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The construction and assembly of an ecological landscape
Journal of Animal Ecology
Abstract
An ecological landscape consisting of discrete interconnected patches was constructed in the laboratory. Each patch in the landscape was a 1-litre aquatic microecosystem containing producers and consumers. Species invaded and spread throughout the landscape in a specific sequence following prescribed invasion pathways. Species distribution among landscape patches was heterogeneous and converged to one of several alternative states despite identical initial conditions. Differences in structure which developed among patches were the result of the assembly processes which occurred in each patch and among interconnected patches. Variance in species composition increased as a function of distance from the patches that served as entry points for invasions into the landscape. The development of organization at the landscape level results from the interplay between the assembly of individual patches and the constraints imposed on each patch by invasion among patches. Differences in invasion success and persistence led to the development of alternative community states. -from Authors
... The realization of one state with deference to another occurs for many reasons ranging from even the most trivial of variations in initial conditions, to chance events with subsequent dynamical consequences, and the relative roles of determinism and indeterminism during development. For example, many studies have shown that simple modifications in the timing of species arrival in a community enable new portions of the attractor space that were previously unavailable (Drake 1991, Drake et al. 1993, 1996, Drake et al. in press, Fukami 2004a. The consequence of this adjustment in the attractor is profound, influencing species diversity, productivity, invasibility, the action of mechanisms, and patterns of species coexistence (Gamarra et al. 2005, Fukami and Morin 2003, Cardinale et al. 2002, McGrady-Steed et al. 1997. ...
... We are cognizant of the pitfalls that accompany assertions that some property or structure is emergent. However, we have additional insight from direct experimentation that we believe points the way to an explanation of emergence (Drake 1991, Drake et al. 1993, Drake et al. in press, Cadotte and Fukami 2005, Cadotte in press, Fukami 2004a, b, Fukami and Morin 2003; see also Drake 1990, Warren et al. 2003. These experiments focused on the construction or assembly of ecological communities in the highly controlled confines of the laboratory (Figure 8.1). ...
... Laboratory experimentation conducted by members of the Complex Systems Group has demonstrated that attractor forcing similar to that above occurs in ecological communities (Cadotte 2005 AQ2 , Cadotte in press, Drake et al. 1993, Fukami and Morin 2003, Cadotte and Fukami 2005. For example, Drake et al. 1993 constructed sets of interconnected community patches that were derived from a common species pool (Figure 8.3). ...
... The realization of one state with deference to another occurs for many reasons ranging from even the most trivial of variations in initial conditions, to chance events with subsequent dynamical consequences, and the relative roles of determinism and indeterminism during development. For example, many studies have shown that simple modifications in the timing of species arrival in a community enable new portions of the attractor space that were previously unavailable (Drake 1991, Drake et al. 1993, 1996, Drake et al. in press, Fukami 2004a). The consequence of this adjustment in the attractor is profound, influencing species diversity, productivity, invasibility, the action of mechanisms, and patterns of species coexistence (Gamarra et al. 2005, Fukami and Morin 2003, Cardinale et al. 2002, McGrady-Steed et al. 1997). ...
... We are cognizant of the pitfalls that accompany assertions that some property or structure is emergent. However, we have additional insight from direct experimentation that we believe points the way to an explanation of emergence (Drake 1991, Drake et al. 1993, Drake et al. in press, Cadotte and Fukami 2005, Cadotte in press, Fukami 2004a, b, Fukami and Morin 2003 see also Drake 1990, Warren et al. 2003). These experiments focused on the construction or assembly of ecological communities in the highly controlled confines of the laboratory (Figure 8.1). ...
... Our search for just such a route to emergence began after noticing a curious experimental and theoretical result (Drake 1990, 1991, Drake et al. 1993 ). Ecological communities can sometimes be assembled that cannot be reconstructed from their constituent parts. ...
The quest to understand animate nature, its origins, current state and future
course, its dynamical underpinnings and interface with the physical world, is
surely the tacit aspiration of contemporary ecology. As a field of inquiry,
academic ecology emerged in earnest from the descriptive realm of natural
history in the late nineteenth century when observers of nature, pondering
similarity and difference, sought cause. Adopting the classic approach to science,
ecologists further described, but also dissected, modeled, conceptualized,
and manipulated the parts of ecological systems. What emerged is the modern
framework of ecology, built upon the struts of population dynamics and tempered
by interactions between species, all within an environmental context.
Yet with the Earth as a laboratory containing millions of extant and evolving
species, the number and variety of plausible ecological studies is functionally
inexhaustible. A quick glance through any ecological journal reveals surprising
novelty and nuance at every turn. Hence, one is compelled to ask whether
progress in ecology is best served by strict adherence to a reductionistic program
and unending description, or whether this approach should be blended with more
general and epiphenomenological complements. This question is apropos to all
of science. Attempts at such a synthesis are well underway sparked by the realities
of a complex and decidedly nonlinear nature. In this complex nature one þ one
need not always equal two, and the simple logical operator if–then requires else. In
ecology, if and then produces a highly contingent mapping of their own accord.
Add else, and we have the fundamental reason for the diversity of nature and the
nuance of its expression.What then are mechanisms, the holy grail of reductionist
ecology, but manifestations of an emergent nature? Nature can be described by
the action of mechanisms explicit in our logical operator. Yet understanding
nature requires a deeper knowledge of how the action of the mechanism emerged.
Here we present a solution to the recalcitrant problem of true or hard
emergence, the existence of which has been debated, demonized, and exalted
for centuries. We suggest that emergence is hidden within the attractor space of
dissipative dynamical systems, or more precisely the interaction between multiple
attractors. Our arguments while tentative, suggest that two independent
systems can interact in the attractor space and produce a persistent attractor
that are essentially offspring of the parents. Emergence in this case is absolute
because no trajectories exist linking the child to either parent. You cannot get
there from here. Our arguments are based on a long series of experimental
studies that have explored the assembly or construction of ecological communities.
We offer this notion of emergence as a general solution to all things
emergent independent of any particular system
... Of these, the effect of the invasion sequence is typically the most understood (Booth and Swanton, 2002). In fact, many studies at the microcosm level (Robinson and Dickerson, 1987;Drake 1991;Drake et al., 1993) and in natural ecosystems (Cole 1983, Abrams et al., 1985, McCune and Allen, 1985 have shown evidences that the order of arrival can influence the ultimate community composition. For example, Drake (1991) introduced species into freshwater microcoms in varied sequence. ...
