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Impacts of Browsing and Grazing Ungulates on Plant Characteristics and Dynamics
Abstract
Plant communities are intricate systems influenced by many factors, including disturbance by ungulates. Differential grazing and browsing preferences, as well as variation in the timing and severity of herbivory, have fitness consequences for plants that lead to shifts in their competitive relationships. In this Chapter, I summarize why different plant types are of variable preference to ungulates. Next, I discuss how plants react to herbivory via altered growth rates and resource reallocation. Then, I describe how these differential responses by individuals and populations of plants lead to change in the overall structure and composition of vegetation communities. I close by addressing vegetation recovery following reduced herbivore pressure, and impediments to recovery such as limited seed availability, recalcitrant layers, invasive species, and the need for site-specific management prescriptions.
... This general and continuous increase in the abundance of large herbivores has led to major changes in forest composition (Ramirez et al. 2019). By consuming plant tissues, herbivores affect the growth, survival and reproduction of plants (Hidding et al., 2012;Sabo, 2019). Heavy browsing generally favours the abundance of species resistant to browsing, at the expense of plant species preferred by herbivores (Tremblay et al., 2006;Hidding et al., 2013;Perea et al., 2014). ...
... The ecological consequences of browsing on plants depend on deer selectivity and on the tolerance of plants to browsing (Long et al., 2007;Perea et al., 2014;Sabo, 2019). Availability of understory vegetation is known to be one of the key factors affecting the sensibility of forest regeneration to browsing damage (Gill, 1992;Vospernik and Reimoser, 2008). ...
Browsing damage in forests relies on a complex interaction between herbivore density and forest understory composition and relative availability. Although variation in the amount of browsed twigs is sometimes used to assess abundance of large herbivores, the potential confounding effect of resource availability on this relationship has not been investigated yet. To fill the gap, we measured how browsing intensity of the woody plants varied in response to changes in both roe deer (Capreolus capreolus) abundance and vegetation availability from an intensive long-term monitoring. We estimated plant availability and consumption by roe deer from a modified Aldous method throughout a 14 year-long period during which we experimentally manipulated population density. The functional response was strongly non-linear and density-dependent. When plant availability was low (<12.5%), browsing intensity strongly increased with plant availability with an increasing rate of roe deer density, whereas beyond this threshold, browsing intensity slightly increased with both plant availability and population density in an additive way. Thus, forest susceptibility to browsing increases with increasing competition for food, especially when plant availability is low. Moreover, the interplay between browsing intensity and population density at low plant availability prevents the use of browsing intensity to monitor roe deer abundance when plant availability is low. Our findings provide clear evidence that relying on key ecological concepts such as functional responses improves the accuracy of management tools when monitoring changes of the herbivore-plant system over time.
... Particularly ungulates show preferred plant heights for browsing, which allow for better vigilance, meet biokinetic demands [23,24], and respond to plant morphological traits [25,26]. On the other hand, plants try to vertically escape ungulate herbivory by height increment, by changing leaf traits (photosynthesis, morphology, and chemistry) at specific height strata [10,78], and plant architecture [79]. Together with the preference or avoidance of specific plant species, the duration and intensity of browsing and the seasonally changing food availability, these height preferences might yield modifications of structural features of plant communities. ...
Different species-specific traits of woody plant species, feeding preferences of herbivores together with resulting effects on plant competition are expected to translate into different plant community structures and expressions of biodiversity. We studied the diversity of woody plant species (trees and shrubs) and structural diversity of forest trees, using a 30-year and an 18-year dataset of ungulate exclosure-control plot pairs in a mixed alpine forest community in Austria. We surveyed the tallest individuals per tree species and plot and analyzed the collective of top-height individuals per plot pair. Incidence data for exclosure and control plots were aggregated. Comparing species diversity and diversity of height classes on the plots throughout time, we calculated diversity profiles based on Hill numbers. Diversity of top height individuals and structural diversity, expressed by height classes, were two diversity aspects that differed between exclosures and control plots. Other diversity estimates of woody plant species showed huge variation without significant differences between plots. Height growth was significantly suppressed by ungulate herbivory. Effects of ungulate herbivores in forest ecosystems are highly complex and context-dependent and thus not reducible to simple top-down forces. Long-term surveys provide data that reflect “ultimate” effects of herbivory interacting with other drivers of community dynamics.
... Although numerous studies have shown that the amount of forage was probably the most important factor determining browsing intensity (Russell et al., 2017), landscape-scale factors matter, including surrounding land cover types, management activities, vegetation structure and preferences of wild browsers in terms of habitat and food sources (Sabo, 2019). Analyses of the browsing behaviours of most important European wild ungulates revealed their preferences for browsing in gaps rather than in closed forests (Kuijper et al., 2009;Tahtinen et al., 2014). ...
Large wild ungulates are a major biotic factor shaping plant communities. They influence species abundance and occurrence directly by herbivory and plant dispersal, or indirectly by modifying plant-plant interactions and through soil disturbance. In forest ecosystems, researchers’ attention has been mainly focused on deer overabundance. Far less is known about the effects on understory plant dynamics and diversity of wild ungulates where their abundance is maintained at lower levels to mitigate impacts on tree regeneration. We used vegetation data collected over ten years on 82 pairs of exclosure (excluding ungulates) and control plots located in a nation-wide forest monitoring network (Renecofor). We report the effects of ungulate exclusion on (i) plant species richness and ecological characteristics, (ii) and cover percentage of herbaceous and shrub layers. We also analysed the response of these variables along gradients of ungulate abundance, based on hunting statistics, for wild boar (Sus scrofa), red deer (Cervus elaphus) and roe deer (Capreolus capreolus). Outside the exclosures, forest ungulates maintained a higher species richness in the herbaceous layer (+15%), while the shrub layer was 17% less rich, and the plant communities became more light-demanding. Inside the exclosures, shrub cover increased, often to the benefit of bramble (Rubus fruticosus agg.). Ungulates tend to favour ruderal, hemerobic, epizoochorous and non-forest species. Among plots, the magnitude of vegetation changes was proportional to deer abundance. We conclude that ungulates, through the control of the shrub layer, indirectly increase herbaceous plant species richness by increasing light reaching the ground. However, this increase is detrimental to forest-specialist species, and contributes to a landscape-level biotic homogenisation. Even at population density levels considered to be harmless for overall plant species richness, ungulates remain a conservation issue for plant community composition. This article is protected by copyright. All rights reserved.
In forests of eastern North America, white‐tailed deer ( O docoileus virginianus ) can directly affect, via herbivory, the presence, abundance and reproductive success of many plant species. In addition, deer indirectly influence understorey communities by altering environmental conditions.
