Interactions retain the co‐phylogenetic matching that communities lost

Both species and their interactions are affected by changes that occur at evolutionary time‐scales, and these changes shape both ecological communities and their phylogenetic structure. That said, extant ecological community structure is contingent upon random chance, environmental filters and local effects. It is therefore unclear how much ecological signal local communities should retain. Here we show that, in a host–parasite system where species interactions vary substantially over a continental gradient, the ecological significance of individual interactions is maintained across different scales. Notably, this occurs despite the fact that observed community variation at the local scale frequently tends to weaken or remove community‐wide phylogenetic signal. When considered in terms of the interplay between community ecology and coevolutionary theory, our results demonstrate that individual interactions are capable and indeed likely to show a consistent signature of past evolutionary history even when woven into communities that do not.     

Evolution of nutrient uptake reveals a trade-off in the ecological stoichiometry of plant-herbivore interactions

Nutrient limitation determines the primary production and species composition of many ecosystems. Here we apply an adaptive dynamics approach to investigate evolution of the ecological stoichiometry of primary producers and its implications for plant-herbivore interactions. The model predicts a trade-off between the competitive ability and grazing susceptibility of primary producers, driven by changes in their nutrient uptake rates. High nutrient uptake rates enhance the competitiveness of primary producers but also increase their nutritional quality for herbivores. This trade-off enables coexistence of nutrient exploiters and grazing avoiders. If herbivores are not selective, evolution favors runaway selection toward high nutrient uptake rates of the primary producers. However, if herbivores select nutritious food, the model predicts an evolutionarily stable strategy with lower nutrient uptake rates. When the model is parameterized for phytoplankton and zooplankton, the evolutionary dynamics result in plant-herbivore oscillations at ecological timescales, especially in environments with high nutrient availability and low selectivity of the herbivores. High herbivore selectivity stabilizes the community dynamics. These model predictions show that evolution permits nonequilibrium dynamics in plant-herbivore communities and shed new light on the evolutionary forces that shape the ecological stoichiometry of primary producers.     

Spatial variation in vegetation damage relative to primary productivity,small rodent abundance and predation

The relative importance of top‐down and bottom‐up mechanisms in shaping community structure is still a highly controversial topic in ecology. Predatory top‐down control of herbivores is thought to relax herbivore impact on the vegetation through trophic cascades. However, trophic cascades may be weak in terrestrial systems as the complexity of food webs makes responses harder to predict. Alternatively, top‐down control prevails, but the top‐level (predator or herbivore) changes according to productivity levels. Here we show how spatial variation in the occurrence of herbivores (lemmings and voles) and their predators (mustelids and foxes) relates with grazing damage in landscapes with different net primary productivity, generating two and three trophic level communities, during the 2007 rodent peak in northern Norway. Lemmings were most abundant on the unproductive high‐altitude tundra, where few predators were present and the impact of herbivores on vegetation was strong. Voles were most common on a productive, south facing slope, where numerous predators were present, and the impacts of herbivores on vegetation were weak. The impact of herbivores on the vegetation was strong only when predators were not present, and this cannot be explained by between‐habitat differences in the abundance of plant functional groups. We thus conclude that predators influence the plant community via a trophic cascade in a spatial pattern that support the exploitation ecosystems hypothesis. The responses to grazing also differed between plant functional groups, with implications for short and long‐term consequences for plant communities.     

Refuge effects of a cactus in grazed short‐grass steppe

Abstract. Question: Which factors influence the effectiveness of biotic refuges for harbouring grazing‐sensitive species in pastures with a long history of grazing by large herbivores? Previous research showed that spiny clumps of the cactus Opuntia polyacantha provided refuges from cattle grazing for plants and for inflorescence production on short‐grass steppe. In this paper, seven factors that may have a potential positive influence on the refuge effects of cactus at a landscape scale were assessed. Location: Short‐grass steppe of the Great Plains of North America. Methods: The study was conducted in eight long‐term grazed pastures and their respective ungrazed controls that were established 60 years ago. Results: Heavy grazing intensities were necessary for some positive effects of cactus to manifest, and some refuge effects changed to negative effects under lower grazing pressure. Refuge effects increased with plant community productivity due to greater abundances of grazing‐sensitive species, and greater grazing intensities in the more productive areas. Cover of cactus cladodes (spine‐covered pads) inside clumps appeared to be the main limiting factor for refuge effects, probably by limiting available space for grazing‐sensitive species in the clumps. Other factors such as size and density of cactus clumps, and the presence of large refuges in the proximity of clumps had minor influence on the effectiveness of cactus refuges. Conclusions: The effects of biotic refuges largely varied with ecological conditions and structural characteristics of the refuge. Refuge effects were mainly influenced by grazing intensity, plant community productivity, and structural characteristics of the biotic refuges. A conceptual model of factors influencing refuge effects at a local landscape scale in plant communities grazed by large herbivores is presented.     