... As mentioned in the previous section, variations in the order of species' arrivals, through temporal variations in the membership of geographic species pool or chance events during dispersal, can strongly influence community assembly, as demonstrated in several empirical and theoretical studies (Blaustein and Margalit, 1996;Drake et al., 1993;Law and Morton, 1996;Grover, 1994;Wilbur, 1997). Historical contingencies result from interactions between the order of species invasions (i.e. ...
Whether biological communities are deterministic or stochastic assemblages of species has long been a central topic of ecology. The widely demonstrated presence of structural patterns in nature may imply the existence of rules that regulate the organization of ecological communities. In this review, I present a compilation of major assembly rules that fundament, in a great proportion, the community assembly theory. Initially, I present a general overview of key concepts associated to the assembly of communities, in particular the origin of assembly rules, definition, the problem of scale and underlying mechanisms in the structure of ecological communities. Subsequently, two major approaches or paradigms (i.e. species-based and trait-based) for the assembly of communities are discussed. Finally, major tested assembly rules are explored and discussed under the light of available published literature.
... In Lotka ÁVolterra model, for example, when the two interspecific competition coefficients are equal and lower than one, two species can coexist regardless of initial conditions; when the two coefficients are equal and greater than one, the initially more abundant species could competitively exclude the other species (Tilman 1982). In line with theory, empirical tests have found alternative stable states of communities under the influence of a variety of historical factors such as perturbations (Morris et al. 2003, Fukami and Lee 2006, initial population density (Hu and Zhang 1993), and colonization sequences (Robinson and Dickerson 1987, Drake 1991, Drake et al. 1993, Lawler and Morin 1993, Shorrocks and Bingley 1994. However, there are also many studies that convincingly demonstrate community convergence, in which outcome of species competition is history free (Tilman and Sterner 1984, Grover 1988, Sommer 1991, Tilman and Wedin 1991. ...
... We propose that biodiversity effects on ecosystem functioning could also be dependent on community assembly history. The early colonists in a habitat may either promote or inhibit the establishment of the late arrivals (Peterson 1984, Robinson and Dickerson 1987, Drake 1991, Drake et al. 1993, Lawler and Morin 1993, Sait et al. 2000, Price and Morin 2004, Fukami et al. 2007, and thus affect the latter's contributions to ecosystem function. For instance, in poor or stressful environments where facilitation among species is found to be important (Callaway 1995, Callaway et al. 2002, early arrival of facilitator species (e.g. ...
Species extinction and immigration are both common in natural communities and the sequence with which species are lost from or added to communities may be crucial to community structure. We experimentally addressed this issue by growing six green algal species in monocultures and all possible two-species mixtures, with two colonization sequences for each mixture. Both convergence and divergence in community structure were observed. The compositions containing particularly productive species were more likely to converge, while those comprising of species with similar monoculture yields were more likely to diverge. The species mixtures with high-yielding initial and low-yielding invading species produced more biomass than monocultures, but mixtures with the opposite assembly order produced only the same level of biomass as monocultures did. To address the diversity–ecosystem functioning issue, we estimate complementarity effect by relative yield total (RYT) and selection effect by the correlation between species’ monoculture yields and their relative yields in mixtures, respectively. We found overall negative complementarity and positive selection effect in mixtures with high-yielding species as initial colonizers, but positive complementarity and negative selection effect in mixtures with low-yielding initial species. Nonetheless, because we used only up to two species in each microcosm, our results are limited in addressing the relationship between species diversity and ecosystem functioning. Future research should study the effects of immigration history with many more species involved in community assembly.
... This predicts the outcome of succession, based on the principle of mutual replenishment. The theory could readily be tested in laboratory experiments where both the support set and the community scale can be defined, along the lines of the pio- neering work of Drake et al. [30]. The outcome of succession is not in general a food web that maximizes, minimizes or opti- mizes any particular whole-community property. ...
We have constructed a model of community dynamics that is simple enough to enumerate all possible foodwebs, yet complex enough to represent awide range of ecological processes. We use the transition matrix to predict the outcome of succession and then investigate how the transition probabilities are governed by resource supplyandimmigration. Low-input regimes leadto simple communities whereas trophically complex communities develop when there is an adequate supplyof both resources andimmigrants. Our interpretation of trophic dynamics in complex communities hinges on a newprinciple of mutual replenishment, defined as the reciprocal alternation of state in a pair of communities linked by the invasion and extinction of a shared species. Such neutral couples are the outcome of succession under local dispersal and imply that food webs will often be made up of suites of trophically equivalent species. When immigrants arrive from an external pool of fixed composition a similar principle predicts a dynamic core of webs constituting a neutral interchange network, although communities may express an extensive range of other webs whose membership is only in part predictable. The food web is not in general predictable from whole-community properties such as productivity or stability, although it may profoundly influence these properties. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
... Experimental studies have manipulated initial abundances or the establishment order of species to explore the occurrence of priority effects[154],[155]. Other experimental studies using a microbial community[47]demonstrate that variation in the timing of species introduction can lead to different community assembly. Additionally, some studies observe that priority effects can also be influenced by abiotic environments. ...
Predicting which species will be present (or absent) across a geographical region remains one of the key problems in ecology. Numerous studies have suggested several ecological forces that can determine species presence-absence: environmental factors (i.e. abiotic environments), interactions among species (i.e. biotic interactions), dispersal and demographic stochasticity. While various ecological forces have been considered, less attention has been given to the problem of understanding how dispersal processes, in interaction with other ecological factors, shape community assembly in the presence of priority effects (i.e. initial abundances determine the presence-absence of species). In this thesis, we investigate the consequences of different dispersal patterns and stochasticity on the occurrence of priority effects and species coexistence in multi-species competitive systems. By employing deterministic and stochastic models in one-dimensional space, our study shows the conditions under which priority effects occur and disappear as local dispersal strength changes. Without dispersal, priority effects emerge in the presence of intense biotic interactions, with only one species surviving at any given location (i.e. coexistence is impossible). For moderate dispersal levels, dispersal enhances priority effects and promotes multiple species coexistence. Further increasing dispersal strength leads to the disappearance of priority effects and causes extinction of some species. We also demonstrate contrasting observations of stochasticity on priority effects: while priority effects are more prevalent in the stochastic individual-based models (IBM) than in the deterministic models for large populations, we observe fewer occurrences of priority effects in IBM for small populations. When non-local dispersal is incorporated into the models, priority effects are more pronounced than in the local dispersal models. We also investigate the effects of different dispersal patterns on species coexistence: although very long-range dispersal leads to species extinctions, intermediate-range dispersal permits more outcomes where multispecies coexistence is possible than short-range dispersal (or purely local dispersal). Finally, we extend our model to consider community dynamics in two-dimensional space. We find that knowledge of species’ environmental requirements is also crucial to improving our ability to predict the occurrence of priority effects across heterogeneous environments.