To examine how deer indirectly influence understorey plants via environmental modification, we sampled vegetation and environmental variables in‐ and outside deer exclosures (10–20 years old) located in temperate forests in northern Wisconsin and the Upper Peninsula of Michigan, USA . We assessed how excluding deer affected understorey community composition and structure, the soil and light environment, and relationships between direct and indirect effects, using non‐metric multidimensional scaling ( NMDS ), mixed linear models and nonparametric multiplicative regression ( NPMR ).
Excluding deer altered sapling communities and several aspects of the understorey environment. Excluding deer from plots with lower overstory basal area increased sapling abundance, decreasing the amount of light available to groundlayer plants. Exclusion also reduced soil compaction and the thickness of the soil E horizon.
The composition of understorey communities covaried in apparent response to the environmental factors affected by exclusion. In several common species and groups, E horizon thickness, compaction, openness, and/or total (sapling and overstory) basal area were significant predictors of plant frequency.
Complementary analyses revealed that deer exclusion also altered the frequency distributions of several species and groups across environmental space.
Synthesis . Deer alter many facets of the understorey environment, such as light availability, soil compaction and thickness of the soil E horizon, which, in turn, appear to mediate variation in plant communities. Those environmental modifications likely compound direct impacts of herbivory as drivers of understorey community change. Our results provide evidence that deer effects on the environment have important implications for forest composition. Thus, we suggest a re‐examination of the common assumption that understorey community shifts stem primarily from tissue removal.
The impacts of introduced or overabundant large herbivores are a concern for the conservation of forest plant communities and the sustainability of ecosystem function. White-tailed deer (Odocoileus virginianus) are considered ecologically overabundant in much of North America. Previous work suggests that impacts of deer overabundance are broadly negative and are consequently degrading forests at multiple ecological and taxonomic levels. However, no quantitative synthesis currently exists to verify the generality or magnitude of these impacts. Here, we report the results of a meta-analysis quantifying the effects of deer exclusion on the diversity, cover and abundance of woody, herbaceous, and whole community components of forest understories in North America. In addition, we explore the relationships of environmental and experimental factors on the direction and magnitude of plant community outcomes using meta-regression. Using 119 calculated effect sizes sourced from 25 peer-reviewed articles, we constructed 10 community-specific datasets and found strongly positive diversity, cover and abundance responses of the woody community to deer exclusion, but no significant effects for the herbaceous or whole community components of forest understories. Local deer density and time since exclusion were significant moderators of both whole community and woody community richness. Local deer density also moderated the effects of deer exclusion on whole community cover. Plot area, in contrast, showed no relationship to any of the community response outcomes. We suggest that the use of inadequate diversity indices, non-native species replacement, or legacy effects of chronic deer overabundance might explain why the herbaceous and whole community components of forest understories showed no diversity or cover responses to deer exclusion. We also suggest some strategies to increase opportunities for future quantitative syntheses of deer impacts on forests, including providing better access to existing and future data. Ultimately, we show that white-tailed deer have strongly negative impacts on forest understory plant communities in North America, but these impacts are not ubiquitous for all components of the plant community.
Changes in habitat and reduction in predation and hunting pressure are two primary causes of high-density populations of white-tailed deer (Odocoileus virginianus) in many areas of eastern North America. Despite increasing recognition of the major effects deer exert on forest communities, deciding when deer are overabundant remains a major challenge to managers charged with conserving native plant communities. In this context, we define overabundance as a condition where deer are causing the local extinction of a native plant species. Because this definition is difficult to apply from a management perspective, we outline an approach using native understory forbs. Indicator species are selected based on combined criteria of palatability to deer, leaf and flower morphology, and demographic characteristics. Four indices related to plant population viability are derived from understory survey data: flowering rate, mean stem height, stage-class distribution, and deer browsing pressure. We apply this analysis to Trillium populations from forests in Minnesota (highly fragmented agricultural landscape with varying deer densities), Pennsylvania (forested and fragmented landscape with long-term high deer densities), and New York (forested and fragmented landscape with long-term low deer densities). We observed two distinct types of deer-affected plant populations. In sites with moderate or recent increases in deer density, Trillium populations were characterized by low mean plant size, high browsing rates, intermediate flowering rates, and a size class distribution lacking large, reproductive plants. Sites affected by long-term high deer densities exhibited low browsing rates on Trillium, low Trillium flowering rates, a population structure lacking both large and small plants, and high summer browsing pressure on woody saplings. We suggest these combined indices be used to assess deer browsing impact, and we discuss the role of landscape structure and deer density in defining deer overabundance.
In 1929, baseline vegetation data were collected in Heart's Content Scenic Area, an old-growth, 50-ha hemlock-northern hardwood tract within the Allegheny National Forest in northwestern Pennsylvania, USA. In 1995, we resampled the ground layer to document the extent of changes in the herb and shrub vegetation in this tract since 1929. We recorded changes in herb and shrub species frequency, abundance, richness, and evenness between 1929 and 1995. Numerous species present in 1929 were absent in 1995: 33 species (80%) were missing from the hemlock-beech stand, and 16 species (59%) were missing from the hemlock stand. More species were missing than were new in the hemlock-beech (sign test, p<0.0001) and in the hemlock stand (sign test, p=0.0001), and more species declined in abundance than increased (sign test p<0.0001) in the hemlock-beech stand. In both stands, species with relative abundance values of <1% in 1929 were more likely than more abundant species to be missing in 1995. Evenness of herbaceous species declined in the hemlock-beech stand but increased in the hemlock stand. High species richness at the quadrat scale was negatively associated with high densities of hay-scented fern (Dennstaedtia punctilobula Michx). The number of families represented declined from 27 in 1929 to 10 in 1995 (sign test, p=0.0001). We attribute declines in species richness to the direct and indirect effects of herbivory by white-tailed deer. We suggest that this tract can and should be restored using a management strategy that includes building a deer exclosure around the entire tract, reducing fern abundance, and reintroducing extirpated species.
White-tailed deer (Odocoileus virginianus Zimmerman) are known to disperse seeds of a hybrid complex of invasive honeysuckle shrubs in northeastern United States. We investigated whether they also disperse seeds of Amur honeysuckle, Lonicera maackii, a problematic invasive shrub in the Midwestern and eastern U.S. We found that deer ingest ripe fruit and void intact seeds of L. maackii. Based on tetrazolium tests, most (68%) of these voided seeds are viable, but the proportion viable was significantly lower than that for seeds taken directly from ripe fruits. White-tailed deer are potentially important in the long-distance dispersal of this invasive shrub.