Regional productivity mediates the effects of grazing disturbance on plant cover and patch‐size distribution in arid and semi‐arid communities

Patch‐size distribution and plant cover are strongly associated to arid ecosystem functioning and may be a warning signal for the onset of desertification under changes in disturbance regimes. However, the interaction between regional productivity level and human‐induced disturbance regime as drivers for vegetation structure and dynamics remain poorly studied. We studied grazing disturbance effects on plant cover and patchiness in three plant communities located along a regional productivity gradient in Patagonia (Argentina): a semi‐desert (low‐productivity community), a shrub‐grass steppe (intermediate‐productivity community) and a grass steppe (high‐productivity community). We sampled paddocks with different sheep grazing pressure (continuous disturbance gradients) in all three communities. In each paddock, the presence or absence of perennial vegetation was recorded every 10 cm along a 50 m transect. Grazing effects on vegetation structure depended on the community and its association to the regional productivity. Grazing decreased total plant cover while increasing both the frequency of small patches and the inter‐patch distance in all communities. However, the size of these effects was the greatest in the high‐productivity community. Dominant species responses to grazing explained vegetation patch‐ and inter‐patch‐size distribution patterns. As productivity decreases, dominant species showed a higher degree of grazing resistance, probably because traits of species adapted to high aridity allow them to resist herbivore disturbance. In conclusion, our findings suggest that regional productivity mediates grazing disturbance impacts on vegetation mosaic. The changes within the same range of grazing pressure have higher effects on communities found in environments with higher productivity, markedly promoting their desertification. Understanding the complex interactions between environmental aridity and human‐induced disturbances is a key aspect for maintaining patchiness structure and dynamics, which has important implications for drylands management.     

Effects of neutrality and productivity on mammal richness and evolutionary history in Australia

Explaining how heterogeneous spatial patterns of species diversity emerge is one of the most fascinating questions of biogeography. One of the great challenges is revealing the mechanistic effect of environmental variables on diversity. Correlative analyses indicate that productivity is associated with taxonomic, phylogenetic, and functional diversity of communities. Surprisingly, no unifying body of theory have been developed to understand the mechanism by which spatial variation of productivity affects the fundamental processes of biodiversity. Based on widely discussed verbal models in ecology about the effect of productivity on species diversity, we developed a spatially explicit neutral model that incorporates the effect of primary productivity on community size and confronted our model's predictions with observed patterns of species richness and evolutionary history of Australian terrestrial mammals. The imposed restrictions on community size create larger populations in areas of high productivity, which increases community turnover and local speciation, and reduces extinction. The effect of productivity on community size modeled in our study causes higher accumulation of species diversity in productive regions even in the absence of niche‐based processes. However, such a simple model is not capable of reproducing spatial patterns of mammal evolutionary history in Australia, implying that more complex evolutionary mechanisms are involved. Our study demonstrates that the overall patterns of species richness can be directly explained by changes in community sizes along productivity gradients, supporting a major role of processes associated with energetic constraints in shaping diversity patterns.     

Plant allometry, stoichiometry and the temperature-dependence of primary productivity