... In the absence of complex feedbacks, our results are broadly consistent with the large body of previous research on the eff ects of dispersal, disturbance, and spatial environmental heterogeneity on community assembly (Drake et al. 1993, Mouquet and Loreau 2003, Jiang and Patel 2008, Fukami 2010, Gravel et al. 2010, Lasky and Keitt 2013. What is novel about this work is the focus on the role of organism -environment feedbacks in directly modifying spatial environmental heterogeneity and, in turn, indirectly altering niche diff erentiation and regional species coexistence. ...
Understanding the factors that determine the extent of biodiversity loss following habitat destruction is central to ecosystem conservation and management. One potential factor is the ecological feedbacks between organisms and local environmental conditions, which can influence how species affect one another and, consequently, whether or not species persist in fragmented landscapes. We investigated this possibility using a spatially explicit individual-based model of plant communities. In this model, plant species affected their own and other species’ competitiveness by modifying local environmental conditions. These plant–environment feedbacks were assumed to vary among species pairs in direction and strength to mimic complex feedbacks observed between plants and soil conditions in real communities. We found that complex feedbacks reduced the extent of diversity loss, effectively buffering species against habitat fragmentation. Our analysis suggested that this buffering effect operated via two mechanisms. First, complex feedbacks decreased the likelihood of immediate extinction by making the spatial distribution of each species less clustered and consequently less likely to be eliminated entirely by fragmentation. Second, complex feedbacks decreased the likelihood of additional extinction by generating negative density dependence among surviving species, thereby keeping low-abundance species from going extinct due to demographic stochasticity and other forces. The buffering effect was particularly strong when species dispersed locally and abiotic environmental conditions varied globally. Our findings highlight the potential importance of organism–environment feedbacks in explaining species extinction by habitat destruction.
... Research into the assembly of ecological communities has shown that the extant composition of communities is strongly influenced by historical factors123. Priority effects occur in communities, when one (or more) species already is present in a habitat and thereby affects the success of later species [4,5], and this effect can be either negative, positive or neutral [6]. ...
Priority effects occur when species that arrive first in a habitat significantly affect the establishment, growth, or reproduction of species arriving later and thus affect functioning of communities. However, we know little about how the timing of arrival of functionally different species may alter structure and function during assembly. Even less is known about how plant density might interact with initial assembly. In a greenhouse experiment legumes, grasses or forbs were sown a number of weeks before the other two plant functional types were sown (PFT) in combination with a sowing density treatment. Legumes, grasses or non-legume forbs were sown first at three different density levels followed by sowing of the remaining PFTs after three or six-weeks. We found that the order of arrival of different plant functional types had a much stronger influence on aboveground productivity than sowing density or interval between the sowing events. The sowing of legumes before the other PFTs produced the highest aboveground biomass. The larger sowing interval led to higher asymmetric competition, with highest dominance of the PFT sown first. It seems that legumes were better able to get a head-start and be productive before the later groups arrived, but that their traits allowed for better subsequent establishment of non-legume PFTs. Our study indicates that the manipulation of the order of arrival can create priority effects which favour functional groups of plants differently and thus induce different assembly routes and affect community composition and functioning.
... The effect of the invasion sequence is the most understood of these (Figure 4). Numerous microcosm (Drake 1991;Drake et al. 1993;Robinson and Dickerson Jr. 1987) and natural ecosystem (Abrams et al. 1985;Cole 1983;McCune and Allen 1985) studies have demonstrated that the order of arrival can influence the ultimate community composition (but seeGrover and Lawton 1994;Sommer 1991). Drake (1991) introduced species (including bacteria, algae, protozoa, and invertebrate and herbivore predators) into freshwater microcosms in varied sequence. ...
Community assembly is a branch of ecology that looks at how communities are assembled as they follow trajectories through time. A trajectory is controlled by biotic and abiotic constraints (filters) that act at multiple scales. From a total species pool, environmental and dispersal constraints control which species enter an ecological species pool. Within this pool, internal dynamics determine which of these species becomes part of the extant community. Environmental filters act by removing species that lack specific traits. Thus, traits are filtered, and with them, species. In this paper, we present the basic ecological theory of community assembly and address how it can be used in conjunction with a trait-based approach to understand and possibly predict how weed community structure changes in response to imposed filters such as tillage or crop rotation. Weed ecologists have struggled with the need to place our practical knowledge of agriculture and weeds into a broader theory, and there have been many calls to integrate ecology with agronomy and weed science. Community assembly might be one way to do so.
... The timing, pattern, and origin of invasions across multiple species and communities are central themes in invasion ecology (e.g., Pimm 1989; Drake et al. 1993; Williamson 1994; Geller 1996; Vermeij 1996; Lockwood et al. 1997; Shigesada and Kawasaki 1997). The intrinsically historical nature of these processes reflects the fact that biological invasions proceed along a spectrum of temporal scales that spans ecological and evolutionary time (Brown 1989; Vermeij 1996). ...
By virtue of their isolation and depauperate faunas, oceanic islands offer unique opportunities to characterize the historical development of ecological communities derived from both natural and anthropogenic invasions. Barbados, an outlying island in the Lesser Antilles, was formed approximately 700,000 YBP by tectonic uplift and was then colonized by birds via natural invasion from the much older volcanic islands in the main Lesser Antillean arc. We investigated the timing and sources of the avian invasion of Barbados by determining levels of mitochondrial DNA (mtDNA) divergence between populations of eight bird species from Barbados and those on the nearby putative source islands of St. Lucia and St. Vincent. Although all Barbados populations appeared to be young relative to the geological age of the island, we found differences among species in their inferred times of colonization and we identified at least two sources of immigrants to Barbados. In contrast to these historical differences across species and populations, our characterization of the mitochondrial genotypes of 231 individual birds suggests that each island population represents the descendants of a single founding maternal lineage. Considered in concert, the results of this molecular survey indicate that the Barbados bird community is composed of species with different invasion histories, which in turn suggests that the island's community composition has changed repeatedly over its 700,000 year history.