Plant communities are often dispersal‐limited and zoochory can be an efficient mechanism for plants to colonize new patches of potentially suitable habitat. We predicted that seed dispersal by ungulates acts as an ecological filter – which differentially affects individuals according to their characteristics and shapes species assemblages – and that the filter varies according to the dispersal mechanism (endozoochory, fur‐epizoochory and hoof‐epizoochory). We conducted two‐step individual participant data meta‐analyses of 52 studies on plant dispersal by ungulates in fragmented landscapes, comparing eight plant traits and two habitat indicators between dispersed and non‐dispersed plants. We found that ungulates dispersed at least 44% of the available plant species. Moreover, some plant traits and habitat indicators increased the likelihood for plant of being dispersed. Persistent or nitrophilous plant species from open habitats or bearing dry or elongated diaspores were more likely to be dispersed by ungulates, whatever the dispersal mechanism. In addition, endozoochory was more likely for diaspores bearing elongated appendages whereas epizoochory was more likely for diaspores released relatively high in vegetation. Hoof‐epizoochory was more likely for light diaspores without hooked appendages. Fur‐epizoochory was more likely for diaspores with appendages, particularly elongated or hooked ones. We thus observed a gradient of filtering effect among the three dispersal mechanisms. Endozoochory had an effect of rather weak intensity (impacting six plant characteristics with variations between ungulate‐dispersed and non‐dispersed plant species mostly below 25%), whereas hoof‐epizoochory had a stronger effect (eight characteristics included five ones with above 75% variation), and fur‐epizoochory an even stronger one (nine characteristics included six ones with above 75% variation). Our results demonstrate that seed dispersal by ungulates is an ecological filter whose intensity varies according to the dispersal mechanism considered. Ungulates can thus play a key role in plant community dynamics and have implications for plant spatial distribution patterns at multiple scales.
Synthesis
Plant communities are often dispersal‐limited and zoochory can be an efficient mechanism for plants to colonize new patches of potentially suitable habitat. Our analysis is the first synthesis of ungulate seed dispersal that compares characteristics from both non‐dispersed and dispersed diaspores, distinguishing the three zoochory mechanisms ungulates are involved in: endozoochory, hoof‐epizoochory and fur‐epizoochory. We confirmed that seed dispersal by ungulates is an ecological filter whose intensity increases from endozoochory, then hoof‐epizoochory to finally fur‐epizoochory. By filtering seed traits through dispersal, ungulates can thus play a key role in plant community dynamics and have implications for plant spatial distribution patterns at multiple scales.
The fact that herbivores and predators exert top-down effects to alter community composition and dynamics at lower trophic levels is no longer controversial, yet we still lack evidence of the full nature, extent, and longer-term effects of these impacts. Here, we use results from a set of replicated experiments on the local impacts of white-tailed deer to evaluate the extent to which such impacts could account for half-century shifts in forest plant communities across the upper Midwest, USA. We measured species' responses to deer at four sites using 10-20 year-old deer exclosures. Among common species, eight were more abundant outside the exclosures, seven were commoner inside, and 16 had similar abundances in- and outside. Deer herbivory greatly increased the abundance of ferns and graminoids and doubled the abundance of exotic plants. In contrast, deer greatly reduced tree regeneration, shrub cover (100-200 fold in two species), plant height, plant reproduction, and the abundance of forbs. None of 36 focal species increased in reproduction or grew taller in the presence of deer, contrary to expectations. We compared these results to data on 50-year regional shifts in species abundances across 62 sites. The effects of herbivory by white-tailed deer accurately account for many of the long-term regional shifts observed in species' abundances (R2 = 0.41). These results support the conjecture that deer impacts have driven many of the regional shifts in forest understory cover and composition observed in recent decades. Our ability to link results from shorter-term, local experiments to regional long-term studies of ecological change strengthens the inferences we can draw from both approaches.
Research reported in this feature identifies a convergence of interpretations regarding the threshold dynamics of complex ecological systems. This convergence has arisen from a diverse set of investigations addressing rangeland ecosystem dynamics, disease transmission, and fluctuations in the populations of insect pests. Effective application of the threshold concept to ecosystem management will require development of more robust linkages between non-equilibrium theory and protocols to identify triggers that initiate threshold conditions, feedback loops that establish system resilience, and developmental trajectories and attributes of potential alternative stable states. Successful implementation of these theory/application linkages has the potential to underpin an operational framework of resilience-based ecosystem management that is founded upon the identification of structural indicators that are correlated with vulnerability or proximity to thresholds, rather than threshold identification per se. Several investigations indicate that thresholds are strongly influenced by scale; multiple cross-scale interactions demonstrate the need for greater knowledge and analyses to address scale-dependent processes, i.e., critical scales and scaling laws. This feature emphasizes the relevance of thresholds and non-equilibrium dynamics in multiple
natural resource management applications and in so doing demonstrates the need for a more comprehensive and integrated ecological framework capable of quantitatively assessing dynamics at multiple scales to inform management and policy recommendations for optimal management and risk assessment.
Some land managers question whether eastern white pine (Pinus strobus L.) will remain an important component of forested landscapes in Minnesota, USA, due to poor establishment and limited recruitment into forest canopies. Browsing by white-tailed deer (Odocoileus virginianus Zimm.) and snowshoe hare (Lepus americana Erxl.) inhibited establishment and recruitment of white pine in an old-growth pine-dominated forest located in a major winter deer yard in northeastern Minnesota. In a nine-year study, we investigated the effectiveness of exclosures to control browsing and improve establishment and early recruitment of eastern white pine. Properly constructed and maintained exclosures virtually eliminated browsing. As a result, density in the largest size class (stems >50 cm tall but <2.5 cm dbh) increased significantly between 1991 and 1997 in two exclosure plots (P < 0.1; two-tailed paired t-test). Reference plots showed no evidence of pine recruitment into the largest size class. We report a little documented factor with potential for aiding eastern white pine establishment: dense patches of young white spruce (Picea glauca L.) and balsam fir (Abies balsamea [L.] Miller), which may buffer white pine seedlings and saplings from browsing. Browsing is just one factor affecting survival and growth of eastern white pine. We propose a model incorporating multiple factors to consider when deciding on effective strategies for the restoration of eastern white pine in the northern Lake States region.
