Summary
1. Plant tolerance to herbivory has been accepted as a mechanism of defense that can be expressed jointly with resistance. Recent advances have partially validated previous theoretical predictions and improved our understanding of tolerance. Here, I highlight the necessity of integrating tolerance into ecological and co-evolutionary theory to better understand the biology of this defensive mechanism.

2. Existing work has been biased toward understanding the adaptive nature and constraints acting on tolerance rather than its ecological implications. Increasing consensus that tolerance may alter the classical antagonistic dynamic between plants and herbivores suggests possible avenues to explore its population and community consequences.

3. In this review, I summarize previous advances in the ecology and evolution of tolerance to herbivory. I also discuss recent evidence that improves our understanding of unresolved issues such as the specificity of tolerance in response to herbivory, its role as an agent of selection on herbivores, and its ecological and evolutionary consequences which include a role of tolerance during biological invasions.

4. Although there is good support for an adaptive role of tolerance in response to herbivory damage, tolerance can itself impose selection on herbivores. Furthermore, tolerance can influence herbivore population dynamics and coexistence, as recent studies show that tolerance responses increase diversity of the associated herbivore communities on individual host plants.

5. Future studies that take into account the role of herbivores on the expression of tolerance will provide novel insight that will help disentangle the mechanisms of tolerance and its ecological and evolutionary implications.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2010.01805.x

Ecological and evolutionary implications of plant tolerance to herbivory

Summary
Large mammalian herbivores are major drivers of the structure and function of terrestrial ecosystems world-wide, and changes in their abundance have resulted in many populations being actively managed. Many empirical studies have identified that abundant mammalian herbivores can have negative impacts on biodiversity, but there has been no specific review of the impacts of native mammalian herbivores.
We assessed the peer-reviewed literature on the effects of large native herbivores on other animals. We aimed to quantitatively synthesize current knowledge, identify gaps and limitations in the literature, and highlight priorities for future research.
Most empirical studies of herbivory effects compared only two levels of herbivory (76%), and meta-analysis showed that very high densities of herbivores, when compared with very low densities, had mostly negative effects on other animal species. These negative effects were usually attributed to changes in the quantity and/or structure of vegetation. Only 24% of papers studied animal responses across a gradient of herbivore densities, and nonlinear responses to herbivory, as well as responses to low and moderate herbivore densities, remain poorly understood.
The literature also was dominated by short-term studies (76% sampled animal responses for 2 years or less), and there was a high incidence of confounding factors among studies (38% of studies). In addition, many studies used only coarse metrics to assess effects (e.g. only 33% of studies assessed species composition) and few included community-level synthesis (only 31% of studies reported results from more than one animal class).
Synthesis and applications. Critical questions remain for both basic ecology and the management of large native herbivores for biodiversity. Key knowledge gaps include (i) nonlinear responses to herbivore pressure, (ii) how responses differ between different herbivores, (iii) the spatial and (iv) the temporal variation of responses, (v) how the effects of herbivores interact with other land management activities and (vi) the mechanisms driving cascading effects through ecosystems. We identify ways to address these gaps and emphasize the need for studies which employ contrasts over a gradient of ecologically relevant herbivore densities and biologically meaningful time frames.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2664.12268

Effects of large native herbivores on other animals

According to the insurance hypothesis, more diverse plant communities are more likely to be resistant to drought. Whilst many experiments have been carried out to determine the effects of plant diversity on plant community insurance, the results are still contradictory. Here, we conducted a drought experiment where we tested whether the presence of subordinate species increases plant community insurance. In Swiss Jura grassland, we combined a removal experiment of subordinate species with a summer drought event using rainout shelters. Plant community composition was determined after the drought and based on biomass measurements; we estimated resistance, recovery and resilience of the plant community for each combination of treatments. Moreover, to assess drought impacts on water-use efficiency (WUE), we analysed carbon isotope ratios (13C values) in plant leaves of two dominants and two subordinates collected at the end of the drought period. We showed that subordinate species are more resistant to drought and increased community resistance by enhancing their above-ground biomass production during the imposed drought. These patterns were associated with decreased competitiveness of dominant species whose biomass decreased during drought. Significant increase in 13C values in plant tissue under drought indicated a better WUE for the measured species. Interestingly, the WUE was significantly higher in plots where subordinates were removed. Recovery and resilience were not affected by the summer drought, but the absence of subordinates reduced overall above-ground biomass in both watered and drought plots. Synthesis. We demonstrated that, independent of plant diversity, the presence of drought-resistant subordinate species increases plant community insurance against drought and, hence, is important for the functioning of grassland ecosystems.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2745.12064

Subordinate plant species enhance community resistance against drought in semi-natural grasslands

Summary 1. Herbivores influence the structure and composition of terrestrial plant communities. However, responses of plant communities to herbivory are variable and depend on environmental conditions, herbivore identity and herbivore abundance. As anthropogenic impacts continue to drive large declines in wild herbivores, understanding the context dependence of herbivore impacts on plant communities becomes increasingly important. 2. Exclosure experiments are frequently used to assess how ecosystems reorganize in the face of large wild herbivore defaunation. Yet in many landscapes, declines in large wildlife are often accompanied by other anthropogenic activities, especially land conversion to livestock production. In such cases, exclosure experiments may not reflect typical outcomes of human-driven extirpations of wild herbivores. 3. Here, we examine how plant community responses to changes in the identity and abundance of large herbivores interact with abiotic factors (rainfall and soil properties). We also explore how effects of wild herbivores on plant communities differ between large-scale herbivore exclosures and landscape sites where anthropogenic activity has caused wildlife declines, often accompanied by livestock increases. 4. Abiotic context modulated the responses of plant communities to herbivore declines with stronger effect sizes in lower-productivity environments. Also, shifts in plant community structure, composition and species richness following wildlife declines differed considerably between exclosure experiments and landscape sites in which wild herbivores had declined and were often replaced by livestock. Plant communities in low wildlife landscape sites were distinct in both composition and physical structure from both exclosure and control sites in experiments. The power of environmental (soil and rainfall) gradients in influencing plant response to herbivores was also greatly dampened or absent in the landscape sites. One likely explanation for these observed differences is the compensatory effect of livestock associated with the depression or extirpation of wildlife. 5. Synthesis. Our results emphasize the importance of abiotic environmental heterogeneity in modulating the effects of mammalian herbivory on plant communities and the importance of such covariation in understanding effects of wild herbivore declines. They also suggest caution when extrapolating results from exclosure experiments to predict the consequences of defaunation as it proceeds in the Anthropocene.