One of the important issues in ecology is to predict which species will be present (or absent) across a geographical region. Dispersal is thought to have an important influence on the range limits of species, and understanding this problem in a multi-species community with priority effects (i.e. initial abundances determine species presence-absence) is a challenging task because dispersal also interacts with biotic and abiotic factors. Here, we propose a simple multi-species model to investigate the joint effects of biotic interactions and dispersal on species presence-absence. Our results show that dispersal can substantially expand species ranges when biotic and abiotic forces are present; consequently, coexistence of multiple species is possible. The model also exhibits ecologically interesting priority effects, mediated by intense biotic interactions. In the absence of dispersal, competitive exclusion of all but one species occurs. We find that dispersal reduces competitive exclusion effects that occur in no-dispersal case and promotes coexistence of multiple species. These results also show that priority effects are still prevalent in multi-species communities in the presence of dispersal process. We also illustrate the existence of threshold values of competitive strength (i.e. transcritical bifurcations), which results in different species presence-absence in multi-species communities with and without dispersal.
The most pervasive image of an ecological system, both in popular and in scientific imagination, surely is that of a highly interacting collective of many parts, each influencing the others in strange, wonderful, and somewhat mysterious ways — an image not absurdly distant from the truth. Some of the most pressing practical problems faced by ecological science require an understanding of exactly how human intervention affects these complex systems, hence of how the systems themselves, the whole systems including all interactions, work. One need seek no further for examples than this morning’s newspaper, which chronicled the scientifically “baffling” collapse of the northwest Atlantic cod fishery, and the not-so-baffling decline of the St. Lawrence River beluga whale population, whose habitat could serve as a catalog of contemporary toxins (The Globe and Mail, Toronto, June 6, 1992).
Summary Habitat restoration is commonly conducted in agricultural landscapes with the aim of restoring biodiversity. Some species, however, might not be able to migrate to restored habitats, and vital habitat elements, such as logs, may be missing. We compared arthropod assemblages under logs amongst different land‐use types: pastures, revegetation and woodland remnants, in south‐eastern Australia. We also supplemented habitats with logs, placed out in different seasons and for different periods of time to determine how season and exposure time affect arthropod composition. We compared assemblages under logs in revegetation adjacent to and isolated from woodland remnants to test the role of habitat connectivity. Arthropod assemblages differed significantly between land‐use types, with pastures most different to remnants. These differences corresponded with differences in log microhabitat. Time was an important determinant of community composition, but habitat (remnant vs revegetated) and revegetation connectivity (adjacent vs isolated patch) were not. Time affected assemblage composition in two distinct ways: first, time of year (November vs January), and second, exposure time of logs (1 vs 3 months) affected composition. Exposure time effects may indicate dispersal limitation, but the proportion of wingless species did not depend on exposure time or connectivity. We conclude that the log fauna in this landscape responds to microenvironments and seasonal change but is not strongly dispersal limited, allowing it to respond rapidly to habitat restoration. The pre‐agricultural landscape likely shared many features with the modified landscape, such that many species possess traits and behaviours that allow them to move through and persist in the matrix.
Priority effects occur when species that arrive first in a habitat significantly affect the establishment, growth, or reproduction of species arriving later and thus affect functioning of communities. However, we know little about how the timing of arrival of functionally different species may alter structure and function during assembly. Even less is known about how plant density might interact with initial assembly. In a greenhouse experiment legumes, grasses or forbs were sown a number of weeks before the other two plant functional types were sown (PFT) in combination with a sowing density treatment. Legumes, grasses or non-legume forbs were sown first at three different density levels followed by sowing of the remaining PFTs after three or six-weeks. We found that the order of arrival of different plant functional types had a much stronger influence on aboveground productivity than sowing density or interval between the sowing events. The sowing of legumes before the other PFTs produced the highest aboveground biomass. The larger sowing interval led to higher asymmetric competition, with highest dominance of the PFT sown first. It seems that legumes were better able to get a head-start and be productive before the later groups arrived, but that their traits allowed for better subsequent establishment of non-legume PFTs. Our study indicates that the manipulation of the order of arrival can create priority effects which favour functional groups of plants differently and thus induce different assembly routes and affect community composition and functioning.
Retreating glaciers and the periglacial areas that they vacate produce a harsh environment of extreme radiation, nutrient limitations and temperature oscillations. They provide a model system for studying mechanisms that drive the establishment and early assembly of communities. Here, we synthesize more than 20 yr of research at the Lyman Glacier forefront in the North Cascades Mountains, comparing the results and conclusions for plant and fungal communities. Compared to plant communities, the trajectories and processes of fungal community development are difficult to deduce. However, a combination of high throughput sequencing, more revealing experimental designs, and phylogenetic community analyses provide insights into mechanisms that shape early microbial communities. While the inoculum is likely to be randomly drawn from regional pools and accumulates over time, our data provide no support for increases in richness over time since deglaciation, as is commonly observed for plant communities. Re-analyses of existing datasets suggest that microbial, and particularly fungal, communities are insensitive to time since substrate exposure from underneath the retreating glacier, but are responsive to plant establishment both in biomass and community composition. Further research on functional aspects, organismal activity, or ecosystem services in early successional environments will provide deeper appreciation of the dynamics of these communities.
The observation of stratigraphic intervals characterized by faunas displaying “coordinated stasis” in the Appalachian Basin of New York State (Brett and Baird, 1995) is based on the existence of biofacies. Thus, any claims as to the significance or unusual nature of patterns of coordinated stasis are really statements about these underlying biofacies — common features of the fossil record that have long been recognized. Evaluation of the bimodal distribution of evolutionary and ecological change through time apparent in biofacies and Ecological Evolutionary subunits must be done in relation to the clearly stated expectations of null models. The evolutionary and ecological bases for such models are explored here using a conceptual approach founded on two ecological and two evolutionary neutral “rules”: (1) communities are capable of considerable compositional flux, (2) multiple stable states are available to communities, (3) allopatry is the main mode of speciation, and (4) stabilizing selection is effective. The aspects of biofacies that appear counter-intuitive when compared to the predictions of this conceptual framework are discussed. Documentation of environmental dynamics, alongside faunal dynamics, seems to be the key to understanding whether biofacies and coordinated stasis fall outside our expectations.
Since the integrity of ecosystems is being challenged worldwide by invading species, there is a growing need to understand their effects on community assembly which is strongly related to community function. Null models, which are important quantitative tools in the search for ecological processes driving local diversity and species distributions, were used to determine whether or not the intact and invaded grassland communities differ from randomly created null communities in respect of species co-occurrence using four co-occurrence indices (C score; number of checkerboard units; number of species combinations and V ratio). In the intact grassland all the indices, except for the number of checkerboards, revealed significant species segregation as expected in competitively structured communities. In comparison, the invaded grassland showed random co-occurrence of species. Besides, decrease in the number of species in the invaded grassland and change in species diversity and dominance is also suggestive of disassembly of the grassland community by the invasive species.