The problem of ascribing adaptive significance to traits that enable graminoids to tolerate or evade ungulate herbivory is examined. Some of these traits may have originally evolved in response to non-grazing selection pressures, thus constituting grazing exaptations rather than true adaptations. The fossil record indicates that semiarid habitats, extensive grasslands and grazers appeared, interacted and evolved together, but several traits that are advantageous in semiarid habitats have beneficial effects for grazed plants. Other traits, such as developmental plasticity, enhance competitive ability in certain environments, but also increase grazing tolerance or resistance. Defoliation responses are embedded in a network of interacting processes, including photosynthesis, transpiration, nutrient uptake and resource allocation. Responses and adaptations to defoliation must be interpreted in this context. Although traits may have arisen due to non-herbivorous selection pressures, they may subsequently have been selected, combined, or amplified through grass-grazer coevolution to form species, polymorphic populations, phenotypically plastic individuals, or communities of species that evade, resist or tolerate herbivory. -from Author
Significance
In ecosystems worldwide, the presence of overabundant ungulates (e.g. deer, cows) and the invasion of exotic plants are disrupting native communities. A recent hypothesis causally links these problems implicating overabundant ungulates in enhancing invaders’ demographic success. We tested this hypothesis in a forest where white-tailed deer are overabundant and garlic mustard is aggressively invading. Using long-term, replicated deer exclusion/deer access plot pairs, we quantified population density, growth, and decline of this invader and native plants. We conclusively demonstrate that deer are required for garlic mustard success; its local extinction is projected where deer are absent. Our findings provide the first definitive support connecting overabundant ungulates to enhanced invader success, with broad implications for biodiversity and ecosystem function.
Soil seed bank composition is important to the recovery of natural and semi-natural areas from disturbance and serves as a safeguard against environmental catastrophe. White-tailed deer (Odocoileus virginianus) populations have increased dramatically in eastern North America over the past century and can have strong impacts on aboveground vegetation, but their impacts on seed bank dynamics are less known. To document the long-term effects of deer browsing on plant successional dynamics, we studied the impacts of deer on both aboveground vegetation and seed bank composition in plant communities following agricultural abandonment. In 2005, we established six 15×15 m fenced enclosures and paired open plots in recently fallowed agricultural fields near Ithaca, NY, USA. In late October of each of six years (2005-2010), we collected soil from each plot and conducted seed germination cycles in a greenhouse to document seed bank composition. These data were compared to measurements of aboveground plant cover (2005-2008) and tree density (2005-2012). The impacts of deer browsing on aboveground vegetation were severe and immediate, resulting in significantly more bare soil, reduced plant biomass, reduced recruitment of woody species, and relatively fewer native species. These impacts persisted throughout the experiment. The impacts of browsing were even stronger on seed bank dynamics. Browsing resulted in significantly decreased overall species richness (but higher diversity), reduced seed bank abundance, relatively more short-lived species (annuals and biennials), and fewer native species. Both seed bank richness and the relative abundance of annuals/biennials were mirrored in the aboveground vegetation. Thus, deer browsing has long-term and potentially reinforcing impacts on secondary succession, slowing succession by selectively consuming native perennials and woody species and favoring the persistence of short-lived, introduced species that continually recruit from an altered seed bank.
Alternative successional trajectories (AST) may result in multiple climax states within an ecosystem when disturbances affect colonization history. In the boreal forest, ungulates have been proposed to drive AST because, under herbivore pressure, preferred species may go extinct and apparent competition may benefit browsing-resistant species. Over a 15-year period following logging, we tested whether deer herbivory altered plant species composition and whether the competitive advantage of resistant species was maintained following herbivore removal. We compared exclosures built immediately after logging with delayed exclosures built eight years later on Anticosti Island, Quebec, Canada. Although the palatable tree Betula papyrifera (paper birch) and some palatable herbs recovered in delayed exclosures, we observed legacies in both tree and herb cover. Woody regeneration in delayed exclosures was dominated by Picea glauca (white spruce), and Poaceae (grasses) were abundant in the field layer. Given that only early-successional species recovered, whereas late-successional broadleaf species and Abies balsamea (balsam fir) remained rare, succession may follow an AST after a limited browsing period during early succession.
Ungulate browsers, when at high densities, are major drivers of vegetation change in forests world‐wide. Their effects operate via a variety of generalizable mechanisms related to plant palatability and relative growth rate with respect to browsing pressure.
Though such impacts are obviously long‐lasting when they determine composition of tree regeneration, we document in a unique long‐term (30 year) experiment that biological legacies of initial deer density persist in the understorey herbaceous vegetation at least 20 years after deer densities were equalized.
We sampled understorey vegetation in former clear‐cut areas where density of white‐tailed deer ( O docoileus virginianus ) was manipulated (3.9–31.2 deer km ⁻² ) for 10 years (1979–1990), and stands experienced ambient deer density (ca. 10–12 deer km ⁻² ) for the next 20 years (1990–2010) to determine whether initial deer density treatments still influenced understorey vegetation in 30‐year‐old, closed‐canopy forests.
Stands initially (1979–1990) exposed to higher deer densities had ca. five times higher fern cover and three times the seedling and forb cover in 2010, as well as significantly lower angiosperm species density, compared to stands initially exposed to lower deer densities.
These results appear driven by deer avoidance of ferns, allowing them to expand at high deer density and sequester sites for decades.
Synthesis . Our long‐term, experimental results show unequivocally that elevated deer densities cause significant, profound legacy effects on understorey vegetation persisting at least 20 years. Of relevance regionally and globally where high deer densities have created depauperate understoreys, we expect that deer density reduction alone does not guarantee understorey recovery; stands may need to be managed by removing recalcitrant understorey layers (e.g. ferns).
Invasive species are often assumed to be the cause (drivers) of declines in native species and alterations of native ecosystems. However, an alternative model suggests that many invasive plants are better described as passengers of altered disturbance regimes or other changes in ecosystem properties. Some species do seem to be easily categorized as passengers or drivers, but others may be better described as “back-seat drivers”. Back-seat drivers require or benefit from disruptions of ecosystem processes or properties that lead to declines of native species but also contribute to changes in ecosystem properties and further declines of native species. Among these possibilities, drivers are a direct cause of the decline of native species through the introduction of novel traits or functions to an ecosystem, whereas back-seat drivers interact with ecosystem change to cause native species declines. Passengers are better considered as a symptom of an underlying problem, rather than the cause of native species declines. Driver, back-seat driver and passenger models suggest different associations between invasive species, ecosystem change and native species declines, and these models provide a framework for predicting and understanding the response of native species to invasive species management.
