although climate change is not expected to favour invasive plants in every case, in Aotearoa-New Zealand a large pool of potential new weeds already exists and this country is predicted to be an ‘invasion hotspot’ under climate change. In particular, ornamental garden plants originating from warmer native ranges are likely to naturalise and become invasive when climatic constraints are lifted. This forum article synthesises research on potential synergistic effects of plant invasion and climate change and discusses the general implications for management of weeds under climate change in New Zealand. A comprehensive assessment of three recently naturalised subtropical species (Archontophoenix cunninghamiana, Psidium guajava and Schefflera actinophylla) illustrates the potential risk of invasions by such species arising with climate change. Despite two recent Department of Conservation reports giving policy recommendations for conserving biodiversity under climate change, in which the potential of new weeds was highlighted as one of the most imminent threats, very little action has been taken. We argue that climate change needs to be accounted for in current weed management policies. Specifically, prediction of potential new weeds needs to be improved, considering up-to-date research and using best-practice modelling tools. Better surveillance for new infestations through spatial prioritisation by habitat suitability and use of well-managed citizen science projects is required. Education campaigns to raise awareness of climate change effects and new weed threats are essential. With an ever-increasing number of potential weeds but limited resources available, it is crucial that more preventative management is taken. Conservation managers, scientists and the general public need to combine their efforts to future-proof New Zealand weed management for the effects of climate change.
... Many authors have emphasized the importance of accounting for stochasticity when analyzing the management of invasive species (e.g. Higgins, Richardson and Cowling 2000;Eiswerth and van Kooten 2002;Grevstad 2005;Bogich and Shea 2008;Hyder, Leung, and Miao 2008). These studies mainly account for stochasticity of spread of the invasive species and ignore the stochasticity of control methods. ...
We analyze the dynamic process of invasive-species control in a spatially explicit and stochastic setting. An integer optimization model is applied to identify optimal strategies to deal with invasive species at a steady state. Optimal strategies depend on the spatial location of invasion as well as on stochastic characteristics of spread and control. Previous studies of invasive-species control have been stochastic or spatial, but not both. We model a landscape as consisting of multiple cells, each of which may be subject to border control or eradication within the cell. Optimal strategies from the model are characterized as eradication, containment, or abandonment of control. Representing the rate of species spread as stochastic rather than deterministic results in less-intensive control becoming optimal at equilibrium. The optimal strategy may switch from eradication to containment or from containment to abandonment. If an infestation occurs at the boundary of the region within which it may spread, it is more likely to be optimal to eradicate or contain the species, compared to an infestation in the interior of the region. If the effectiveness of border control is stochastic, then containment is not feasible in the long term, but it is still optimal as a temporary measure in some scenarios.
... We found that if the goal is to reduce invasive tree occupancy as much as possible within the time of the treatment period (2002–2009), given its budget of 35 million rand during this period, Working for Water would have been able to reduce the highest amount of invasive tree occupancy on transformed and abandoned plantation areas because the treatments were most cost-effective there. However, over a longer time-frame, and given that invasive trees will likely continue to spread on untransformed areas, it would be more cost-effective to target untransformed areas (Higgins et al. 2000). If the goal of the program were to avoid new areas from becoming invaded (e.g. to minimize biodiversity loss), then the program would have been better off targeting untransformed areas exclusively. ...
Prioritizing limited conservation funds for controlling biological invasions requires accurate estimates of the effectiveness of interventions to remove invasive species and their cost-effectiveness (cost per unit area or individual). Despite billions of dollars spent controlling biological invasions worldwide, it is unclear whether those efforts are effective, and cost-effective. The paucity of evidence results from the difficulty in measuring the effect of invasive species removal: a researcher must estimate the difference in outcomes (e.g. invasive species cover) between where the removal program intervened and what might have been observed if the program had not intervened. In the program evaluation literature, this is called a counterfactual analysis, which formally compares what actually happened and what would have happened in the absence of an intervention When program implementation is not randomized, estimating counterfactual outcomes is especially difficult. We show how a thorough understanding of program implementation, combined with a matching empirical design can improve the way counterfactual outcomes are estimated in nonexperimental contexts. As a practical demonstration, we estimated the cost-effectiveness of South Africa's Working for Water program, arguably the world's most ambitious invasive species control program, in removing invasive alien trees from different land use types, across a large area in the Cape Floristic Region. We estimated that the proportion of the treatment area covered by invasive trees would have been 49% higher (5.5% instead of 2.7% of the grid cells occupied) had the program not intervened. Our estimates of cost per hectare to remove invasive species, however, are three to five times higher than the predictions made when the program was initiated. Had there been no control (counterfactual), invasive trees would have spread on untransformed land, but not on land parcels containing plantations or land transformed by agriculture or human settlements. This implies that the program might have prevented a larger area from being invaded if it had focused all of its clearing effort on untransformed land. Our results show that, with appropriate empirical designs, it is possible to better evaluate the impacts of invasive species removal and therefore to learn from past experiences.
... We found that if the goal is to reduce invasive tree occupancy as much as possible within the time of the treatment period (2002–2009), given its budget of 35 million rand during this period, Working for Water would have been able to reduce the highest amount of invasive tree occupancy on transformed and abandoned plantation areas because the treatments were most cost-effective there. However, over a longer time-frame, and given that invasive trees will likely continue to spread on untransformed areas, it would be more cost-effective to target untransformed areas (Higgins et al. 2000). If the goal of the program were to avoid new areas from becoming invaded (e.g. to minimize biodiversity loss), then the program would have been better off targeting untransformed areas exclusively. ...
Invasive plant management is a costly endeavor, with new areas constantly being invaded and new invaders perpetually appearing. Managers need to be efficient in how they use the limited resources they have for management activities, especially when managing large areas or extensive invasions. While a large body of developed mathematical models predicting plant invasion spread and impact can help determine optimal strategies to achieve the maximum return from management investment, we still lack adequate knowledge to generalize optimal strategies in the field since little empirical work has been done to validate existing conclusions from the modeling approaches. To address critical gaps between invasive plant management and modeling, we discuss how a variety of developed models can be used to inform on-the-ground management and identify major challenges that inhibit field application of modeling findings. We propose potential solutions to these challenges, including (1) developing models that share mutual objectives with field managers; (2) using model input parameters that are easily measured in the field to improve communication and understanding between modelers and managers; (3) moving away from models that rely on data- and time-intensive individual-based measurements and towards models that are more patch-based and/or hierarchical in nature; and (4) encouraging managers to better integrate model output/results into management activities by developing field-applicable models with output parameters that are familiar to land managers, e.g., socioeconomic or logistical factors. These solutions are expected to facilitate communication and create a culture in which long-term interactions and interdisciplinary teams between managers and modelers can be built, which will ultimately contribute to more efficient incorporation of relevant modeling results into management activities and improve our ability to manage invasive plant species.
Dispersal drives invasion dynamics of nonnative species and pathogens. Applying knowledge of dispersal to optimize the management of invasions can mean the difference between a failed and a successful control program and dramatically improve the return on investment of control efforts. A common approach to identifying optimal management solutions for invasions is to optimize dynamic spatial models that incorporate dispersal. Optimizing these spatial models can be very challenging because the interaction of time, space, and uncertainty rapidly amplifies the number of dimensions being considered. Addressing such problems requires advances in and the integration of techniques from multiple fields, including ecology, decision analysis, bioeconomics, natural resource management, and optimization. By synthesizing recent advances from these diverse fields, we provide a workflow for applying ecological theory to advance optimal management science and highlight priorities for optimizing the control of invasions. One of the striking gaps we identify is the extremely limited consideration of dispersal uncertainty in optimal management frameworks, even though dispersal estimates are highly uncertain and greatly influence invasion outcomes. In addition, optimization frameworks rarely consider multiple types of uncertainty (we describe five major types) and their interrelationships. Thus, feedbacks from management or other sources that could magnify uncertainty in dispersal are rarely considered. Incorporating uncertainty is crucial for improving transparency in decision risks and identifying optimal management strategies. We discuss gaps and solutions to the challenges of optimization using dynamic spatial models to increase the practical application of these important tools and improve the consistency and robustness of management recommendations for invasions.