We review the literature on patterns, causes, processes and impacts of exotic plants, primarily in the mediterranean region of Chile, considering three major non-independent drivers of the invasion process: (a) Availability of exotic species propagules, (b) attributes of the local communities in which exotic species establish and through which they will eventually spread out, and (c) attributes of exotic species that either facilitate or constraint their spread into new sites. Regarding availability of propagules, central Chile matorral presents the communities with the greatest incidence of naturalized herbs, followed by the sclerophyllous forest and the espinal scrubland in the coastal range. In contrast, north-central communities have lower numbers and proportions of naturalized species of herbs in their seed banks. Availability and persistence of naturalized herbs do not differ between aboveground vegetation and seed bank. Regarding attributes of local communities associated with the establishment and the spread of exotics, grazing regime and land use emerge as the most prominent causes that render them more prone to invasion by exotics. Evidence on the effect of the fire regime is contradictory and native species richness does not seem to be an important factor. Regarding attributes of exotic species seeds, results suggest that naturalized annuals germinate within a wide temperature range, are highly resistant to cold and dry conditions, and show some degree of physiological dormancy. Additionally, naturalized annuals are highly tolerant to poor soils, but are generally intolerant to shade. These general attributes have largely determined the invasion process in the mediterranean region of Chile. Historical data indicate that an important number of exotic species were intentionally introduced, and that the spread of exotic is uncontrolled. It has been demonstrated that arrival time of exotics is of great relevance to understand present day spread of exotics in Chile, independent of their biogeographic origin. Exotic species may cause strong disruptions of ecosystem processes and functions in Chile, as exemplified by exotic tree plantations, which have altered soil chemistry, nutrient cycling, water cycle, hydrology, microclimate, and fire frequency and intensity.
Channelization is the creation of man-made structures that resect, realign, or enclose aquatic systems in order to prevent flooding or to modify their flows for various land uses. The present study examined the impact of this human pressure on the spatial and temporal dynamics of small streams. The physical and floristic characteristics of seventeen reaches in four French rivers were surveyed six times over the course of two years. Macrophyte communities were divided into three biological groups: vascular plants, macroalgae and bryophytes. We used plant functional traits to understand the effects of channelization on community structure. Our results suggest that channelization affected the spatio-temporal dynamic of physical and floristic composition. Channelized reaches were shallower and narrower than non-channelized reaches, i.e. control reaches. They also exhibited different substrate types and dominant species. Differences were mainly observed at the macrohabitat scale, i.e. the pool/riffle scale, within the selected reaches. Alterations in spatio-temporal dynamics of physical and plant composition could be linked to species biological traits. Vascular plants and macroalgae in channelized reaches used a variety of adaptive strategies (e.g. small versus tall size) which allowed them to persist despite environmental differences, whereas plants in control reaches showed a combination of intermediate strategies. Bryophytes were mainly found in control reaches with the exception of Fontinalis antipyretica. These findings could serve as guidelines for future channelization projects and for conservation measures to preserve the dynamics of natural streams.
Experimentation in landscape ecology is widely conducted using diverse approaches to answer a broad range of questions. By assessing the response to controlled manipulations alternate hypotheses can be clearly refuted, model parameters quantified, and conditions are often ripe for unexpected insights.
Results from landscape experiments complement the many well developed observational and modeling approaches more commonly used in landscape ecology. To better understand how landscape experimentation has been conducted and to identify future research directions, we reviewed and organized the diversity of experiments. We identified fifteen distinct landscape experiment types, which we categorized into four broad groups including (I) identifying landscape structure, (II) identifying how ecological processes vary within existing landscapes, (III) identifying how landscape structure influences ecological processes, and (IV) identifying landscape pattern formation factors. Experiment types vary along axes of scalable to real landscapes and generalizability, suitability for analysis through traditional experimental design and flexibility of experimental setup, and complexity of implementation and resource requirements. The next generation of experiments are benefiting from more explicit inclusion of scaling theories and tighter coupling between experiments and cyberinfrastructure. Future experimental opportunities for landscape ecologists include expanded collaborations among experiments, better representations of microbial- soil structure relationships at microscales, and direct evaluations of landscape interactions with global changes. The history, current practice, and future needs of landscape ecological research strongly support an expanded role of experimental approaches that complements the rich observational and modeling strengths of the field.
Abstract An area of dry grassland in New Zealand, comprising an equal mixture of native and exotic species, was subject to perturbations of irrigation, fertilization and cessation of grazing. The vegetation response was recorded for 3 years. Total cover, and the contribution of native species to that cover, fluctuated between years even in the control plots. Irrigation increased total cover, but decreased the cover of native species. Fertilization produced the same effects, only less strongly, and also reduced species richness, the loss being in native species. In spite of overall effects of treatments on native and exotic cover, when individual species’ responses to irrigation, fertilization or exclosure were calculated, there was no significant difference between the native and exotic plant guilds. Species differed in their responses, but the native and exotic guilds overlapped. When grouped by morphology, the only significant difference between the responses to perturbation was that forbs and graminoids responded more positively to irrigation than woody and cryptogamic species. The realized responses of the species to the perturbations described here showed little correlation with their physiological responses as determined in previous greenhouse experiments. It is suggested that the realized responses are strongly, and currently unpredictably, influenced by competition from the other species present. Soil nutrients and soil water were both important controls on the community. The relative similarity in the nature of the response to these two factors – nutrients and water – suggests that they affect species in similar ways, possibly because the greater growth rate of the exotic species mediates the short-term response to both. Grazing has less effect on the current community than either nutrients or water, although it may have been historically important in shaping the species pool. From the poor predictability of field responses from morphological guilds or from ecophysiological responses, it is suggested that the ‘functional types’ approach, although conceptually attractive, lacks experimental support in these grasslands. It is concluded that the exotic species have invaded by being pre-adapted to the environment with the same environmental responses as the natives, but with the advantage of generally higher growth rates.
Metapopulation dynamics have been postulated as a possible mechanism giving rise to positive interspecific relationships between local abundance and regional occupancy. These may operate through the carrying capacity or the rescue effect hypotheses. However, both are based on single species models that sum independent occurrences of species to form an assemblage, ignoring interspecific interactions.
Here we test experimentally whether interspecific interactions and dispersal influence the formation of the abundance–occupancy relationship in metapopulation systems in laboratory microcosms containing protists and bacteria.
The effect of species interactions was tested by comparison of the abundance–occupancy relationship in multiple habitat patch systems containing all species together, with the relationship formed by combining data from equivalent systems containing each protist species alone.