The introduction of Sitka black-tailed deer (Odocoileus hemionus sitkensis Merriam) to Haida Gwaii (Queen Charlotte Islands, BC, Canada) in the late 19th century, provided an opportunity to understand the long-term eVects of deer populations on the vegeta- tion of temperate rain forests in the absence of their natural predators wolves (Canis lupus L.), and cougars (Puma concolor L.). Using seven small islands with diVerent browsing histories (no deer, deer for 50 years), we tested the long-term eVects of high deer densities on plant cover and species richness in the understorey of forest interior and forest edge habitats. Overall vege- tation cover exceeded 80% in the lower vegetation layers on islands without deer and was less than 10% on the islands with deer for more than 50 years. Although overall plant species richness was similar on islands with or without deer, plant species richness at the plot scale (314 m2) was reduced by 20-50% on islands with deer for >50 years. The diVerences were most pronounced for the species- rich edge communities and among herb and shrub species. These results suggest that in the absence of predators, deer have the poten-
Moose and reindeer occur in large populations in the Fennoscandian boreal forests, and also roe deer occurs in dense populations in Sweden and Norway. These large herbivores affect the structure and function of the forest ecosystems. During periods of high densities discussions arise about the impact of these herbivores on e.g. economic forest trees and preservation of biodiversity. The aim of this study is to review the present knowledge of the disturbance caused by moose in the boreal forest. First, we give a quantitative estimate of the different disturbance factors (feeding, trampling, defecation and urination). Second, we discuss the ecological impact of the different disturbances.
Methane (CH4) emissions by human activities have more than doubled since the 1700s, and they contribute to global warming. One of the sources of CH4 is produced by incomplete oxidation of feed in the ruminant's gut. Domestic ruminants produce most of the emissions from animal sources, but emissions by wild ruminants have been poorly estimated. This study (i) scales CH4 against body mass in 503 experiments in ruminants fed herbage, and assesses the effect of different sources of variation, using published and new data; and (ii) it uses these models to produce global estimates of CH4 emissions from wild ruminants. The incorporation of phylogeny, diet and technique of measuring in to a model that scales log10 CH4 g d− 1 against log10 body mass (kg), reduces the slope, from 1.075 to 0.868, making it not significantly steeper than the scaling coefficient of metabolic requirements to body mass. Scaling models that include dry matter intake (DMI) and dietary fiber indicate that although both increase CH4, dietary fiber depresses CH4 as the levels of DMI increases. Cattle produces more CH4 per unit of DMI than red deer, sheep or goat, and there are no significant differences between CH4 produced by red deer and sheep. The average estimates of global emissions from wild ruminants calculated using different models are smaller (1.094–2.687 Tg y− 1) than those presented in the reports of the Intergovernmental Panel on Climate Change (15 Tg yr− 1). Potential causes to explain such discrepancy are the uncertainty on the world's wild ruminant population size, and the use of methane output from cattle, a high methane producer, as representative methane output of wild ruminants. The main limitation researchers' face in calculating accurate global CH4 emissions from wild ungulates is a lack of reliable information on their population sizes.
Local, short- to medium-term studies make clear that white-tailed deer can greatly suppress tree growth and survival in palatable tree species. To assess how deer have broadly affected patterns of tree recruitment across northern Wisconsin, we analyzed recruitment success in 11 common trees species that vary in palatability across 13,105 USFS - FIA plots sampled between 1983 and 2013. We also examined how recruitment in these species covaried with estimated deer densities here. Saplings of five palatable species were scarce relative to less palatable species and showed highly skewed distributions. Scarcity and skew provide reliable signals of deer impacts even when deer have severely reduced recruitment and/or no reliable deer density data are available. Deer densities ranged from 2.3 to 23 deer per km2 over a 30 year period. Sapling numbers in two maples (Acer) and aspen (Populus) with intermediate palatability declined sharply in apparent response to higher deer density. Path analysis also reveals that deer act to cumulatively depress sapling recruitment in these species over successive decades. Together, these approaches show that deer have strongly depressed sapling recruitment in all taxa except Abies and Picea. As these impacts are now propagating into larger sized trees, deer are also altering canopy composition and dynamics. The tools developed here provide efficient and reliable indicators for monitoring deer impacts on forest tree recruitment using consistent data collected by public agencies.
Because of the modular structure and enormous phenotypic plasticity of plants, herbivores influence plant architecture in diverse ways that range from superficial modifications to transformations of the entire plant form. Potential consequences for the plant of these structural changes are numerous, and may result in repercussions at the organismal, population, or community levels. The importance of plant structure in determining the species richness of insect herbivore communities is well documented (Lawton and Schröder, 1977, 1978; Strong and Levin, 1979; Moran, 1980; Southwood et al., 1982; Lawton, 1983). In general, these studies found that larger, more structurally complex plants, support a greater number of herbivore species because they provide a greater diversity of habitats.
Due to an extremely high density of sika deer population (50 deer/km2), the vegetation on Kinkazan Island, northern Japan, is heavily affected. Unpalatable plants including forbs, ferns, and shrubs containing secondary compounds such as Senecio cannabifolis (Aleutian ragwort), Primula japonica (primrose), Pteridium aquilinum (bracken fern), and Leucothoe grayana (Ericaceae) are abundant on Kinkazan Island. The forest structure is modified, the shrub layer is quite poor, and regeneration of the forest is prevented by deer browsing. Forest gaps are formed by strong winds, and the gaps are invaded by Miscanthus sinensis (silver grass). At the density of 100-200 deer/km2, the Miscanthus community is maintained, but it is often concentrated on by sika deer because of great forage biomass, and the deer density often exceeds this level. Thus, Miscanthus cannot tolerate heavy graz ing, and the Miscanthus community is replaced by the Zoysia japonica (Japanese lawngrass) community which is the most grazing-tolerant lawn-type community. The Zoysia community is favored by deer grazing because grazing removes other competitive tall plants while Zoysia japonica can produce leaves under a good light condition. Z. japonica also owes seed dispersal to sika deer. Thus, forest is decreasing and grassland communities, particularly the Zoysia community, are increasing on Kinkazan Island.
Both defoliation and higher leaf placement can reduce growth in herbaceous plants. We examined variation in relative growth rate (RGR) and reproduction in response to defoliation and leaf height in Trillium grandiflorum at 4 sites in northern Wisconsin, USA. Plant biomass and the probability of flowering increased with increased leaf height. Analyses of covariance revealed that experimental defoliation and leaf height separately and independently reduced individual RGR, and accounted for about 42% of the total variance in RGR. Defoliation in 1998 had no effect on flowering in 1999, but leaf height in 1998 was positively associated with the likelihood of flowering in 1999. RGR and plant responses to defoliation varied among sites. We conclude that defoliation and leaf height reduce RGR over short time scales.