Abstract: Invasive alien species are responsible for several negative impacts worldwide. Managing biological invasions is often difficult and the success rate is quite low but with good planning it is possible to achieve good results. Besides employing the correct methods and techniques, an overall strategy based on landscape dynamics and expected spatial patterns can be fundamental to achieve success. The decision of where to act can be embedded in a general strategy based on several criteria/goals such as control of large populations, connectivity disruption, etc. This work focused on Acacia dealbata in a Natura 2000 site in Portugal, how the current amount and distribution can affect the spread pattern and different possible strategies to approach the management. Based on the species dispersal traits, we argue that not only the area but also the perimeter (therefore, the shape) and location of the patches should be considered when fighting the invasion. Three scenarios were designed and compared using the perimeter-area ratio, a landscape dynamics analysis and a connectivity index. Results show that the removing the patches with higher perimeter-area ratio (mostly small satellite patches) would be more impactful than removing the larger patch or removing random intermediary perimeter-area patches first. After this approach based on landscape dynamics, the employment of a connectivity assessment provided an ordered list of patches to remove sequentially. Overall, this approach can be valuable in the early steps of the planning process, supporting better decisions regarding the available resources and contributing to maximize the effectiveness of the action.
Citation: Machado, R., Duarte, L.N., Gil, A., Sousa-Neves, N., Pirnat, J., Santos, P. (2021). Supporting the spatial management of invasive alien plants through assessment of landscape dynamics and connectivity. Restoration Ecology. First Published: 27 October 2021, e13592. https://doi.org/10.1111/rec.13592
This study forms part of the Natural Capital Accounting and Valuation of Ecosystem Services (NCAVES) project and builds on the compilation of pilot physical and monetary ecosystem services accounts for KwaZulu-Natal for 2005-2011. The analysis demonstrates the utility of ecosystem accounts – consistent with the System of Environmental Economic Accounting – Ecosystem Accounting (SEEA EA) framework – to inform policymaking. The study investigates the economic case for ecosystem management and restoration interventions in the Thukela river basin over the period 2021-2030 to achieve or exceed land degradation neutrality relative to a 2015 baseline. This study weighs the benefits of ecosystem restoration – in terms of the monetary value of improved provision of selected ecosystem services against the costs of interventions.
In 2013, the U.S. Fish and Wildlife Service (USFWS) Conservation Objectives Team (COT) identified wildfire and the associated conversion of low‐ to mid‐elevation sagebrush (Artemisia tridentata) habitats to invasive annual grass‐dominated vegetation communities as the two primary threats to the sustainability of Greater sage‐grouse (Centrocercus urophasianus, hereafter GRSG) in the western portion of the species range. To facilitate the examination and evaluation of the role fire and invasive plants play in the conservation of GRSG, the USFWS solicited the assistance of the Western Association of Fish and Wildlife Agencies (WAFWA) to conduct a collaborative assessment of the conservation challenges associated with the fire and invasive threat. The results of this collaborative effort led by WAFWA can be found in a series of publications between 2014 and 2019. With respect to the management of invasive plants within the range of the GRSG, WAFWA assessed conditions and activities within the geographic range of GRSG, including parts of 11 states in the western United States and two Canadian provinces represented by the northwestern Great Plains, Great Basin, Snake River Plain, Colorado Plateau, and Columbia Basin. This chapter highlights the results of the assessment and identifies key challenges associated with invasive plant management across the range of the GSRG.
Invasive alien plants negatively impact ecosystems, necessitating intricate management actions. In a critically endangered vegetation type within the fynbos biome of South Africa, a study was performed comparing different management interventions over plots invaded by Acacia saligna. A dynamic modelling approach was designed to analyze field data and simulate effectiveness of several restoration methods. Field data for vegetation recovery rates over the course of two years were fed into the model, which allowed the extrapolation of multiple recovery trajectories over a long time‐span, not possible to obtain from traditional short‐term field surveys. Our model simulations show that different treatments in similarly degraded states at the time of clearing can result in vastly different recovery trajectories. Active seed sowing was initially most expensive but resulted in most successful native shrub recovery, decreasing costs of longer‐term follow‐up acacia clearing. Clearing without burning was cheapest but resulted in limited establishment of both native and acacia cover, providing an opportunity for secondary invasion by alien forbs. In this case, biotic thresholds may have been crossed which prevented recovery of certain vegetation components. Active sowing can partially reverse thresholds by restoring shrub cover but not structural diversity. Therefore even applying this treatment did not resemble vegetation structure of the reference condition after an extended period of 30 years, but does show how restoration can be improved by native seed sowing compared to passive restoration alone. Our model simulations provide a useful tool to support decision‐making by providing management recommendations for optimizing alien plant clearing protocols.
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Owing to their potentially wide-ranging adverse effects, invasive species are a growing global problem. The common ragweed (Ambrosia artemisiifolia var. elatior (L.) Desc) is one of the most important invasive plants, necessitating management because of its tendency to “spread.” Various studies and management strategies are being conducted based on the concept of “density” because of the increasing importance of the spatial perspective in this application. Although eradicating from the outliers (low-density regions) has a high efficiency, there is a lack of validation methodology for deriving both spatial and statistical results. We formed a general validation methodology by assessing various removal scenarios based on two removal strategies, namely Outside-in and Inside-out. These approaches exhibited several removal rates, and take into account the spatial perspective by considering species density. The Outside-in strategy entails the removal of species, which commences from the low-density regions, whereas the Inside-out removal initiates from the high-density regions. To classify the spatial regions for priority removal using each strategy, we defined the density level and then processed the removal of the occurrence points for each strategy to derive generalized results. We used the species distribution model MaxEnt to determine the predicted distribution of the target species for each removal strategy applied; subsequently, the final randomly generalized occurrence point results were used as model input data. Assessment analyses were conducted based on the final probability distribution and appearance level for each scenario, which included a newly proposed index was termed the “removal effect index.” Results indicated that the efficacy of the Outside-in removal strategy exceeded that of the Inside-out strategy for all assessment analyses, with the removal effect index showing a difference of about 2–5 times between strategies in each removal rate. In addition, through numerical analysis of the changed area of each scenario, the Outside-in strategy showed a successful removal effect in the “removal management priority spatial range,” whereas the Inside-out strategy showed limitations. We confirmed the efficacy of the Outside-in strategy as an optimal removal approach that takes into account spatial information of the priority spatial range for eradication in terms of the removal effect.
Decay rates in seed populations of Acacia cyclops and A. saligna of known viability and dormancy were determined after one and two years of burial at different densities. Decay rates differed significantly between the two species, but there was no clear effect of density. Decay rates were high, despite an initially high level of dormancy: an average of 97% and 45% of A. cyclops and A. saligna seeds, respectively, were lost in the first year. Decay rates were significantly lower in the second than in the first year and it is predicted that decay rates would continue to decline in seeds surviving more than two years. Acacia seed populations in the soil do not have a continuous and constant death risk and consequently do not fit the log-linear (i.e. Deevey Type II) survivorship curve reported in the literature for soil-stored seeds. As a small proportion of the Acacia seed population may have great longevity, clearing operations should incorporate a programme of periodic follow-up control.