Abundance–occupancy relationships in interacting communities were better defined than those in non‐interacting communities. The inclusion of interspecific interactions was found to cause a reduction in the abundance and occupancy of the majority of species, as a consequence of which the position of species within the relationship changed drastically.
In simple microcosm systems, the form of interspecific abundance–occupancy relationships has been found to be dependent on biotic interactions. However, in more complex systems such effects may be obscured by those of habitat heterogeneity. Here we test this proposal using laboratory microcosms of protists and bacteria.
The independent effects of species interactions and heterogeneity were tested by comparison of the abundance–occupancy relationship formed in multiple habitat patch systems containing all species together, with that relationship formed by combining data from equivalent systems containing each protist species alone and between homogeneous and heterogeneous environments.
There was more residual variation about positive interspecific abundance–occupancy relationships formed in heterogeneous environments in interacting and non‐interacting communities as the majority of species were more restricted in the number of patches they could occupy compared to homogeneous landscapes.
Abundance–occupancy relationships in interacting communities were better defined than those in non‐interacting communities. The inclusion of interspecific interactions caused a reduction in the abundance and occupancy of the majority of species and changed the position of species within the relationship.
Our results show that biotic interactions influence the abundance–occupancy relationships even with imposed environmental heterogeneity. However, in heterogeneous environments, for some species these processes occurred in fewer patches, causing increased residual variation about positive interspecific abundance–occupancy relationships compared to homogeneous environments.
In this paper, I examine the dynamics of species richness in a model system in which multiple species compete in a metacommunity (multiple patches linked by dispersal). Patches lie along an environmental gradient, and new species are introduced into the system by speciation of existing species. This model is used to explore how the ecological similarity of species influences the patterns in community structure that result and to determine whether patterns in fossil and systematics data may be signatures for different types of community structure. Making species more similar overall along the entire gradient or making new species that have more similar optimal positions along the gradient to their progenitor both increase the time required to drive species extinction. As a result, making species more similar ecologically to one another increases overall species richness because of an increased frequency of transient species in the system. Having more transient species in systems shifted the longevity distributions of species in the fossil record towards having a greater frequency of shorter duration species, and the age distribution of extant species that would be estimated from molecular phylogenies also had a higher frequency of younger aged species. Comparisons of these results with species longevity distributions extracted from two data sets and with species ages derived from species-level molecular phylogenies strongly suggest that transient species are an important component of real biological communities.
The community conditioning hypothesis is used as a framework in which to place the layers of effects during and after pesticide intoxication. Community conditioning states that information about the history of a system can be and is written at a variety of organismal and ecological levels. This historical component or etiology determines the future dynamics of a system. The storage of information concerning prior stressor events has been observed in a variety of compartments. Fish populations have been observed to have different genetic structures in populations that have been exposed to toxicant stressors. Analysis of biomarker data from field experiments reveals a variety of patterns, some due to the location of the field plots. Treatment groups within a series of microcosm experiments maintain their identities long after the degradation of the toxicant. The dynamics of the treatment groups in multivariate ecological space are characteristic of a particular treatment. Other microcosm systems differentially respond to invasion depending upon the order of the inoculation of the biotic components, even though at the time of the invasion the systems are indistinguishable. A major factor in the uncertainty of pesticide risk assessment will be the unknown etiology of the system of interest.
Summary 1. Natural aquatic communities or habitats cannot be fully replicated in the wild, so little is known about how initially identical communities might change over time, or the extent to which observed changes in community structure are caused by internal factors (such as interspecific interactions or traits of individual species) versus factors external to the local community (such as abiotic disturbances or invasions of new species).
2. We quantified changes in seven initially identical fish assemblages, in habitats that were as similar as possible, in seminatural artificial streams in a 388-day trial (May 1998 to May 1999), and compared the change to that in fish assemblages in small pools of a natural stream during a year. The experimental design excluded floods, droughts, immigration or emigration. The experimental fish communities diverged significantly in composition and exhibited dissimilar trajectories in multivariate species space. Divergence among the assemblages increased from May through August, but not thereafter.
3. Differences among the experimental assemblages were influenced by differences that developed during the year in algae cover and in potential predation (due to differential survival of sunfish among units).
4. In the natural stream, fish assemblages in small pools changed more than those in the experimental units, suggesting that in natural assemblages external factors exacerbated temporal variation.
5. Our finding that initially identical assemblages, isolated from most external factors, would diverge in the structure of fish assemblages over time suggests a lack of strong internal, deterministic controls in the assemblages, and that idiosyncratic or stochastic components (chance encounters among species; vagaries in changes in the local habitat) even within habitat patches can play an important role in assemblage structure in natural systems.
Stones along the shore of a eutrophic English lake in summer have one of two epilithic assemblages. Many stones are covered with dense filaments of the macro‐alga Cladophora glomerata and the tubes of detritivorous/browsing chironomids. Others have few algal filaments and are dominated by gallery‐building caddis larvae ( Tinodes waeneri ). These communities are patchily distributed along the shore, macro‐algae dominating in areas away from riparian trees and on tall, rather than flat stones. The origins of this dichotomy were assessed in a field experiment that provided new surfaces for colonization, according to a planned schedule in time and space.
To examine temporal heterogeneity, artificial substrata (clay tiles) were introduced into the littoral each month from February to October 1995 and harvested on several occasions from 1 to 9 months later. ‘Large’ scale spatial heterogeneity was investigated by introducing tiles into three locations along the shore that differed with respect to shoreline trees. At a smaller scale, tiles were introduced with varying degrees of accessibility to dispersing insect larvae by placing them directly on the lake bed, above the bed on platforms, or as stacks of several tiles (to mimic tall stones).
There was a distinct temporal separation in the colonization of invertebrates, with tube‐building chironomid larvae dominating substrata in spring, before being displaced by Tinodes in summer. A strong temporal pattern in algal productivity, with spring and autumn maxima and a summer minimum, was attributable to nutrient limitation in summer.
In summer, there was a strong spatial separation of organisms at large and small scales. In patches under or near trees, chironomids and Cladophora were replaced by Tinodes larvae. Cladophora remained abundant away from trees, where tiles were never colonized by the crawling Tinodes larvae, originating from egg masses laid beneath trees. Areas distant from oviposition sites may act as a grazing refugium for Cladophora and its associated chironomids. At a smaller scale, the inability of Tinodes to colonize the upper surfaces of tall stones also created spatial refugia for algae and chironomids, even within patches of generally high Tinodes density.
There was also evidence of a temporal grazing refugium for Cladophora . Cladophora filaments which colonized tiles introduced in April subsequently resisted grazing by Tinodes newly recruiting in June. Filaments colonizing tiles introduced before or after April, however, were vulnerable to Tinodes . The precise mechanism for this is unclear, however.
