Herbivores form an important link in the transfer of energy within a food web and are strongly influenced by bottom‐up trophic cascades. Current hypotheses suggest that herbivore consumption and impact on plants should scale positively with plant resource availability. However, depending on the effect of resources on plant quantity and quality, herbivore impact may vary with different types of resources. We test four alternative hypotheses for the relationship between plant biomass, herbivore impact on plant biomass and plant resource gradients, each based on how resources might affect plant abundance and quality to herbivores. We measured plant biomass for four non‐consecutive years in a long‐term grazing exclosure experiment in the Serengeti National Park that includes seven sites that vary substantially in rainfall and soil and plant nitrogen (N) and phosphorus (P). Our data supported the hypothesis that herbivore impact is controlled by plant quality, in this case driven by plant P, as herbivore effects on biomass decreased with higher rainfall but increased with greater plant P, but not N content. To our knowledge, this is the first experimental study to indicate that wild mammalian herbivory is associated with P availability rather than N.
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Abstract Synthesis . Our results suggest that P, in addition to water and N, may play a more important role in driving trophic interactions in terrestrial systems than previously realized. Given the uncertainties in rainfall due to climate change and increasing anthropogenic manipulations of global N and P cycles, our findings emphasize the need to consider multiple resources for understanding how trophic interactions might be influenced by environmental variables. -
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Abstract Herbivory is a major energy transfer within ecosystems; an open question is under what circumstances it can stimulate aboveground seasonal primary production. Despite multiple field demonstrations, past theory considered herbivory as a continuous process and found stimulation of seasonal production to be unlikely. Here, we report a new theoretical model that explores the consequences of discrete herbivory events, or episodes, separated in time. We discovered that negative density (biomass) dependence of plant growth, such as might be expected from resource limitation of plant growth, favors stimulation of seasonal production by infrequent herbivory events under a wide range of herbivory intensities and maximum plant relative growth rates. Results converge to those of previous models under repeated, short‐interval herbivory, which generally reduces seasonal production. Model parameters were estimated with new and previous data from the Serengeti ecosystem. Patterns of observed frequent and large magnitude stimulated production in these data agreed generally with those predicted by the episodic herbivory model. The model thus may provide a new framework for evaluating the sustainability and impact of herbivory.
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Abstract Ubiquitous declines in biochemical reaction rates above optimal temperatures (
T opt ) are normally attributed to enzyme state changes, but such mechanisms appear inadequate to explain pervasiveT op twell below enzyme deactivation temperatures (T den ). Here, a meta-analysis of 92 experimental studies shows that product formation responds twice as strongly to increased temperature than diffusion or transport. This response difference has multiple consequences for biochemical reactions, such as potential shifts in the factors limiting reactions as temperature increases and reaction-diffusion dynamics that predict potential product inhibition and limitation of the reaction by entropy production at temperatures belowT den . Maximizing entropy production by the reaction predictsT opt that depend on enzyme concentration and efficiency as well as reaction favorability, which are patterns not predicted by mechanisms of enzyme state change. However, these predictions are strongly supported by patterns in a meta-analysis of 121 enzyme kinetic studies. Consequently, reaction-diffusion thermodynamics and entropy production may constrain organism performance at higher temperatures, yielding temperature optima of life that may depend on reaction characteristics and environmental features rather than just enzyme state changes. -
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Abstract Questions Grasslands support ecosystem services, promote diversity, and assist in carbon sequestration. However, grasslands worldwide are diminishing in area, and understanding the drivers shaping the remaining grasslands is critical for their maintenance. The North American tallgrass prairie covers approximately 13% of its historical range and is shaped by fire and herbivory. Fire frequency negatively correlates with plant species richness, while bison (
Bos bison ) — the historical grazers — offset this effect. However, bison populations have declined, and large browsers are increasing in density. Few studies though have examined the role of large browsers — particularly white‐tailed deer (Odocoileus virginianus ) — and their interaction with fire frequency in tallgrass prairies. Here, we addressed two questions: (a) What are the impacts of deer on plant diversity, species identities, and relative abundances; and (b) is there an interactive effect between the pressures exerted by deer and the well‐documented effects of fire in driving plant community responses?Location This study took place at the Konza Prairie Biological Station in northeastern Kansas, USA.
Methods Using a 22‐year deer exclosure experiment, we examined differences in plant species richness, evenness, and plant community composition between plots that were either accessible or inaccessible to deer, in areas burned annually or once every four years.
Results We did not find significant effects of deer or interactive effects between deer and burning frequency on any metric of the plant community measured, including plant species richness, evenness, and plant community composition.
Conclusions Contrary to the impact that deer have in other ecosystems (e.g. forests), our results indicate that deer do not affect the plant community of herbaceous‐dominated tallgrass prairies. These results indicate that while the loss of bison‐grazers has shifted tallgrass prairie plant communities to C4grass‐dominated systems, the shift to browsing‐dominated herbivore pressure from deer has a minimal effect on the plant community.