This study describes changes in the abundance of shrub species after two fires in 1979 and 1980 on Otay Mountain in San Diego County, California. The 1979 fire burned a large area of dense chaparral and coastal sage scrub. The 1980 fire burned a portion of the 1979 fire area that had been seeded with annual ryegrass (Lolium multiflorum) as an erosion protection measure. Changes in the vegetation caused by the 1979 fire alone were similar to those commonly seen in chaparral wildfire, but the reburning of the vegetation in 1980 caused drastic changes in some areas. Ceanothus oliganthus was almost completely eliminated from the area of the 1980 burn. Adenostoma fasciculatum, the most abundant shrub at the study site, was reduced in density by up to 97%. Even Xylococcus bicolor, which normally resprouts with complete success after fire, suffered substantial mortality with reburning. It is concluded that the changes brought about by the 1980 fire will certainly persist for many decades. While sudden shifts in vegetation composition probably occurred without human intervention, we believe that human activity, especially after the introduction of aggressive annual grasses 200 yr ago, has caused an increase in the instances of abrupt change.
We describe and apply a correspondence between two major modeling approaches to forest dynamics: transition markovian models and Sap models or JABOWA-FORET type simulators. A transition model can be derived from a gap model by defining states on the basis of species, functional roles, vertical structure, or other convenient cover types. A gap-size plot can be assigned to one state according to dominance of one of these cover types. A semi-Markov framework is used for the transition model by considering not only the transition probabilities among the states, but also the holding times in each transition. The holding times are considered to be a combination of distributed and fixed time delays. Spatial extensions are possible by considering collections of gap-size plots and the. proportions of these plots occupied by each state. The advantages of this approach include: reducing simulation time, analytical guidance to the simulations, direct analytical exploration of hypothesis and the possibility of fast computation from closed-form solutions and formulae. These advantages can be useful in the simulation of landscape dynamics and of species-rich forests, as well as in designing management strategies. A preliminary application to the H. J. Andrews forest in the Oregon Cascades is presented for demonstration.
Recent data on biological invasion show that range expansion is driven by various modes of dispersal such as neighborhood diffusion and long-distance dispersal that occur side by side within a species. In such a stratified dispersal process, the initial range expansion mainly occurs by neighborhood diffusion. However, as the range of the founder population expands, new colonies created by long-distance migrants increase in number to cause an accelerating range expansion in the later phase. We classify several well-documented examples of geographical expansions into three major types depending on the nonlinearity of the range-versus-time curve. To examine how long-distance dispersal produces accelerating range expansion, we construct a stratified diffusion model, which describes the dynamics of the size distribution of colonies created by long-distance migrants. The model consists of a von Foerster equation combined with a Skellam model. Analyzing the model provides an estimate of range expansion in terms of the rate of expansion due to neighborhood diffusion, the leap distance, and the colonization rate of long-distance migrants. The results explain various types of nonlinear range expansion observed in biological invasions.
Recruitment, the addition of new individuals into a community, is an important factor that can substantially affect community composition and dynamics. We present a method for calibrating spatial models of plant recruitment that does not require identifying the specific parent of each recruitment. This method calibrates seedling recruitment functions by comparing tree seedling distributions with adult distributions via a maximum likelihood analysis. The models obtained from this method can then be used to predict the spatial distributions of seedlings from adult distributions. We calibrated recruitment functions for 10 tree species characteristic of transition oak-northern hardwood forests. Significant differences were found in recruitment abundances and spatial distributions. Predicted seedling recruitment limitation for test stands varied substantially between species, with little recruitment limitation for some species and strong recruitment limitation for others. Recruitment was limited due to low overall recruit production or to restricted recruit dispersion. When these seedling recruitment parameters were incorporated into a spatial, individual-based model of forest dynamics, called SOR-TIE, alterations of recruitment parameters produced substantial changes in species abundance, providing additional support for the potential importance of seedling recruitment processes in community structure and dynamics. 45 refs., 9 figs., 3 tabs.
South African fynbos ecosystems are under threat from alien plant invasions and transformations to alternative land uses. If large-scale habitat loss and species extinction are to be halted, restoration actions are urgently required. We postulate that by adopting an approach in which an understanding of community and ecosystem dynamics is applied to restoration practices, protocols can be developed which will lead to more efficient restoration. This understanding is based on a review of the relevant ecological literature, focusing on recruitment dynamics, community structure, and ecosystem function, which are particularly relevant to restoration. We develop a conceptual framework for restoration and apply our protocols to a case study area on the Cape Peninsula. Before ecological restoration can begin, the cause of transformation must first be removed or ameliorated. The next step is to ensure that the important ecological processes are functioning. We contend that a fully functioning community requires a good balance of the major growth-form, regeneration, and nutrient acquisition guilds. Fire is the natural disturbance event initiating recruitment in fynbos. It is, therefore, essential to either burn a site or provide fire-related germination cues in order to stimulate recruitment. Where guilds are under-represented, corrective reintroductions will further improve the long-term resilience of the restored community. Many taxa have persistent soil-stored seed banks, so it is important to conserve topsoil and optimize use of this local species pool. Seed dispersal distances are generally very short, and in highly transformed sites it will be necessary to reintroduce seed of the major guilds in order to restore community structure and functioning. Post-fire succession in fynbos begins with the full complement of species; species gradually die out from the vegetation according to their respective life spans. In order to stimulate germination and promote successful establishment, it is important to sow seed after fire or site clearing in late summer or autumn. Introducing seed or plants at a later stage in the succession is very likely to fail. Because of the localized distributions of many taxa, extreme care must be taken when selecting species for reintroduction if local gene pools are to be conserved.
Landscape ecologists deal with processes that occur at a variety of temporal and spatial scales. The ability to make predictions at more than one level of resolution requires identification of the processes of interest and parameters that affect this process at different scales, the development of rules to translate information across scales, and the ability to test these predictions at the relevant spatial and temporal scales. This paper synthesizes discussions from a workshop on Predicting Across Scales: Theory Development and Testing that was held to discuss current research on scaling and to identify key research issues.
Crown fires create broad-scale patterns in vegetation by producing a patch mosaic of stand age classes, but the spread and behavior of crown fires also may be constrained by spatial patterns in terrain and fuels across the landscape. In this review, we address the implications of landscape heterogeneity for crown fire behavior and the ecological effects of crown fires over large areas. We suggest that fine-scale mechanisms of fire spread can be extrapolated to make broad-scale predictions of landscape pattern by coupling the knowledge obtained from mechanistic and empirical fire behavior models with spatially-explicit probabilistic models of fire spread. Climatic conditions exert a dominant control over crown fire behavior and spread, but topographic and physiographic features in the landscape and the spatial arrangement and types of fuels have a strong influence on fire spread, especially when burning conditions (e.g., fuel moisture and wind) are not extreme. General trends in crown fire regimes and stand age class distributions can be observed across continental, latitudinal, and elevational gradients. Crown fires are more frequent in regions having more frequent and/or severe droughts, and younger stands tend to dominate these landscapes. Landscapes dominated by crown fires appear to be nonequilibrium systems. This nonequilibrium condition presents a significant challenge to land managers, particularly when the implications of potential changes in the global climate are considered. Potential changes in the global climate may alter not only the frequency of crown fires but also their severity. Crown fires rarely consume the entire forest, and the spatial heterogeneity of burn severity patterns creates a wide range of local effects and is likely to influence plant reestablishment as well as many other ecological processes. Increased knowledge of ecological processes at regional scales and the effects of landscape pattern on fire dynamics should provide insight into our understanding of the behavior and consequences of crown fires.