Aim While physical constraints influence terrestrial primary productivity, the extent to which geographical variation in productivity is influenced by physiological adaptations and changes in vegetation structure is unclear. Further, quantifying the effect of variability in species traits on ecosystems remains a critical research challenge. Here, we take a macroecological approach and ask if variation in the stoichiometric traits (C: N: P ratios) of plants and primary productivity across global‐scale temperature gradients is consistent with a scaling model that integrates recent insights from the theories of metabolic scaling and ecological stoichiometry. Location This study is global in scope, encompassing a wide variety of terrestrial plant communities. Methods We first develop a scaling model that incorporates potentially adaptive variation in leaf and whole‐plant nutrient content, kinetic aspects of photosynthesis and plant respiration, and the allometry of biomass partitioning and allocation. We then examine extensive data sets concerning the stoichiometry and productivity of diverse plant communities in light of the model. Results Across diverse ecosystems, both foliar stoichiometry (N : P) and ‘nitrogen productivity’ (which depends on both community size structure and plant nutrient content) vary systematically across global scale temperature gradients. Primary productivity shows no relationship to temperature. Main conclusions The model predicts that the observed patterns of variation in plant stoichiometry and nutrient productivity may offset the temperature dependence of primary production expected from the kinetics of photosynthesis alone. Our approach provides a quantitative framework for treating potentially adaptive functional variation across communities as a continuum and may thus inform studies of global change. More generally, our approach represents one of the first explicit combinations of ecological stoichiometry and metabolic scaling theories in the analysis of macroecological patterns.     

Disproportionate effects of non‐colonial small herbivores on structure and diversity of grassland dominated by large herbivores

The response of semiarid grasslands to small, non‐colonial herbivores has received little attention, focusing primarily on the effects of granivore assemblages on annual plant communities. We studied the long‐term effects of both small and large herbivores on vegetation structure and species diversity of shortgrass steppe, a perennial semiarid grassland considered marginal habitat for small mammalian herbivores. We hypothesized that 1) large generalist herbivores would affect more abundant species and proportions of litter‐bare ground‐vegetation cover through non‐selective herbivory, 2) small herbivores would affect less common species through selective but limited consumption, and 3) herbivore effects on plant richness would increase with increasing aboveground net primary production (ANPP). Plant community composition was assessed over a 14‐year period in pastures grazed at moderate intensities by cattle and in exclosures for large (cattle) and large‐plus‐small herbivores (additional exclusion of rabbits and rodents). Exclusion of large herbivores affected litter and bare ground and basal cover of abundant, common and uncommon species. Additional exclusion of small herbivores did not affect uncommon components of the plant community, but had indirect effects on abundant species, decreased the cover of the dominant grass Bouteloua gracilis and total vegetation, and increased litter and species diversity. There was no relationship between ANPP and the intensity of effects of either herbivore body size on richness. Exclusion of herbivores of both body sizes had complementary and additive effects which promoted changes in vegetation composition and physiognomy that were linked to increased abundance of tall and decreased abundance of short species. Our findings show that small mammalian herbivores had disproportionately large effects on plant communities relative to their small consumption of biomass. Even in small‐seeded perennial grasslands with a long history of intensive grazing by large herbivores, non‐colonial small mammalian herbivores should be recognized as an important driver of grassland structure and diversity.     

Intraspecific variation overrides origin effects in impacts of litter-derived secondary compounds on larval amphibians

Vertebrate herbivores can be key determinants of grassland plant species richness, although the magnitude of their effects can largely depend on ecosystem and herbivore characteristics. It has been demonstrated that the combined effect of primary productivity and body size is critical when assessing the impact of herbivores on plant richness of perennial-dominated grasslands; however, the interaction of site productivity and herbivore size as determinants of plant richness in annual-dominated pastures remains unknown. We experimentally partitioned primary productivity and herbivore body size (sheep and wild rabbits) to study the effect of herbivores on the plant species richness of a Mediterranean semiarid annual plant community in central Spain over six years. We also analyzed the effect of grazing and productivity on the evenness and species composition of the plant community, and green cover, litter, and plant height. We found that plant richness was higher where the large herbivore was present at high-productivity sites but barely changed at low productivity. The small herbivore did not affect species richness at either productivity site despite its large effects on species composition. We propose that adaptations to resource scarcity and herbivory prevented plant richness changes at low-productivity sites, whereas litter accumulation in the absence of herbivores decreased plant richness at high productivity. Our results are consistent with predictions arising from a long history of grazing and highlight the importance of both large and small herbivores to the maintenance of plant diversity of Mediterranean annual-dominated pastures.     

Herbivore impact on grassland plant diversity depends on habitat productivity and herbivore size

Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies.     

Defoliation of a grass is mediated by the positive effect of dung deposition,moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment

Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing‐tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily‐grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long‐lasting effects on plant productivity and ecosystem functioning after grazing has ceased.     