Spatiotemporal patterns in the epilithic community were caused by the external factors of a seasonally fluctuating nutrient supply and a patchy riparian strip of trees and the intrinsic factors of habitat choice, and the life history phenology of the organisms concerned. Our results infer strong direct biotic interactions among species (particularly grazing of Cladophora by Tinodes ), indirect interactions between Tinodes and several chironomids via habitat engineering (i.e. the removal of Cladophora as chironomid habitat) and the crucial role of spatiotemporal heterogeneity in decoupling those interactions.
Invasions are widely regarded as a significant current problem in population, ecosystem and habitat management, with consequences including global homog-enization of floras and faunas, extirpation of native species and interference with ecosystem functioning (Drake et al. 1989, Williamson 1996, Mack et al. 2000, Pimentel et al. 2000, Sakai et al. 2001). Such invasions occur, and have effects, at large scales, in real ecosystems. So why consider investigating such phenomena in artificial microcosm systems in the laboratory? We believe that there are several compelling reasons why laboratory microcosm systems have a valuable role in the study of biological invasions; these centre on the issues of manipulation, time scale and replication.
A central goal oi ecology is to understand what determines the number and identity of species in ecological communities. Of the many species that could potentially occupy an area, why do only a particular subset of species actually co-occur and what determines the identity of those species? And to what extent are these patterns the product of deterministic processes rather than stochastic events? Much interest has focused on interspecific competition as a process shaping cooccurrence patterns (Strong et al, 1984, Diamond and Case 1986): some species capable of joining a local community may be excluded by the presence of competitors. If competition is sufficiently strong and pervasive enough to structure ecological communities then certain 'assembly rules' should govern how communities are put together (Diamond 1975, Weiher and Keddy 1999). In particular, competition should be more intense, and competitive exclusion more likely, among species of similar size and morphology that compete for similar resources. In these circumstances we expect co-occurring species to be morphologically different from each other and to exhibit a pattern of 'morphological overdispersion' (Pimm 1991).
The effect of manipulation of between-habitat dispersal rates in multiple patch systems was examined experimentally using protist communities in laboratory microcosms. Replicate landscapes of eight microcosms (patches) at two spatial scales (patch sizes) were inoculated with 13 species of protists. Dispersal was carried out by transferring a small random sample of medium and protists from one randomly selected microcosm to another within a landscape. Four dispersal rates (24, 6, 2 and 0 transfers very 3 days) were used, and the microcosms were sampled after 6 and 12 weeks. Patch size had a consistent effect on within-path (community) and within-landscape (metacommunity) diversity, both being lower in small patch systems. Higher dispersal rates had a slight effect on community and metacommunity diversity after 12 weeks, with a tendency for higher dispersal to slightly offset the rate of loss of species. Both dispersal and patch size had effects on the abundance of many individual species, though in a variety of ways. The individual species results suggest that extinction is selective with respect to both patch size and dispersal rate treatments, and may be influenced by species interactions. It seems likely that in metacommunity systems of this sort, rather than mainland-island systems, the potential effect of between-patch dispersal rate in rescuing and recolonizing where local extinctions occur may be much reduced by the effect of selective extinction, relative to that expected under the assumption of random extinction.
1 Competing theories of community assembly are very difficult to test. Four main theories exist. The Stochastic theory sees species assembly as being random. The Humpty Dumpty/Alternative Stable States (ASS) theory suggests that a community may be unable to reassemble itself from its constituent species. The Deterministic theory suggests there will be convergence to one stable state. The Pre‐adaptation theory is similar to the Deterministic theory but emphasizes that many species fit the stable state because of characters acquired elsewhere.
2 The reassembly of a flora into new communities in a different country, or its assimilation as a major component of such communities, offers a means to test these theories. The invasion of British plant species into New Zealand, and their reassembly into roadside communities there, is a good example of such a natural experiment.
3 Plant communities of NZ roadsides were compared to the communities of the British National Vegetation Classification (NVC). British roadside communities were also compared to the NVC as a control. New Zealand roadside communities provided a fit to the NVC communities of only 54.7% on average. After excluding species that are not present in NZ, and therefore could not possibly reassemble, the fit increased to 61.1%. British roadsides gave a 65.8% fit. The NZ figures are similar to the fit obtained with random data (58.7%), indicating that the NZ communities bear little relation to the ones formed by the same species in Britain.
4 Similarity between roadside communities in NZ and Britain was low, forming two almost distinct sets of communities.
5 Some of the predictions of the Stochastic, Humpty Dumpty / ASS and Deterministic models are borne out, but others are not. It is concluded that British species have reassembled into communities in NZ most of which are new, i.e. distinct from those that occur in the native range of the species in Britain. The evidence points to a process of community assembly by pre‐adaptation.
Community structure is defined as the distribution and frequency of occurrence of some ecological traits in a set of coexisting species. Many palaeoecological studies of mammal communities assume as valid a model of Community Structure Convergence (CSC), i. e. communities from similar environments should converge to a similar structure. However convincing evidences are known of the existence of multiple Alternative Stable States in ecological communities and similar structures in dissimilar environments have been found. The model of community convergence and the existence of multiple Alternative Stable States are tested here using data from a set of 24 Middle Pleistocene and 50 recent European mammalian communities. Community structure is compared using a multivariate approach. Species are assigned to one of 19 possible ecological groups and a multidimensional “eco-space” is computed based on the abundance of these groups of species in each community, using Principal Components Analysis. The dispersion of the communities in the “eco-space” is used as a measure of their community structure differences. While results indicate the existence of different structures in Glacial and Interglacial northern paleocommunities the considerable intra-group heterogeneity observed contradicts the predictions of the CSC model. The results support the existence of structural continuity (conservation of community structure despite concurrent changes in species composition) in the Iberian and Italian peninsulas during the Middle Pleistocene, as well as a model of cyclic disruption and assembly of paleocommunities with multiple Alternative Stable States in Northern Europe. Northern European assembly processes differed one from the other, giving rise to a community structure more varied than that found in Southern Europe.