With 2285 species of higher plants crammed into 471 km2, the flora of South Africa's Cape Peninsula is exceptionally rich. Similar sized areas in other Mediterranean-climate region biodiversity hot-spots support between 4.7 and 2.7 times fewer species. The high plant species richness of the Cape Peninsula is due to the exceptionally high turnover between moderately species-rich sites in different habitats (beta diversity) and between sites in similar habitats along geographical gradients (gamma diversity). Highest beta diversity, encompassing almost complete turnover, was recorded along soil fertility gradients. Although similar patterns for these independent components explain the richness of other regions in the Cape Floristic Region, it is the very long and steep habitat gradients of the Cape Peninsula that makes this region exceptionally rich. Furthermore, the flora is characterized by a high degree of rarity, a phenomenon that undoubtedly influences the turnover. Future research should focus on developing a biological and ecological understanding of the different forms of rarity and integrating this into management plans for the maintenance of biodiversity.
The Cape Peninsula, a 470 km2 area of rugged scenery and varied climate, is located at the southwestern tip of the Cape Floristic Region, South Africa. The Peninsula is home to 2285 plant species and is a globally important hot-spot of biodiversity for higher plants and invertebrates. This paper provides a broad overview of the physiography, biological attributes and history of human occupation of the Peninsula. The Peninsula is characterized physiographically by extremely high topographical heterogeneity, very long and steep gradients in annual rainfall, and a great diversity of nutrient-poor soils. Thus, the Peninsula supports a high number of habitats and ecological communities. The predominant vegetation is fynbos, a fire-prone shrubland, and 12 broadly characterized fynbos types have been described on the Peninsula. Animal community structure, especially with regard to invertebrates, is poorly known. Vertebrate community structure is probably strongly influenced by nutrient poverty and recurrent fire. Generally, most vertebrates are small and typically occur in low numbers. Some invertebrates play keystone roles in facilitating ecological processes. Human occupation of the Peninsula was limited, until relatively recently, by nutrient poverty. After Dutch colonization in 1652, direct and indirect impacts on the natural ecosystems of the Peninsula escalated dramatically, and by 1994, some 65% of original natural habitat was either transformed by urbanization and agriculture, or invaded by alien plants. Nonetheless, there is still excellent potential to conserve the Cape Peninsula's remaining biodiversity.
The authors suggest that biological invasions of non-native species, for example the zebra mussel, have become so widespread as to constitute a significant component of global environmental change. They list a number of nonnative species that have invaded US National Parks in both terrestrial and aquatic ecosystems and describe the damage caused by these invasions. Consequences of invasions throughout the world are then described, including: their acting as vectors of disease; the economic damage they can cause; their altering of ecosystem processes; the reduction of biological diversity; and the promotion of extinctions in other species. Finally, the authors discuss what measures need to be taken in order to attempt to control invasions in the future.
This book is the second of two volumes in a series on terrestrial and marine comparisons, focusing on the temporal complement of the earlier spatial analysis of patchiness and pattern (Levin et al. 1993). The issue of the relationships among pattern, scale, and patchiness has been framed forcefully in John Steele’s writings of two decades (e.g., Steele 1978). There is no pattern without an observational frame. In the words of Nietzsche, “There are no facts… only interpretations.”
Examined stands of the invasive Australian Acacia saligna and surrounding sand plain lowland fynbos vegetation after an autumn fire. Indigenous forms regenerated less successfully in acacia-infested than in natural habitats. Seedling:parent ratios of proteoids were 32%, restioids 42% and erocoids 65% of those in natural vegetation. Seedling mortalities of all plant forms were not different in the two habitats. The risk of local extinction after fire was three to four times greater for ericoids, five times greater for proteoids and seven times greater for restioids in acacia-infested than in natural habitats. Within a 10m2 acacia stand area, the probability of extinction of proteoids was high (99.7%) and moderate (56-57%) for ericoid and restioid forms. Soil N, Ca, Mg, K, Mn and B concentrations were higher and soil Fe concentrations lower in acacia-infested habitats, but the only significant concentration increases recorded for all plant forms were in the N and K of seedling leaves in acacia stands. Increased shoot:root ratios observed among indigenous forms in acacia-infested habitats reflected responses to shading by burnt acacia parental remnants rather than soil mineral enrichment. Acacia shoot:root ratios were unaffected by the different environmental conditions in acacia stands. The depletion of indigenous taxa, particularly obligate reseeding forms, beneath acacia stands results mainly from their poor seed regeneration success and associated increased risk of local extinction from stochastic causes after fire. -from Author
Models of plant migration based on estimates of biological parameters severely underestimate the rate of spread when compared to empirical estimates of plant migration rates. This is disturbing, since an ability to predict migration and colonization rates is needed for predicting how native species will distribute themselves in response to habitat loss and climate change and how rapidly invasive species will spread. Part of the problem is the difficulty of formally including rare long‐distance dispersal events in spread models. In this article, we explore the process of making predictions about plant migration rates. In particular, we examine the links between data, statistical models, and ecological predictions. We fit mixtures of Weibull distributions to several dispersal data sets and show that statistical and biological criteria for selecting the most appropriate statistical model conflict. Fitting a two‐component mixture model to the same data increases the spread‐rate prediction by an average factor of 4.5. Data limit our ability to fit more components. Using simulations, we show that a small proportion (0.001) of seeds moving long‐distances (1–10 km) can lead to an order of magnitude increase in predicted spread rate. The analysis also suggests that most existing data sets on dispersal will not resolve the problem; more effort needs to be devoted to collecting data on long‐distance dispersal. Although dispersal had the strongest effect on the predicted spread rate, we showed that dispersal interacts strongly with plant life history, disturbance, and habitat loss in influencing the predicted rate of spread. The importance of these interactions means that an approach that integrates local and long‐distance dispersal with plant life history, disturbance, and habitat availability is essential for predicting migration rates.
Alien plants invade many ecosystems worldwide and often have substantial negative effects on ecosystem structure and functioning. Our ability to quantitatively predict these impacts is, in part, limited by the absence of suitable plant-spread models and by inadequate parameter estimates for such models. This paper explores the effects of model, plant, and environmental attributes on predicted rates and patterns of spread of alien pine trees (Pinus spp.) in South African fynbos (a mediterranean-type shrubland). A factorial experimental design was used to: (1) compare the predictions of a simple reaction-diffusion model and a spatially explicit, individual-based simulation model; (2) investigate the sensitivity of predicted rates and patterns of spread to parameter values; and (3) quantify the effects of the simulation model's spatial grain on its predictions. The results show that the spatial simulation model places greater emphasis on interactions among ecological processes than does the reaction-diffusion model. This ensures that the predictions of the two models differ substantially for some factor combinations. The most important factor in the model is dispersal ability. Fire frequency, fecundity, and age of reproductive maturity are less important, while adult mortality has little effect on the model's predictions. The simulation model's predictions are sensitive to the model's spatial grain. This suggests that simulation models that use matrices as a spatial framework should ensure that the spatial grain of the model is compatible with the spatial processes being modeled. We conclude that parameter estimation and model development must be integrated procedures. This will ensure that the model's structure is compatible with the biological processes being modeled. Failure to do so may result in spurious predictions.
1. The invasion of fynbos shrublands by woody weed species can reduce the water yield from catchment areas dramatically. We modelled the consequences of uncontrolled invasion on water yield using a geographical information system (Arc/Info). 2. Five important processes were recognized: the occurrence of fire; the spread and establishment of alien plants after fire; rainfall-to-run-off ratios; growth and changes in biomass between fires; and effects of these changes on streamflow. 3. The simulations of water yield were modelled with the Arc/Info GRID module using a 200 x 200-m grid. It was assumed that the interval between fires was 15 years and that proliferation and dispersal of alien plants took place only after fires. 4. Between fires, the model simulated the growth of the vegetation and its effects on streamflow, using relationships between rainfall and run-off, and run-off and above-ground biomass. 5. Results for the Kogelberg area in the Western Cape Province showed that alien plants invaded about 40% of the grid cells within 50 years. Cover of alien plants increased from an initial estimate of 2.4% to 62.4% after 100 years. 6. Invasion of catchment areas would result in an average decrease of 347 m3 of water per hectare per year over 100 years, resulting in average losses of more than 30% of the water supply to the city of Cape Town. In individual years, where large areas would be covered by mature trees, losses would be much greater. 7. In addition, invasion of fynbos by alien plants will cause the extinction of many plant species, increase the intensity of fires, destabilize catchment areas with resultant erosion and diminished water quality, and decrease the aesthetic appeal of mountain areas. 8. Control of alien weed species is necessary to avert the above impacts, and the costs of control operations could be justified by the savings achieved in maintaining adequate water run-off from stable catchments in the long term.