Trait convergence and trait divergence in herbaceous plant communities: Mechanisms and consequences

In landscapes subject to intensive agriculture, both soil fertility and vegetation disturbance are capable of impacting strongly, evenly and simultaneously on the herbaceous plant cover and each tends to impose uniformity on the traits of constituent species. In more natural and ancient grasslands greater spatial and temporal variation in both productivity and disturbance occurs and both factors have been implicated in the maintenance of species‐richness in herbaceous communities. However, empirical data suggest that disturbance is the more potent driver of trait differentiation and species co‐existence at a local scale. This may arise from the great diversity in opportunities for establishment, growth or reproduction that arise when the intensity of competition is reduced by damage to the vegetation. In contrast to the diversifying effects of local disturbances, productivity‐related plant traits (growth rate, leaf longevity, leaf chemistry, leaf toughness, decomposition rate) appear to be less variable on a local scale. This difference in the effects of the productivity and disturbance filters arises from the relative constancy of productivity within the community and the diversity in agency and in spatial and temporal scales exhibited by disturbance events. Also, evolutionary responses to disturbances involve minor adaptive shifts in phenological and regenerative traits and are more likely to occur as micro‐evolutionary steps than the shifts in linked traits in the core physiology associated with the capacity to exploit productive and unproductive habitats. During the assembly of a community and over its subsequent lifespan filters with diversifying and convergent effects may operate simultaneously on recruitment from the local species pool and impose contrasted effects on the similarity of the trait values exhibited by co‐existing species. Moreover, as a consequence of the frequent association of productivity with the convergence filter, an additional difference is predicted in terms of the effects of the two filters on ecosystem functioning. Convergence in traits selected by the productivity filter will exert effects on both the plant community and the ecosystem while divergent effects of the disturbance filter will be restricted to the plant community.     

GRAZING OPTIMIZATION,NUTRIENT CYCLING,AND SPATIAL HETEROGENEITY OF PLANT‐HERBIVORE INTERACTIONS: SHOULD A PALATABLE PLANT EVOLVE?

Abstract.— Can the evolution of plant defense lead to an optimal primary production? In a general theoretical model, Loreau (1995) and de Mazancourt et al. (1998, 1999) have shown that herbivory could increase primary production up to a moderate rate of grazing intensity through recycling of a limiting nutrient, provided several conditions are fulfilled. In the present paper, we assume: (1) grazing intensity is controlled by plants through their level of palatability; and (2) plant fitness is determined by its productivity. We explore the conditions under which such an optimal production may be reached through natural selection. We model two competing plant types that differ only in palatability and are distributed in a patchy landscape determined by the plant‐herbivore interaction. Patch size is determined by herbivore behavior: herbivores recycle nutrient homogeneously within patches, but recycle nutrient proportionally to consumption between patches. The model shows that a strategy of intermediate palatability can be adaptive in response to a small herbivore that lives on and recycles nutrient around one or a few individual plants. For moderately small herbivores, plant palatability may evolve towards one of two local convergent strategies, depending on the initial conditions. For medium‐ to large‐sized herbivores, the nonpalatable strategy is always selected. We discuss the functional and evolutionary implications of these results, and suggest that the traditional dichotomy describing antagonistic and mutualistic interactions may be misleading.     

Extending the stress‐gradient hypothesis – is competition among animals less common in harsh environments?

The role of positive interactions has become widely accepted as a mechanism shaping community dynamics. Most empirical evidence comes from plant communities and sessile marine organisms. However, evidence for the relative role of positive interactions in organizing terrestrial animal communities is more limited, and a general framework that includes positive interactions among animals is lacking. The ‘stress gradient hypothesis’ (SGH) developed by plant ecologists predicts that the balance between positive and negative interactions will vary along gradients of biotic and abiotic stress, with positive interactions being more important in stressful environments. Paralleling the SGH, stress gradients for terrestrial herbivores could be equated to inverse primary productivity gradients, so we would expect positive interactions to prevail in more stressful, low productivity environments. However, this contradicts the typical view of terrestrial animal ecology that low primary productivity systems will foster intense competition for resources among consumers. Here we use alpine herbivores as a case study to test one of the predictions of the SGH in animal communities, namely the prevalence of positive interactions in low productivity environments. We identify potential mechanisms of facilitation and review the limited number of examples of interspecific interactions among alpine herbivores to assess the role of positive and negative interactions in structuring their communities. A meta‐analysis showed no clear trend in the strength and direction of interactions among alpine herbivores. Although studies were biased towards reporting significant negative inter actions, we found no evidence of competition dominating in harsh environments. Thus, our results only partially support the SGH, but directly challenge the dominant view among animal ecologists. Clearly, a sound theoretical framework is needed to include competition, positive and neutral interactions as potential mechanisms determining the structure of animal communities under differing environmental conditions, and the stress‐gradient hypothesis can provide a solid starting point.     