Understanding drivers of plant invasions is essential to predict and successfully manage invasions. Across forests in North America, increased white-tailed deer (Odocoileus virginianus) abundance and non-native earthworms may facilitate non-native plant invasions. While each agent may exert independent effects, earthworms and deer often co-occur and their combined effects are difficult to predict based solely on knowledge of their individual effects. Using a network of twelve forested sites that differ in earthworm density, we evaluated deer exclusion effects (30 × 30 m; with an adjacent similar sized unfenced control plot) on cover, growth and reproduction of three non-native plant species: Alliaria petiolata, Berberis thunbergii and Microstegium vimineum. In addition, we assessed interactive effects of deer exclusion and earthworm invasions on B. thunbergii ring-growth. Five years after fence construction, A. petiolata frequency and density, B. thunbergii height, and M. vimineum cover were all significantly lower in fenced compared to open plots. In addition, B. thunbergii ring-growth was significantly lower in fenced compared to open plots, and ring-growth was positively correlated with earthworm density. Moreover, deer access and earthworm density synergistically interacted resulting in highest B. thunbergii ring-growth in open plots at sites with higher earthworm density. Results indicate facilitative effects of deer on non-native plant species and highlight the importance of understanding interactions among co-occurring factors in order to understand non-native species success. Successful long-term control of invasive plants may require a reduction in deer abundance, rather than just removing invasive plant species.
In spring, migrating Brent Geese (Branta bernicla) stage on the salt-marsh of Schiermonnikoog, a Dutch island in the Wadden Sea. During May, their principal food plants on the marsh are Puccinellia maritima and Plantago maritima. The Brent graze in large groups, and regraze the same places every four days. This interval seems to be related to the rate of regrowth of their food plants.A close study of the grazing of Plantago by the Brent revealed that the bite size is about 14 mm and is almost exclusively confined to the youngest leaves. Each time the geese harvest about 30 % of the rosette's leaf material.A clipping experiment on the salt-marsh with Plantago, showed that a regime of harvesting about 30 % of the rosette once every four days, like the geese do, gave the highest regrowth of new plant tissue, compared with no clipping, or with other clipping treatments.The selective advantage to the individual in taking part in flock grazing movements is discussed in relation to the effects of Brent grazing on the vegetation.
Vertebrate herbivores and omnivores modify forest regeneration not only via direct and indirect trophic pathways, but also via nontrophic pathways such as litter and soil disturbances that may favor the regeneration of some species but not others. Deer are overabundant throughout vast portions of the eastern deciduous forest, and turkeys, once nearly extirpated, are now far more common; their foraging habits disturb litter over large areas, though this has rarely been evaluated. We quantified the size and frequency of litter disturbances created by both deer and turkeys. In addition, we tested the hypothesis that excluding vertebrates via fences would cause a reduction in the abundance and size of litter disturbances. We tested this hypothesis inside and outside six exclosures and adjacent control plots in an old-growth deciduous forest in Pennsylvania. Bare soil patches were similar to 60% smaller and between 50% and 90% less abundant inside exclosures. The mean size of turkey litter disturbances was large (mean >30 m(2)) and significantly greater than deer disturbances (p = 0.002), though turkey disturbances were less frequent. Our findings should apply broadly and are some of the first to demonstrate the extent of turkey disturbances. In addition, we demonstrate that exclosure studies reduce physical disturbances as well as browsing, both of which may synergistically act to cause changes in forest communities. We also caution that many deer fences exclude other vertebrates such as turkeys, which are important herbivores, seed predators, and major agents of disturbance. Consequently, we argue that studies that use fences to exclude deer should explicitly consider nontrophic indirect effects, particularly leaf disturbances and the potential impact of other large consumers as well (e.g., turkeys).
Exotic species are widely assumed to thrive because they lack natural enemies in their new ranges. However, a meta-analysis
of 63 manipulative field studies including more than 100 exotic plant species revealed that native herbivores suppressed exotic
plants, whereas exotic herbivores facilitated both the abundance and species richness of exotic plants. Both outcomes suggest
that plants are especially susceptible to novel, generalist herbivores that they have not been selected to resist. Thus, native
herbivores provide biotic resistance to plant invasions, but the widespread replacement of native with exotic herbivores eliminates
this ecosystem service, facilitates plant invasions, and triggers an invasional “meltdown.”
Feeding strategies of ungulates are usually classified along a browser - grazer continuum which ranges from browsing through to grazing ungulates, but does not accurately include frugivores. However, to understand the evolution of ungulate feeding it is necessary to have a classification that realistically incorporates the full range of ungulate feeding strategies. Such a classification can be described as a linear continuum that ranges from fruit feeders through to browsers and then grazers. Purely frugivorous ungulates are restricted to tropical forests and have consistently small body sizes. Pure grazers on the other hand are absent from tropical forests and are found most commonly in grassland and savannas. Browsing is the most common ungulate feeding strategy and is found in ungulates with a wide range of body sizes and is common in all habitat types. Fruit differs greatly from browses and grasses and adds additional support to the proposed frugivore-browser-grazer classification.
African pastoral ecosystems have been studied with the assumptions that these ecosystems are potentially stable (equilibrial) systems which become destabilized by overstocking and overgrazing. Development policy in these regions has focused on internal alterations of system structure, with the goals of restoring equilibrium and increasing productivity. Nine years of ecosystem-level research in northern Kenya presents a view of pastoral ecosystems that are non-equilibrial but persistent, with system dynamics affected more by abiotic than biotic controls. Development practices that fail to recognize these dynamics may result in increased deprivation and failure. Pastoral ecosystems may be better supported by development policies that build on and facilitate the traditional pastoral strategies rather than constrain them.
We measured legume abundance following 13- and 5-yr experiments testing for the effects of mammalian herbivores, nutrients, and climate on plant communities in an old field and a savanna in east central Minnesota. Total legume abundance was significantly greater in plots with herbivores excluded. Within herbivore exclosures, legumes were more abundant in plots to which P, K, S, Mg, Mn, Ca, Na, and trace minerals had been added. Legumes were significantly more abundant in the savanna than in the old field. Lathyrus venosus, a rapidly growing early-maturing species, was largely responsible for these results. Two other common legumes at our study site, Amorpha canescens and Lespedeza capitata, were not significantly affected by herbivore exclosures. However, within herbivore exclosures, addition of nutrients significantly reduced Lespedeza. Late summer total legume biomass within herbivore exclosures increased strongly following exclosure establishment in 1982, declined dramatically following a severe drought in 1988, and then increased again following the drought. This trend suggested that herbivore effects we measured in 1994 resulted from long-term accumulation of legumes following the establishment of exclosures. Our results suggest that herbivores and nutrients other than nitrogen can dramatically limit the abundance of some legume species that might otherwise dominate grassland plant communities on nitrogen-poor soils. Limitation of legumes by colonization and drought may also be important. Thus, herbivores and nutrients other than nitrogen may be critical in structuring grassland plant communities and influencing succession, even on nitrogen-poor soils.