1. The impact of dense stands of the alien invasive species Acacia saligna (Labill) Wendl. on the guild structure of indigenous fynbos vegetation was investigated at three sites on the Cape Peninsula, South Africa. Two Acacia stands of either recent (1-2 fire cycles) or longer (>2 fire cycles) origin were compared with neighbouring uninvaded vegetation. 2. At one site fynbos recruitment was monitored following complete removal of vegetation in the three different stands in order to assess the restoration potential of invaded vegetation. 3. Fynbos species richness, cover and frequency all declined through each invasion stage in the standing vegetation. Guild structure also changed: species with vertebrate-dispersed seeds were relatively more frequent in long-invaded stands and at two of the three sites long-invaded stands had relatively more tall shrubs and fewer shrubs with leptophyllous (ericoid) leaves compared to fynbos. Serotinous shrubs were virtually absent in long-invaded stands. 4. Richness per plot of species recruited by seedlings was highest in fynbos and declined both with stage of invasion and time after clearing. However, the total stand species richness was nearly as high in recently invaded as in uninvaded fynbos. 5. Ephemeral forbs constituted the majority of seedlings in all stands, but after two years they remained the most important growth form in terms of density and cover only in the long-invaded stand. 6. At 18 months after clearing, indigenous canopy cover was about 50% in all stands, but only in uninvaded fynbos did this continue to increase over the second summer. After two years, resprouters formed about half the canopy cover in fynbos and recently invaded stands, but were insignificant in the long-invaded stand. 7. After clearing, many species recruiting in the long-invaded stand were not present in the standing vegetation, indicating that persistent seed banks exist. 8. As representatives of all the major fynbos growth forms were recruited into the long-invaded stand after clearing, albeit at a low density, such stands could revert to vegetation resembling fynbos in structure. However, in order to speed up the restoration process and to improve the probability of a fully functioning ecosystem being established, guilds which have been eliminated, such as serotinous Proteaceae, should be reintroduced.
Plant invasions are a serious threat to natural and managed ecosystems worldwide. The number of species involved and the extent of existing invasions renders the problem virtually intractable, and it is likely to worsen as more species are introduced to new habitats and more existing invaders move into a phase of rapid spread. We contend that current research and management approaches are inadequate to tackle the problem. The current focus is mostly on the characteristics and control of individual invading species. Much can be gained, however, by considering other important components of the invasion problem. Patterns of weed spread indicate that many species have a long lag phase following introduction before they spread explosively. Early detection and treatment of invasions before explosive spread occurs will prevent many future problems. Similarly, a focus on the invaded ecosystem and its management, rather than on the invader, is likely to be more effective. Identification of the causal factors enhancing ecosystem invasibility should lead to more-effective integrated control programs. An assessment of the value of particular sites and their degree of disturbance would allow the setting of management priorities for protection and control. Socioeconomic factors frequently play a larger part than ecological factors in plant invasions. Changes in human activities in terms of plant introduction and use, land use, and timing of control measures are all required before the plant invasion problem can be tackled adequately. Dealing with plant invasions is an urgent task that will require difficult decisions about land use and management priorities. These decisions have to be made if we want to conserve biodiversity worldwide.
Plant species continue to be introduced in North America for various purposes. If the trend continues, it is probable that some will escape cultivation and become invasive in native ecosystems. We present a retrospective analysis of several structural, life history, and biogeographical attributes of woody plants introduced in North America to determine which traits characterize species that have and have not invaded. Predictive models derived from discriminant analysis correctly classified 86.2% of the species in cross-validation, whereas those derived from classification and regression trees classified 76% correctly. From these models we created a hierarchical predictive tree that allows the user to divide species into three categories: admit (low risk of invasiveness), deny admission (high risk of invasiveness), or delay admission for further analyses and/or monitor intensively (risk cannot adequately be assessed based on only the included attributes). We recommend that species that are highly invasive elsewhere not be allowed into the U.S. and that a more conservative introduction policy using a hierarchical predictive method be employed.
A large data set on the biophysical characteristics of north-eastern New South Wales, a region of 7.6 × 106 ha, was analysed to measure the level of protection of the region's forests from extractive uses. A classification of 81 environmental units was derived by subdividing the region according to combinations of mean annual rainfall, mean annual temperature, soil fertility and slope. All environmental units contain forest, although some also contain other vegetation formations. The units were used to separate the region's forests into 81 classes. This approach produced a consistent classification of environments for the entire region that could be applied across all land tenures and uses. Derivation of the classification from physical data also enabled the extent of clearing of each environmental unit to be accurately assessed.
The reduction of species richness of indigenous plants is one of the major problems associated with the presence of dense stands of invasive alien trees and shrubs in the Fynbos Biome of the Cape Province, South Africa. A synthesis was made of published and unpublished data on plant species richness in fynbos with different levels of invasion and different histories of control. Linear regressions of species richness on the log of quadrat size were significant for both uninvaded fynbos and fynbos under dense stands of alien trees and shrubs. The slopes of the regression equations did not differ significantly between invaded and uninvaded sites, but elevations were significantly different, indicating a marked reduction in richness of indigenous plant species in invaded areas. The linear regression of species richness on quadrat size for cleared areas was not significant, but quadrats at most cleared sites showed species richness values intermediate to those of uninvaded fynbos and dense stands of aliens. Reductions in species richness at the scale of the sample quadrats used in this study (4–256 m) occur once the canopy cover of aliens exceeds about 50% and there is evidence of reduced species richness with increased time of suppression. For this reason, stands should be cleared before canopy closure is achieved.
SUMMARY (I) We evaluated through simulation the spatial growth of an invading terrestrial plant population and various strategies for its control. The initial population comprised a single large expanding focus but had the potential for the continual establishment of new foci. (2) We compared the area occupied through the establishment and expansion of these "satellite" foci to the area occupied by the initially large or main focus under varying regimens of repeated control, in which either the area of the main focus was reduced or some satellites were destroyed, or both. (3) Whether varying growth rates for the foci, rates of satellite establishment, the level of reduction of the main focus or the intensity of satellite detection and destruction, the overall effectiveness of control measures was greatly improved by destroying even 30% of the satellites. (4) These predictions contrast with much current practice in the control of alien plants, where large or otherwise conspicuous infestations are often treated at the expense of eradicating isolated populations while they still remain small. As supported by empirical precedents, consistent implementation of the general strategy suggested by our model should improve the control of alien plants.
Process‐based models, and spatially explicit models in particular, will play an important role in predicting the impacts of future environmental change. Enthusiasm for the rich potential of these models, however, is tempered by the realization that their parameterization is often challenging and time consuming. Moreover, these models are seldom validated; this makes their predictive value in applied contexts uncertain.
In this paper we describe the process of parameterizing and validating a spatial demographic model of a spreading alien plant population. The model, a spatially explicit individual‐based simulation, has modest data requirements (for a spatial simulation model) in that it concentrates on simulating recruitment, dispersal, mortality and disturbance and ignores the environmental and biotic heterogeneity of the receiving environment.