How community adaptation affects biodiversity–ecosystem functioning relationships

Evidence is growing that evolutionary dynamics can impact biodiversity–ecosystem functioning (BEF) relationships. However the nature of such impacts remains poorly understood. Here we use a modelling approach to compare random communities, with no trait evolutionary fine‐tuning, and co‐adapted communities, where traits have co‐evolved, in terms of emerging biodiversity–productivity, biodiversity–stability and biodiversity–invasion relationships. Community adaptation impacted most BEF relationships, sometimes inverting the slope of the relationship compared to random communities. Biodiversity–productivity relationships were generally less positive among co‐adapted communities, with reduced contribution of sampling effects. The effect of community‐adaptation, though modest regarding invasion resistance, was striking regarding invasion tolerance: co‐adapted communities could remain very tolerant to invasions even at high diversity. BEF relationships are thus contingent on the history of ecosystems and their degree of community adaptation. Short‐term experiments and observations following recent changes may not be safely extrapolated into the future, once eco‐evolutionary feedbacks have taken place.     

Importance of grazing and soil acidity for plant community composition and trait characterisation in coastal dune grasslands

Question: Does grazing by large herbivores affect species composition or community‐wide variation in plant functional traits? Location: Dune grasslands at the Belgian coast. Methods: Plant cover and soil data were collected in 146 plots that were randomly selected at 26 grazed and ungrazed grassland sites. Plant community composition was assessed by Detrended Correspondence Analysis and mean values of plant trait categories were calculated across the plots. Results: Differentiation of plant composition and community‐wide plant trait characteristics was largely determined by grazing, soil acidity and their interaction. In ungrazed situations, a clear floristic distinction appears between acidic (non‐calcareous) and alkaline (calcareous) grasslands. In grazed situations, these floristic differences largely disappeared, indicating that grazing results in a decrease of natural variation in species composition. At higher soil pH, a larger difference in plant community composition and community‐wide plant traits was observed between grazed and ungrazed plots. In ungrazed situations, shifts in plant functional traits along the acidity gradient were observed. Conclusions: Grazing is responsible for shifts in plant community composition, and hence a decrease in plant diversity among grasslands at opposing acidity conditions in coastal dune grasslands. Therefore, care should be taken when introducing grazing as a system approach for nature conservation in dune grasslands as it may eliminate part of the natural variation in plant diversity along existing abiotic gradients.     

Soil nutrients indirectly influence intraspecific plant selection in white-tailed deer

Few attempts have been made to determine how soil productivity influences diet selection in herbivores, likely because environmental characteristics known to influence diet selection such as plant community structure and herbivore nutritional demands are often confounded with changes in soil productivity. We designed a soil-amendment experiment to isolate the effects of soil productivity on diet selection by manipulating soil productivity and quantifying intraspecific plant selection within a population of white-tailed deer (Odocoileus virginianus). We hypothesized soil productivity would indirectly influence deer plant selection by directly affecting plant tissue chemistry. Soil productivity indeed influenced diet selection indirectly because soil amendments only affected deer plant selection when palatable plants were present. Soil amendments increased plant phosphorus concentrations, and plant phosphorus concentrations explained 47% of the variation in diet selection. Thus, our data indicate plant nutritional quality mediates the indirect effects of soil productivity on herbivore diet selection. Previous research demonstrating differential influences of herbivory on plant communities across a soil productivity gradient may in part be explained by indirect effects of soil productivity on diet selection.     