We recorded food selection by free-ranging kudus (Tragelaphus strepsiceros), impalas (Aepyceros melampus), and domestic goats in the Nylsvley Nature Reserve in South Africa. The predominant vegetation is a deciduous wooded savanna growing on infertile sandy soils; patches of Acacia savanna occur on the more fertile sites of former human settlements. Woody species fall into two basic categories of acceptability to the animals: (1) species favored year-round; and (2) species generally rejected, except during certain periods. Among the latter, some species increase in acceptability during the dry season; others are favored temporarily while new leaves predominate. Some species remain low in acceptability year-round. We propose a palatability classification of woody species, considering features of leaf retention (deciduous vs. evergreen) and spinescence, as well as seasonal variations in acceptability. Estimated annual foliage losses to browsing ungulates were 1-3% for unpalatable deciduous species, compared with 10-30% for most palatable unarmed deciduous and evergreen species. Unpalatable deciduous species dominate the woody foliage biomass on the infertile soils, while palatable but spinescent species make up most of the leaf biomass of trees and shrubs on the more fertile sites. These patterns seem widely typical of African savanna. Plants known to have chemical defenses against vertebrate herbivory are prominent on nutrient-deficient soils, while those with structural defenses are prominent on fertile soils.
A general model of simple grazing systems can be based on the general form of the functions relating both plant growth and herbivore consumption to vegetation biomass. Graphical considerations lead from this model to predictions on the existence and number of plant-herbivore equilibrium points, whether they are stable or unstable and the relations between herbivore density, plant biomass and productivity at these points. Some predictions can be transferred directly from the existing theory of predator-prey graphs to plant-herbivore systems, while others need to be modified. The stability of grazing systems depends on animal density and plant biomass either continuously, or discontinuously, depending on the properties of the vegetation and the herbivore. Evidence from real pasture and range systems suggests that at least some of them are discontinuously stable, with significant implications for their management.
Examined potential effects of herbivory on temperate forest ecosystems using a forest succession simulation model with the capacity for treating various hierarchical levels for long time periods. Two species of trees were chosen for evaluating herbivory effects: white oak Quercus alba, a relatively slow-growing shade-tolerant species, and tulip poplar Liriodendron tulipifera, a fast-growing shade-intolerant species. The study focused on four hierarchical levels, covering individual, phenotypic, interspecific, and community interactions. White oak tended to shown a greater sensitivity to interspecific interactions; tulip poplar showed a higher sensitivity to intraspecific interactions. Changes in growth rates associated with the switching strategies (an ultimate factor) were more important in answering variations in productivity than was impact imparted by annual changes in incidence of herbivore presence (a proximate factor) for these two species. While results suggest that herbivore stress can explain a large degree of the variation in long-term community dynamics, ecological interactions between herbivore and climate effects must be more closely linked in such long-term studies. -from Authors
Invasive browsing ungulates can have strong impacts on the structure and composition of forest ecosystems, particularly where ungulates are not native ecosystem components as in New Zealand. Ungulate impacts on plant communities have been considered mostly from an above‐ground perspective. However, understanding below‐ground effects of these invasive herbivores is critical as they may drive feedbacks to above‐ground ecosystem components.
We measured growth responses of seedlings of five common tree species in a greenhouse experiment in soils collected from 26 plots fenced to exclude invasive ungulates for at least 17 years and from paired, unfenced control plots. We then further investigated soil‐mediated effects of ungulates on one tree species, M elicytus ramiflorus , by partitioning these effects into soil abiotic and biotic components, as well as measuring arbuscular mycorrhizal fungal ( AMF ) root infection.
Biomass of seedlings of all five species was greater in soils from within exclosures, although this was only significant for two species. These soil‐mediated effects were partially driven by changes in physical and chemical soil properties; soil bulk densities were lower inside exclosures than in controls.
Effects of invasive ungulates on seedling biomass of M . ramiflorus were positively related to effects on per cent AMF root infection. The biomass of M . ramiflorus seedlings was positively related to the AMF infection of its roots, which in turn was related to greater organic matter content and lower bulk density of soils from within exclosures. Results for M . ramiflorus indicated that soil‐mediated effects of ungulates on seedling biomass were of abiotic origin, but were mediated by the biotic soil component, that is, through effects on AMF .
Synthesis . Invasive herbivores may potentially impact on plant performance and community structure not only directly but also indirectly through influencing soil abiotic and biotic properties. Our results show that shifts in plant–soil interactions and feedbacks represent important but understudied pathways by which invasive ungulates can have wide‐ranging impacts on forest ecosystems. Future studies should consider the importance of soil‐mediated effects of invasive ungulates relative to direct effects of herbivory.
The last several decades have seen dramatic increases in ungulate populations worldwide, and white-tailed deer in the eastern United States currently exist at unprecedented densities in many areas. Numerous studies have demonstrated the effects of high densities of white-tailed deer on forest communities. However, few studies have simultaneously examined the effects of deer on multiple components of forest communities across trophic levels. Here, we simultaneously examine effects of excluding white-tailed deer on responses of woody and herbaceous vegetation, terrestrial and subterranean animals, mycorrhizal fungi, and soil characteristics. This study was conducted in a forest preserve with high deer densities in the central hardwoods region of the Midwestern US, using a series of replicated deer exclosures (15x15 m) and adjacent unfenced controls that ranged in age from two to seven years. Despite significant tree recruitment inside exclosures, we recorded no native tree seedling recruitment in control plots. In addition, the growth rate of existing tree seedlings was significantly greater in exclosures than in controls, and the growth rate of invasive shrubs was approximately 30 times higher inside exclosures. Exclosures also had increased height, species diversity, and abundance of spring plants, and increased vegetation density in summer. We also found differences in terrestrial animals with higher densities of white-footed mice (Peromyscus leucopus) and dog ticks (Dennacentor variabilis) inside deer exclosures. However, there were no differences in salamanders or earthworms. Soil inside exclosures was significantly less compacted than in control plots despite the short period of deer exclusion, but there were no significant differences in soil nutrients or arbuscular mycorrhizal fungi. These results indicate that there are strong effects of high deer densities on all classes of understory vegetation and indirect effects on animals and soils. However, most belowground effects were nonsignificant, suggesting that responses of belowground communities to deer exclusion are weaker or slower to develop than aboveground effects.