We tested the model using the invasion of Acacia cyclops and Pinus pinaster into fynbos, the mediterranean shrublands of South Africa, as a case study. Dispersal, recruitment and mortality data were collected for each species at six different sites. Aerial photographs from six independent sites (two sites for A. cyclops and four sites for P. pinaster) were used to reconstruct the invasion histories of the two species between 1938 and 1989. Demographic data were used to parameterize the model, and the 1938 distribution of alien plants, derived from aerial photography, was used to initialize the model.
The empirically estimated indices of rate and pattern of invasion fell within the range of model predictions made at all six sites studied. The indices of rate and pattern of invasion predicted by the model did not differ significantly from the empirically estimated indices for 76% of the model data comparisons made. These analyses suggested that the model predictions are good, given the variance in parameter estimates.
The proportion of grid locations where the model correctly predicted alien plant distribution was typically above 0·75 and always above 0·5 for both species. A permutation test showed that locations of invasive plants predicted by the model were significantly better than random for P. pinaster , but not always for A. cyclops; this may be because A. cyclops is bird dispersed, and its dispersal may be biased towards perch sites, whereas P. pinaster is wind dispersed.
We conclude that, although spatial simulation models are often more difficult to parameterize and validate than statistical or analytical models, there are situations where such effort is warranted. In this case the validation process provides confidence to use the model as a tool for planning the control of invasive plants. In a more general sense we believe that the approach outlined here could be used for model parameterization and validation in situations where spatial simulation models seem appropriate.
Invasive alien organisms pose a major threat to global biodiversity. The Cape Peninsula, South Africa, provides a case study of the threat of alien plants to native plant diversity. We sought to identify where alien plants would invade the landscape and what their threat to plant diversity could be. This information is needed to develop a strategy for managing these invasions at the landscape scale. We used logistic regression models to predict the potential distribution of six important invasive alien plants in relation to several environmental variables. The logistic regression models showed that alien plants could cover over 89% of the Cape Peninsula. Acacia cyclops and Pinus pinaster were predicted to cover the greatest area. These predictions were overlaid on the current distribution of native plant diversity for the Cape Peninsula in order to quantify the threat of alien plants to native plant diversity. We defined the threat to native plant diversity as the number of native plant species (divided into all species, rare and threatened species, and endemic species) whose entire range is covered by the predicted distribution of alien plant species. We used a null model, which assumed a random distribution of invaded sites, to assess whether area invaded is confounded with threat to native plant diversity. The null model showed that most alien species threaten more plant species than might be suggested by the area they are predicted to invade. For instance, the logistic regression model predicted that P. pinaster threatens 350 more native species, 29 more rare and threatened species, and 21 more endemic species than the null model would predict. Comparisons between the null and logistic regression models suggest that species richness and invasibility are positively correlated and that species richness is a poor indicator of invasive resistance in the study site. Our results emphasize the importance of adopting a spatially explicit approach to quantifying threats to biodiversity, and they provide the information needed to prioritize threats from alien species and the sites that need urgent management intervention.
Resumen: Organismos invasores poseen una amenaza grave para la biodiversidad global. La Península Cape, en Sudáfrica provee un caso de estudio de la amenaza de plantas invasoras sobre la diversidad nativa de plantas. Intentamos identificar si las plantas pueden invadir el paisaje y cual sería su amenaza sobre la diversidad de plantas. Esta información es necesaria para desarrollar una estrategia para el manejo de estas invasiones a escala de paisaje. Mediante el uso de modelos de regresion logística predecimos la distribución potencial de seis especies de plantas invasoras importantes en relación con diversas variables ambientales. Los modelos de regresión logística mostraron que las plantas invasoras podrían cubrir mas de un 89% de la Península Cape. Acacia cyclops y Pinus pinaster fueron predecidas como las especies que cubrirían mas área. Estas predicciones fueron sobrepuestas en un mapa de distribución actual de la diversidad de plantas nativas de la Península Cape con la intención de cuantificar la amenaza de las especies invasoras sobre la diversidad de especies nativas. Definimos la amenaza a la diversidad de plantas nativas como el número de especies de plantas nativas (divididas como: todas las especies, especies raras y amenazadas y especies endémicas) que tienen su rango total cubierto por la distribución predecida de plantas invasoras. Un modelo nulo que asume una distribucióñ aleatoria de sitios invadidos se empleó para evaluar si el área invadida estaba siendo confundida con la amenaza de la diversidad sobre plantas nativas. De hecho, el modelo de regresión logística predijo que Pinus pinaster amenaza a 350 especies nativas, 29 consideradas raras y amenazadas y 21 especies endémicas mas de lo que el modelo nulo predijo. Las comparaciones entre los modelos nulos y logísticos sugieren que la riqueza de especies y la invisabilidad estan positivamente correlacionadas y que la riqueza de especies es un indicador pobre de la resistencia contra invasiones en el área de estudio. Nuestros resultados hacen énfasis en la importancia de adoptar aproximanciones espacialmente explícitas para cuantificar las amenazas contra la biodiversidad y proveer la información necesaria para priorizar amenazas por especies invasoras y los sitios que necesitan una intervención de manejo urgente.
Current theories of plant invasion have been criticized for their limited heuristic and predictive value. We explore the heuristic and predictive potential of a model which explicitly simulates the mechanisms of plant invasion. The model, a spatially-explicit individual-based simulation, is applied to the invasion of pine trees (Pinus spp.; Pinaceae) in three vegetation types in the southern hemisphere. The model simulates factors which have been invoked as major determinants of invasive success: plant traits, environmental features and disturbance level. Results show that interactions between these determinants of invasive success are at least as important as the main effects. The complexity of invasions has promoted the belief that many factors must be invoked to explain invasions. This study shows that by incorporating interactions and mechanisms into our models we can potentially reduce the number of factors needed to predict plant invasions. The importance of interactions, however, means that predictions about invasions must be context-specific. The search for all-encompassing rules for invasions is therefore futile. The model presented here is of heuristic value since it improves our understanding of invasions, and of management value since it defines the data and models needed for predicting invasions.
The Cape Peninsula, a landscape of profound scenic beauty, is also botanically exceptionally species-rich and has high concentrations of both endemic and threatened plant species. Alien invasive trees, urban expansion and growing tourism are impacting increasingly on the landscape and biota. Three reserve scenarios were modelled, the primary objective being to maximize the conservation of biodiversity in a manner which takes both cost and efficiency into account. A comprehensive plant species database, an endemic animali species database, a vegetation type database, land-tenure and land-use data were used in this process. The resolution of all databases was by 1 km cells. The first scenario investigated the effectiveness of the existing reserve system in conserving the Peninsula's biodiversity. The second assessed the benefit of adding all publicly owned and to the existing reserves. In scenario three, a reserve-selection algorithm was applied to conserve those plant species outside existing reserves at least once. Where endemic animal species, and areas with high concentrations of threatened and endemic plant species were not adequately conserved, extra cells were added for their inclusion. Finally, one cell was added to cater for one inadequately conserved vegetation type. Fifty-one cells were needed to satisfy the requirements stipulated for scenario three. Analyses showed that 22% of plant species have all their records within existing reserves. Adding all public land improves the status to 43% with 97% having >50% of their records included in reserves. In scenario three, these figures are 32 and 87% respectively. In terms of animal species, four species are unconserved in scenario one, two in scenario two, and all species are conserved in scenario three. We conclude that scenarios two (including all public areas) and three (iterative selection to conserve each species once) provide practicable options for conserving the Peninsula's remaining biodiversity.