/pdf/effects-of-mammalian-herbivore-declines-on-plant-communities-2d4dkhm0vh.pdf

Effects of mammalian herbivore declines on plant communities: observations and experiments in an African savanna

Summary

1. Herbivores consume a large portion of the biomass produced by plants in virtually all ecosystems, which has dramatic effects on both the ecology and evolution of plants. In response to this threat, plants have evolved a diverse arsenal of direct and indirect defences to reduce herbivory and the impacts of damage on plant performance.



2. This special feature is a broad synthesis of the evolution and ecology of plant defences. The first objective of this special feature is to provide a review of what we have learned about plant defences against herbivores. The second objective is to stimulate debate and sow fresh ideas for the future research.



3. The 11 articles in this issue address three fundamental questions: (i) How do plants defend themselves against a diverse array of enemies? (ii) Why do plant species vary in defence? And (iii) What are the ecological and ecosystem-level consequences of plant defence? In addressing these questions the articles cover the interdisciplinary nature of plant–herbivore evolutionary ecology, from genes to global patterns.



4. The articles contained in the special feature question existing paradigms and provide new analyses of data. In some cases, influential hypotheses are firmly supported with new analyses (e.g. the Resource Availability Hypothesis), whereas in other instances conventional wisdom is called into question (e.g. the importance of secondary metabolites in the microevolution of resistance) and popular hypotheses are rejected (e.g. the Apparency Hypothesis, the Latitudinal Biotic Interaction Hypothesis).



5. This is an exciting time for research on the evolutionary ecology of plant defences. The articles in this special feature provide a guide to how we can move forward in resolving existing problems and tackling new questions.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2011.01838.x

Evolutionary ecology of plant defences against herbivores

http://www.vliz.be/imisdocs/publications/258447.pdf

Grazing as a management tool in dune grasslands: Evidence of soil and scale dependence of the effect of large herbivores on plant diversity

We tested whether differences in the herbivory tolerance of plant species is related to their abundance in grassland communities and how herbivory and nutrient availability affect competitive balances among plant species through changes in their tolerance. The experimental approach involved a simulated grazing treatment (clipping) of two competitive grass species (Arrhenatherum elatius and Holcus lanatus) and two subordinate forb species (Prunella vulgaris and Lotus corniculatus) along a gradient of nutrient availability and under conditions of competition. Total standing, aboveground, root, and regrowth biomass were evaluated at the end of the experiment as an estimate of the capacity to compensate for twice removing aboveground biomass at different nutrient levels (NPK). Although clipping had a more pronounced negative effect on dominant plant species (Arrhenatherum and Holcus) than on subordinate species, the negative effects on dominant species were offset by the application of fertilizer. The combined effect of fertilizer and competition had more negative effects on the performance of Lotus and Prunella than on the dominant species. In terms of competition, the regrowth ability of Arrhenatherum and Holcus increased with the application of fertilizer, while the opposite pattern was observed for Lotus and Prunella. The addition of fertilizer has a positive effect on both grass species in terms of growth in clipped pots and competition, while subordinate species did not respond to the addition of fertilizer to the clipped pots and were negatively affected by competition with both grass species. The results suggest (1) that species replacement towards subordinate species as a function of herbivory is partially dependent on the herbivory tolerance of that species, (2) competitive relations between competitive grass species and subordinate forb species change under different environmental conditions, and (3) although grazing disturbance significantly influences competitive relations in favor of less competitive species, increasing nutrient levels counteract the negative effect of grazing on dominant competitive plant species.

Differential herbivory tolerance of dominant and subordinate plant species along gradients of nutrient availability and competition

Background and aims Re-introduction of large grazers in the few remaining natural and semi-natural grasslands are thought to be an effective management tool to prevent dominance of late successional plant species and restoration of plant biodiversity. The main objective of this study was to test whether the introduction of large herbivores retard the succession by reducing the abundance of highly competitive tall species and whether it is accompanied with changes in plant community composition and spatial distribution of plant species. 
Methods In order to test this hypothesis, we studied the effect of grazing by large herbivores on vegetation at three hierarchical levels: individual plant species, emergent groups of functionally similar herbaceous plant species, and the main gradients of plant community composition. Study sites were thirteen spatially separated, dry coastal dune grasslands in western Belgium and north-western France. 
Key results Grazing had a predominantly negative effect on high competitive dominant species and led to changes in composition of emergent groups toward less competitive plant species. Additionally, these changes in plant community composition were accompanied with changes in spatial distribution patterns of individual plant species and community richness. 
Conclusions Our results suggest that the current grazing management applied in these nature reserves is able to prevent the expansion of dominant highly competitive species and establishment of functionally different plant species.

Pejman Tahmasebi Kohyani

Papers

Grazing as a management tool in dune grasslands: Evidence of soil and scale dependence of the effect of large herbivores on plant diversity

Differential herbivory tolerance of dominant and subordinate plant species along gradients of nutrient availability and competition

Grazing impact on plant spatial distribution and community composition