The term “legacy effect” has been used in ecology since the early 1990s by authors studying plant succession, invasive-plant impacts, herbivory impacts, ecosystem engineering, and human land-use impacts. Although there is some variability in usage, the term is normally used to describe impacts of a species on abiotic or biotic features of ecosystems that persist for a long time after the species has been extirpated or ceased activity and which have an effect on other species. For example, human agricultural activities may have a legacy effect on soil structure and vegetative communities that lasts for centuries and which alters current communities. The concept may be related to the idea of ecological inheritance in evolutionary biology but would refer only to a subset of the features of this concept. In particular, legacy effects could refer to those kinds of ecological inheritance where a physical or biological change in ecosystem state is caused by one species, where this change persists after the extirpation of the causal species and alters selection pressure of another species much later in time.
Although the immediate impacts of elevated deer (Odocoileus spp.) browsing on forest regeneration have been well documented, few studies have examined the longer term consequences. A deer exclosure experiment was initiated in 1991 in an old-growth northern mixed mesic forest in northeastern Minnesota, and resampled in 2008 to examine changes in composition, structure and productivity. Decades of overbrowsing by white-tailed deer have led to almost complete recruitment failure in size classes >2.5 cm dbh for preferred deer browse species Thuja occidentalis and Pinus strobus in unprotected plots. Other palatable browse species have been severely limited in understory development (Populus tremuloides, Betula papyrifera, Fraxinus nigra). Within exclosures, P. strobus gained in all size classes <20 cm dbh, while F. nigra, B. papyrifera, T. occidentalis all showed significant gains. Non-preferred Picea glauca increased outside exclosures, but has also gained within exclosures. The increase in P. glauca across treatments indicates a long-term legacy effect of preferential browsing. Browsing induced suppression of subcanopy density of preferred species and failure of canopy tree replacement may lead to a more open woodland structure dominated by P. glauca. Browsing pressure may negatively impact productivity, as whole tree biomass in exclosures increased at a rate twice that of unprotected plots. The low biomass levels recorded in 2008 (unprotected: 98.0 mg/ha−1, exclosure: 104 mg/ha−1) are approximately 1/2 of values typically recorded in later successional forests in this region indicating lower productivity may be another longer-term legacy of elevated deer population. Continued high browsing pressure is one of many factors contributing to the restructuring of northern Great Lakes forests away from historical variability conditions towards a novel and more homogeneous forested landscape. These simplified forests may be less resilient to the suite of emerging stressors such as climate change and less able to provide ecosystem services such as carbon storage, biological diversity and forest products. Sustained restoration efforts, along with reductions in deer density will be needed to restore species and structural diversity.
Large mammalian herbivores significantly alter the functioning of a range of ecosystems worldwide. One family of herbivores in particular, deer (Cervidae), have become widespread throughout temperate regions over the last century. Culling is frequently used to reduce the ecological impacts of deer, but recent studies demonstrate that the recovery of herbivore-disturbed vegetation is often a protracted process on the order of decades. Here, we ask why increases in deer-preferred plants occur slowly following culling and what generalities can inform management of the responses of vegetation to deer culling? Vegetation changes following deer population reduction may be slow because of the: (i) slow growth rate of plants relative to the amount of biomass consumed by herbivores; (ii) depletion of seed sources of deer-preferred plants; (iii) formation of alternate vegetation types under high deer browsing; (iv) preferential browsing of forage even under low deer densities; (v) variation in browse damage of deer-preferred species with plant community composition (e.g. associational resistance or susceptibility); (vi) suppression of trophic cascades; and (vii) changes in abiotic conditions associated with other ecological processes. Managers can accelerate recovery through different active management strategies that directly address the reasons behind slow recovery. However, while the ecology behind the impacts of deer, and to a certain extent their mitigation, are largely understood, a significant obstacle to achieving recovery often lies in implementing science-based management recommendations, e.g. culling, and their societal acceptance. Bridging the science-policy-practice gap requires that applied ecologists share common language and values with policymakers and the public. Recovery may hinge as much on bridging this communication gap as on improving our ecological understanding of deer impacts and ecosystem recovery.
Consumption of Pinus sylvestris twig biomass by Alces alces was highest on pines from the most productive habitat and lowest on those from the moderately productive habitat with an overstory of mature pines. Consumption was intermediate on pines from the least productive habitat and from the moderately productive habitat without an overstory. Pines from the highly productive habitat had greater N, P and K content and more readily degradable dry matter than did pines from the less productive habitats. Even though they provided a lower quality food for moose, pines in the less productive habitats generally suffered most from moose browsing. The slender twigs and the slow growth rate of these pines allow the moose to browse the main part of the needle-bearing twigs, following simulated winter browsing in living pines, pine mortality was greater in the less productive habitats. These findings largely explain why pines growing in habitats of low and medium productivity suffer more than pines in highly productive habitats, where the possibility for growth is better. Severe damage to stands growing on poor soils most probably arises where moose density is high and where moose have already harvested most of the available twigs from pines growing in the more productive habitats. The moose are then forced to utilize the suboptimal food resources available in less productive habitats. -from Authors
Predicting the diet of large mammalian herbivores is difficult due to the many factors at multiple scales that influence diet selection. One approach to understanding diet selection is to relate diet choices to the foliar and structural traits of forage species. Using data on diet selection by red deer (Cervus elaphus scoticus) in Fiordland National Park, New Zealand, we determined the extent to which interspecific differences in the palatability of 46 plant species could be explained by 11 chemical and structural characteristics of plant foliage. Woody species were more palatable to deer than non-woody species (mostly ferns) and palatability declined with increasing concentrations of foliar cellulose. High concentrations of cellulose in fern leaves may increase toughness and resistance to tearing by herbivores, thus contributing to the low palatability of these plants. When two species that were consumed only in the winter were removed from the analysis, palatability was also negatively correlated with phenolic content. High phenolic concentrations may deter herbivory in summer, when forage with low phenolic concentrations is available (e.g. herbaceous dicots), whereas deer may have little choice but to consume species rich in phenolics during winter, when less forage is available. The relatively high concentrations of phenolics in our dataset compared to other floras, despite the absence of large native mammalian herbivores in New Zealand, suggest that these compounds are multifunctional.