The biodiversity of the Cape Peninsula (49127 ha in extent) has been considerably affected by various factors since European settlement in 1652. Urbanization and agriculture have transformed 37% of the original area of natural vegetation. Lowland vegetation types have been worst affected, with almost half of the transformation occurring in one of the 15 recognized vegetation types. Vegetation at high altitudes has been little affected by urbanization and agriculture, but alien trees and shrubs are now threatening biodiversity in these areas. Of the area not affected by urbanization and agriculture 10.7% is currently under dense stands (>25% canopy cover) of alien plants and another 32.9% is lightly invaded. Dense stands of Acacia cyclops, the most widespread invader, cover 2510 ha, 76% of the total area under dense alien stands. This paper assesses the impacts of these factors on aspects of the plant biodiversity of the area, namely, the distribution of major vegetation types and of endemic, rare and threatened plant taxa and of taxa in the Proteaceae (a prominent element in almost all communities, taken as an indicator of overall plant biodiversity).
Possible future impacts on biodiversity are assessed by considering the effects of several scenarios involving increased urbanization and changes to alien plant control strategies and further spread over the next 50–100 years. The worst-case scenario for urbanization sees the area under natural vegelation reduced to 12163 ha (39.3% of its extent in 1994, or 24.8% of its original extent). Under this scenario almost a quater of the 161 endemic, rare and threatened (‘special’) taxa are confined totally to urban areas; 57.4% of the known localities of these taxa, and 40.1% of the remaining localities of Proteaceae taxa are transformed. Dense alien stands currently affect 29.8% of the localities of special taxa known from herbarium records and 8.4% of these taxa currently occur only in areas at present affected by aliens. Clearing all dense stands would result in 62.9% of special taxa having less than half of their known localities affected (49.1% at present). Under this scenario, 92% of Proteaceae taxa have more than 75% of their localities unaffected by aliens. If clearing is confined to specific areas (the Cape Peninsula Protected Natural Environment or all publicly-owned land) and the aliens spread further outside these areas, the area of natural vegetation remaining shrinks (to 82.4% of the current extent if control is confined to public land). The further losses in biodiversity associated with these scenarios are described. If control programmes collapse and all potentially invadable land is occupied by dense alien stands, only 407 ha of natural vegetation would remain (1.5% of the current extent).
The probability of the various scenarios materializing is discussed. To minimize further losses in biodiversity it is essential that: (1) a major initiative is launched immediately to clear (firstly) the 10184 ha of lightly invaded vegetation and then the 3313 ha of densely invaded vegetation; (2) no urban development be permitted within the boundaries of the Cape Peninsula Protected Natural Environment; (3) a systematic programme of prescribed burning (linked to the alien control programme) is initiated; and (4) contingency measures are implemented to improve the status of seriously threatened taxa, habitats and vegetation types.
The Cape Peninsula (area: 471 km2), situated at the south-western extremity of the Cape Floristic Region, has exceptionally high plant species richness (2285 species and infraspecific taxa) and numbers of endemic (90; 88 species and two infraspecific) and threatened (141; 138 species and three infraspecific) taxa (termed species from here on). This biodiversity is threatened by urban development and the spread of invasive alien plants. Peninsula endemics are concentrated in a few, predominantly species-rich families and these correspond well with endemic-rich families in other areas of the Cape Floristic Region. A high level of similarity exists between families with threatened and families with endemic species. A frequency analysis of the biological traits of both endemic and threatened species shows that low growing, ant-dispersed shrubs are over-represented in both groups. Endemics are most likely to be non-sprouters, but threatened plants do not have a specific post-fire regeneration strategy. Threatened species have higher frequencies of geophytes, sprouters and wind-dispersed species compared to endemic species. Numbers of endemic and threatened species are not randomly distributed with regard to occurrence in vegetation types and patterns are similar for both groups. The habitat and biological profiles of both endemic and threatened species suggest that they are highly vulnerable to extinction as a result of increasing rates of alien plant infestation, urbanization and inappropriate fire regimes.
The use of large grid cell databases (1/2 to 5) to drive nonlinear ecosystem process models may create an incompatibility of scales which can often lead to biased outputs. Global simulations of net primary production (NPP) often assume that bias due to averaging of sub-grid variations in climate, topography, soils, and vegetation is minimal, yet the magnitude and behavior of this bias on estimates of NPP are largely unknown. The effects of averaging sub-grid land surface variations on NPP estimates were evaluated by simulating a 1 1 land surface area as represented by four successive levels of landscape complexity, ranging from a single computation to 8,456 computations of NPP for the study area. Averaging sub-grid cell landscape variations typical of the northern US Rocky Mountains can result in overestimates of NPP as large as 30 %. Aggregating climate within the 1 cell contributed up to 50 % of the bias to NPP estimates, while aggregating topography, soils, and vegetation was of secondary importance. Careful partitioning of complex landscapes can efficiently reduce the magnitude of this overestimation.
In the coastal areas of northern Australia, the introduced shrub Mimosa pigra has been spreading rapidly in native ecosystems during the past decade. An almost monospecific tall shrubland replaces sedgeland initially, then riparian, aquatic, paperback and monsoon forest communities. The flora and fauna of two areas, one infested for about five years and the other for three, were compared with uninfested areas nearby. Although small mammals were more abundant at least in the short term, and frogs seemed little affected, many birds and lizards were lower in abundance. Furthermore, it is likely that the advantage of the new habitat for small mammals is in the form of shelter from avian predation and that this would disappear as the shrubland spreads and removes the foods resources of nearby native habitats. A massive loss of animal and plant species may well occur if the spread of this aggressive weed is not halted.
The development and application of an expert system is described for screening alien woody plants for their invasive potential in South African fynbos. The system is proposed for use by potential introducers to demonstrate low invasive risk before importing woody alien species for cultivation. Rules for the system were derived from empirical evidence by quantifying invasion windows and barriers that have limited the set of widespread woody invaders (trees and shrubs) in fynbos to fewer than 20, out of several hundred introduced species. The system first compares broad-scale environmental conditions (climate and soil) between the home environment of a species and fynbos. Features of the plant in its home environment (basic life history traits, population characteristics, regeneration biology, habitat preferences) are then assessed. Finally, an assessment is made of life history adaptations to the prevailing fire regime in fynbos (juvenile period, fire-survival capacity of adult plants, seed bank longevity). The reasoning is explicit and the steps leading to a conclusion (high risk/low risk) can be retraced.Besides the obvious application in identifying species with a high risk of invading, the system has considerable potential for modelling, and for teaching the concepts of biological invasions. The rules provide an explicit conceptualization of invasion processes in fynbos and identify multiple paths to invasive success (not all of which have been realized yet). The system can therefore be used in planning control operations (for optimal allocation of control effort to critical stages in invasion), and for predicting the outcome of changes (e.g. in fire frequency) on the dimensions of invasion windows, and for assessing what changes are needed to prevent or reduce the extent of invasion by a given taxon.Application of the system is demonstrated on Pinus and Banksia taxa and a selection of species from Californian chaparral.
Alien plants invade many ecosystems worldwide, often having substantial negative effects on ecosystem structure and functioning. The apparent complexity of invasions has impaired the development of a predictive framework of alien plant spread. Such a framework requires both a conceptual understanding of the ecology of invasions and appropriate modelling tools. We demonstrate, using a simple conceptual model and illustrative examples from the literature, that a predictive understanding of invasions can be established. Potential modelling tools are reviewed by categorizing models of plant spread as either simple-demographic, spatial-phenomenological or spatial-mechanistic, based on the model's data inputs and outputs. The assumptions, predictive potential, knowledge and data requirements of these modelling tools are discussed in the context of selecting the most appropriate alien plant spread model for a given case.