Temporal scales of density‐dependent habitat selection in a large grazing herbivore

Habitat selection is a density‐dependent process, but little is known regarding how this relationship may vary across different temporal scales. Over long time scales, grazing shapes the structure, diversity and functioning of terrestrial ecosystems, and grazing‐induced changes in forage production over time are likely to affect the level of density dependence in habitat selection. In this fully‐replicated, landscape‐scale experiment, we investigated how density‐dependent habitat selection by a large grazing herbivore, sheep Ovis aries, develops over the time scale of a decade. We also address an often‐neglected challenge in habitat selection studies; namely, whether there is variation in use within a particular habitat or vegetation type and why. We found clear evidence of density dependence in habitat selection, with a wider use of habitats at high density. Despite a change in the standing biomass of high‐productivity vegetation at high herbivore density over the years, with herb biomass declining and graminoid biomass increasing, there was no clear evidence that these grazing‐induced changes in habitat over the years were strong enough to affect the level of density‐dependent habitat selection. The difference in selection for high versus low‐productivity habitats remained similar, despite annual fluctuations in the strength of selection. We found strong variation in selection within each vegetation type, even when vegetation types were mapped at a fine‐resolution scale. Our study shows that despite the interactive effects of herbivores and habitats, they are not always sufficiently strong enough to affect the level of density‐dependent habitat selection.     

Large predators limit herbivore densities in northern forest ecosystems

There is a lack of scientific consensus about how top-down and bottom-up forces interact to structure terrestrial ecosystems. This is especially true for systems with large carnivore and herbivore species where the effects of predation versus food limitation on herbivores are controversial. Uncertainty exists whether top-down forces driven by large carnivores are common, and if so, how their influences vary with predator guild composition and primary productivity. Based on data and information in 42 published studies from over a 50-year time span, we analyzed the composition of large predator guilds and prey densities across a productivity gradient in boreal and temperate forests of North America and Eurasia. We found that predation by large mammalian carnivores, especially sympatric gray wolves (Canis lupus) and bears (Ursus spp.), apparently limits densities of large mammalian herbivores. We found that cervid densities, measured in deer equivalents, averaged nearly six times greater in areas without wolves compared to areas with wolves. In areas with wolves, herbivore density increased only slightly with increasing productivity. These predator effects are consistent with the exploitation ecosystems hypothesis and appear to occur across a broad range of net primary productivities. Results are also consistent with theory on trophic cascades, suggesting widespread and top-down forcing by large carnivores on large herbivores in forest biomes across the northern hemisphere. These findings have important conservation implications involving not only the management of large carnivores but also that of large herbivores and plant communities.     

Introduced and native herbivores have different effects on plant composition in low productivity ecosystems

Questions

Understanding how livestock grazing alters plant composition in low productivity environments is critical to managing livestock sustainably alongside native and introduced wild herbivore populations. We asked four questions: (1) does recent livestock and rabbit grazing reduce some plant attributes more strongly than others; (2) does grazing by introduced herbivores (i.e. livestock and rabbits) affect plants more strongly than native herbivores (i.e. kangaroos); (3) do the effects of recent livestock grazing differ from the legacy effects of livestock grazing; and (4) does the probability of occurrence of exotic plants increase with increasing net primary productivity (NPP)?

Location

South‐eastern Australia.

Methods

We measured the recent grazing activity of co‐occurring livestock (cattle, sheep, goats), rabbits and kangaroos by counting faecal pellets; historic grazing activity by measuring livestock tracks; and derived NPP from satellite imagery. We used a hierarchical GLMM to simultaneously model the presence or absence (i.e. probability of occurrence) of all plant species as a function of their attributes (growth form, lifespan and origin) to assess their average response to recent grazing, historic grazing and productivity in a broad‐scale regional study.

Results

Recent and historic livestock grazing, rabbit grazing and increasing NPP reduced the average probability of occurrence of plant species, although responses varied among plant attributes. Both recent and historic livestock grazing strongly reduced the average probability of occurrence of native species, and forbs and geophytes, but differed in their relative effects on other growth forms. Recent livestock grazing, rabbit grazing and NPP had similar effects, strongly reducing native species and forbs, geophytes, shrubs and sub‐shrubs. The overall effects of recent kangaroo grazing were relatively weak, with no clear trends for any given plant attribute.

Conclusion

Our results highlight the complex nature of grazing by introduced herbivores compared with native herbivores on different plant attributes. Land managers need to be aware that domestic European livestock, rabbits and other free‐ranging introduced livestock such as goats have detrimental impacts on native plant communities. Our results also show that kangaroo grazing has a relatively benign effect on plant occurrence.