Mosaic cycles were originally understood as cyclical regeneration phases in forests. In this review, we shall examine how far the concept can be extended towards cyclical mosaics of habitat quality in patterned landscapes as a special case of ‘dynamic landscapes’. We will concentrate on habitats and plants in European temperate agricultural landscapes and grasslands in particular. Mosaic cycles of habitat quality are characterised by spatiotemporal shifts between disturbance and secondary succession. We found evidence for mosaic cycles in traditional agricultural systems, modern crop farming, and in recent conservation management. The relevant disturbance parameters to describe land-use drivers of mosaic cycles are spatial extent, frequency, and magnitude (biomass loss). Land-use-related drivers are usually regular and deterministic in space and time, with the exception of year-round grazing by free-ranging large herbivores. Fluctuating soil resources such as water and nutrients in interaction with climate variability add a stochastic component to these (land-use-related) drivers. The proportion of deterministic and stochastic components and their autocorrelation in time and space divides purely deterministic mosaic cycles from purely stochastic dynamic landscapes. In a second part, we briefly review plant life-history traits that may facilitate survival of plants in mosaic cycles of habitat quality. Theoretical studies emphasise (i) dispersal functions for extinction and recolonisation processes of metapopulations, (ii) storage effects as a component of buffered population growth in response to temporal fluctuations of habitat quality, and (iii) competitive ability in metacommunities. We propose a simple scheme relating these functions to the temporal and spatial correlation of patterned landscapes. There are only a very limited number of field studies available that give some support for the proposed scheme. We provide perspectives for further research in this field.ZusammenfassungMosaikzyklen sind ursprünglich als zyklische Regenerationsphasen von naturnahen Wäldern aufgefasst worden. In dieser Übersicht dehnen wir das Konzept auf zyklisch wiederkehrende Habitatqualitäten in Landschaften aus und betrachten es als Spezialfall von ‘dynamischen Landschaften’. Wir konzentrieren uns auf Agrarlandschaften der europäischen gemäßigten Zone, speziell auf Grünland, aus dem Blickwinkel von Naturschutz und Landschaftspflege. Mosaikzyklen sind durch räumliche und zeitliche Wechsel zwischen Störungen und Sekundärsukzessionen gekennzeichnet. Wir fanden Belege für Mosaikzyklen in traditionellen agrarischen Anbausystemen und in aktuellen Richtungen der Landschaftspflege. Räumliche Ausdehnung, Frequenz und Biomasseentzug sind wesentliche Störungsparameter für die Charakterisierung des Einflusses der Landnutzung auf Mosaikzyklen.Landnutzungen als Treiber von Mosaikzyklen sind üblicherweise regelmäßig und deterministisch, mit der Ausnahme von ganzjähriger Beweidung durch freilaufende Großherbivoren. Jährlich schwankende Wasser- und Nährstoffangebote fügen dem eine stochastische Komponente hinzu. Die Anteile deterministischer und stochastischer Komponenten und ihre Autokorrelationsstruktur in Zeit und Raum trennen rein deterministische Mosaikzyklen von rein stochastischen dynamischen Landschaften.Im zweiten Teil geben wir einen kurzen Überblick über biologische Merkmale (Life-History Traits) von Pflanzen, welche das Überleben in Mosaikzyklen ermöglichen. Theoretische Studien betonen die Bedeutung von (i) Ausbreitungsmechanismen für Rekolonisationsprozesse im Rahmen von Metapopulationen, (ii) Speichereffekten reproduktiver Kapazität als Puffermechanismus von Populationen bei zeitlicher Änderung der Habitatqualität, sowie (iii) Konkurrenzkraft im Rahmen von Meta-Gemeinschaften. Wir schlagen ein einfaches Schema vor, das diese Funktionen in Abhängigkeit von der räumlichen und zeitlichen Korrelation des Landschaftsmusters darstellt. Bisher gibt es nur eine sehr begrenzte Zahl von empirischen Studien zu diesem Thema. Diese unterstützen das vorgeschlagene Schema zum Teil. Wir geben Hinweise für weitere Forschungen in diesem Feld.
Over the past few years, ecologists have increasingly recognized the existence of strong self-reinforcing (or self-organizing) interactions within systems at a variety of scales. Positive feedback within food chains has been reported from terrestrial and aquatic ecosystems. Accumulating evidence supports the existence within communities of cooperative guilds - tit-for-tat relationships based on diffuse mutualisms and favored by environmental unpredictability. At the landscape level, both real world experience and models indicate that processes such as hydrology and the propagation of disturbance can be strongly self-reinforcing (i.e. the landscape structure supports the process, and vice versa). Hence the picture emerges of a hierarchy of self-organizing systems that span food chains, communities and landscapes/regions.
Community assembly theory asserts that the contemporary composition of ecological communities may depend critically on events that occur during the formation of the community; a phenomenon termed "historical contingence." We tested key aspects of this theory using plant communities in over 200 experimentally created vernal pools at a field site in central California, USA. The experiment was initiated in 1999 with construction of vernal pool basins into which different seeding treatments were imposed to evaluate the effects of dispersal limitation, order of colonization ("priority effects"), and frequency of colonization on plant community composition. We tracked the abundance and distribution of five focal species for seven years following seeding and observed strong but transient effects of seeding, as well as order and frequency of colonization. All five species occurred with higher frequency in seeded pools vs. unseeded control pools, demonstrating dispersal limitation. Three of four species exerted strong priority effects, with much higher abundance in pools in which they were seeded first compared to pools in which they were seeded in the second year of the study, one year after other species were seeded. We tested for effects of frequency of colonization using one species, the endangered Lasthenia conjugens, and observed much higher abundance in frequently vs. infrequently seeded pools for the first four years following seeding. Finally, we observed that the strength of priority effects varied significantly with water depth for one of the species groups, which demonstrates that abiotic context can strongly influence species interactions. We conclude that several aspects of historical contingence play key roles in the early formation of vernal pool plant communities. But we also observed reversals in community trajectories, suggesting that in this system historical effects may be lost within a decade.
Large-scale processes are known to be important for patterns of species richness, yet the ways in which local and larger scale processes interact is not clear. I used metacommunities consisting of five interconnected microbial aquatic communities to examine the manner in which processes at different scales affect local and metacommunity richness. Specifically, I manipulated the potential dispersal rate, whether dispersal was localized or global, and variation in initial community composition. A repeated-measures ANOVA showed that a low dispersal rate and intermediate distance dispersal enhanced local richness. Initial assembly variation had no effect on local richness, while a lack of dispersal or global dispersal reduced local richness. At the metacommunity scale, richness was enhanced throughout the time course of the experiment by initial compositional variation and was reduced by high or global dispersal. The effects of dispersal were contingent on the presence of initial compositional variation. The treatments also affected individual species occupancy patterns, with some benefiting from large-scale processes and others being adversely impacted. These results indicate that the effects of dispersal on species richness have a complex relationship with scale and are not solely divisible into "regional" vs. "local" scales. Finally, predictions of the manner in which dispersal rate structures communities appear dependent upon species compositional variation among communities.
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