Argues that the problem of pattern and scale is the central problem in ecology, unifying population biology and ecosystems science, and marrying basic and applied ecology. Applied challenges, such as the prediction of the ecological causes and consequences of global climate change, require the interfacing of phenomena that occur on very different scales of space, time, and ecological organization. Systems generally show characteristic variability on a range of spatial, temporal, and organizational scales. The observer imposes a perceptual bias, a filter through which the system is viewed. This has fundamental evolutionary significance, since every organism is an "observer' of the environment, and life history adaptations such as dispersal and dormancy alter the perceptual scales of the species, and the observed variability. The key to prediction and understanding lies in the elucidation of mechanisms underlying observed patterns. Typically, these mechanisms operate at different scales than those on which the patterns are observed; in some cases, the patterns must be understood as emerging from the collective behaviors of large ensembles of smaller scale units. In other cases, the pattern is imposed by larger scale constraints. Examination of such phenomena requires the study of how pattern and variability change with the scale of description, and the development of laws for simplification, aggregation, and scaling. -from Author
Prior to the arrival of white settlers, fire-return intervals in the sagebrush Artemisia steppe probably varied between 60-110 yr but much of the region now burns at intervals of <5yr. Cheatgrass Bromus tectorum, an introduced annual, increases fire frequencies by creating a more continuous fuelbed. More-frequent fires and reduced patchiness prevent, or greatly retard, normal vegetation replacement sequences leading to vegetation resembling less-frequently burned areas. Reducing the frequency and size of fires on these areas should be a primary management objective. -Author
Myrica faya, an introduced actinorhizal nitrogen fixer, in invading young volcanic sites in Hawaii Volcanoes National Park. We examined the population biology of the invader and ecosystem-level consequences of its invasion in open-canopied forests resulting from volcanic cinder-fall. Although Myrica faya is nominally dioecious, both males and females produce large amounts of fruit that are utilized by a number of exotic and native birds, particularly the exotic Zosterops japonica. In areas of active colonization, Myrica seed rain under perch trees of the dominant native Metrosideros polymorpha ranged from 6 to 60 seeds mâ»Â² yrâ»Â¹; no seeds were captured in the open. Planted seeds of Myrica also germinated an established better under isolated individuals of Metrosideros than in the open. Diameter growth of Myrica is > 15-fold greater than that of Metrosideros, and the Myrica population is increasing rapidly. Rates of nitrogen fixation were measured using the acetylene reduction assay calibrated with ¹âµN. Myrica nodules reduced acetylene at between 5 and 20 μmol gâ»Â¹ hâ»Â¹, a rate that extrapolated to nitrogen fixation of 18 kg haâ»Â¹ in a densely colonized site. By comparison, all native sources of nitrogen fixation summed to 0.2 kg haâ»Â¹ yrâ»Â¹, and precipitation added < 4 kg haâ»Â¹ yrâ»Â¹. Measurements of litter decomposition and nitrogen release, soil nitrogen mineralization, and plant growth in bioassays all demonstrated that nitrogen fixed by Myrica becomes available to other organisms as well. We conclude that biological invasion by Myrica faya alters ecosystem-level properties in this young volcanic area; at least in this case, the demography and physiology of one species controls characteristics of a whole ecosystem.
The statistical expectation operator can be used as a rigorous method for translating fine-scale relationships to coarser scales without aggregation errors, but this method is too cumbersome to be applied in most cases, and alternative methods must be used. These alternative methods are typically partial corrections for the variation in only a few of the fine-scale attributes. Therefore, for these methods to be effective, the attributes that are the most severe sources of error must be identified a priori. The authors present a procedure for making these identifications based on a Monte Carlo sampling of the fine-scale attributes, then present four methods of translating fine-scale knowlege so it can be applied at coarser scales: 1) partial transformations using the expectation operator, 2) moment expansions, 3) partitioning, and 4) calibration. -from Authors
Recent invasion research can be categorized broadly into two types: studies of the spatial spread, and those of the biological impact the invader has on the native biota. The first type is the most factual, and the second the most theoretical. So far, however, it is difficult to connect the two, implying that neither the spatial spread nor the species interactions can be explained in terms of each other. Recent models, analysing spatial spread, progress steadily by making small steps.
Mountain fynbos ecosystems in South Africa are threatened by alien plant invasions and by a lack of funding for effective management of these invasions. This paper develops an ecological-economic argument for the effective management of plant invasions in mountain fynbos ecosystems. We do this by building a dynamic ecological economic model which values the ecosystem services that fynbos ecosystems provide under different management regimes. We propose that the services that mountain fynbos ecosystems provide fall into six components: water production, wildflower harvest, hiker visitation, ecotourist visitation, endemic species and genetic storage. A scenario analysis based on a hypothetical 4 km(2) mountain fynbos ecosystem in the western part of the fynbos biome estimated that the ecosystem's value varies from R19 million (under low valuation and poor management scenario) to R300 million (under high valuation and good management scenario) [R4.50=US$1]. Water production and genetic storage were the most valuable ecosystem services. The model showed that the cost of clearing alien plants (under the proactive management scenario) was a tiny (0.6-5%) proportion of the value of mountain fynbos ecosystems. This result motivates an injection of funds for clearing alien plants from mountain fynbos ecosystems.
Reid's paradox describes the fact that classical models cannot account for the rapid (10(2)-10(3) m yr-1) spread of trees at the end of the Pleistocene. I use field estimates of seed dispersal with an integrodifference equation and simulation models of population growth to show that dispersal data are compatible with rapid spread. Dispersal estimates lay to rest the possibility that rapid spread occurred by diffusion. The integrodifference model predicts that, if the seed shadow has a long 'fat' tail, then rapid spread is possible, despite short average dispersal distances. It further predicts that velocity is more sensitive to life history than is classical diffusion. Application of such models is frustrated because the tail of the seed shadow cannot be fitted to data. However, the data can be used to test a 'long-distance' hypothesis against alternative ('local') models of dispersal using Akaike's Information Criterion and likelihood ratio tests. Tests show that data are consistent with >10% of seed dispersed as a long (10(2) m) fat-tailed kernel. Models based on such kernels predict spread as rapid as that inferred from the pollen record. If fat-tailed dispersal explains these rapid rates, then it is surprising not to see large differences in velocities among taxa with contrasting life histories. The inference of rapid spread, together with lack of obvious life-history effects, suggests velocities may have not reached their potentials, being stalled by rates of climate change, geography, or both.
The ecological impact of non-indig-enous plants. Pages 39-61 in Impact and management of non-indigenous species in Florida Modeling stratified diffusion in biological invasions
- D C Schmitz
- D Simberloff
- R H Hofstetter
- W Haller
- D Sutton
Schmitz, D. C., D. Simberloff, R. H. Hofstetter, W. Haller, and D. Sutton. 1997. The ecological impact of non-indig-enous plants. Pages 39-61 in D. Simberloff, D. C Schmitz, and T. C. Brown, editors. Strangers in paradise. Impact and management of non-indigenous species in Florida. Island Press, Washington, D.C., USA. Shigesada, N., K. Kawasaki, and Y. Takeda. 1995. Modeling stratified diffusion in biological invasions. American Nat-uralist 146:229-251.
The IUCN plant red data book. International Union for the Conservation of
- S Lucas
- H Synge
Lucas, S., and H. Synge. 1978. The IUCN plant red data book. International Union for the Conservation of Nature and Natural Resources, Morges, Switzerland.
Predicting in-vasions of woody plants introduced into North America
- S H Reichard
- C W Hamilton
Reichard, S. H., and C. W. Hamilton. 1997. Predicting in-vasions of woody plants introduced into North America. Conservation Biology 11:193-203.
Priority conservation areas: towards an operational definition for regional assessments. Pages 338-357 in Natural parks and protected areas: selection, delimitation and manage-ment
- R L Pressey
Pressey, R. L. 1997. Priority conservation areas: towards an operational definition for regional assessments. Pages 338-357 in J. J. Pigram and R. C. Sundell, editors. Natural parks and protected areas: selection, delimitation and manage-ment. University of New England, Armidale, New South Wales, Australia.