Top-down control of biotic stability mechanisms by large mammalian herbivores

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Carmen Ebel, Department of Biology, University of Oregon, Eugene, OR, Madelon Case, Institute of Ecology and Evolution, University of Oregon, Eugene, OR, Duncan M. Kimuyu, Natural Resource Management and Environmental Studies, Karatina University, Karatina, Kenya, Ryan E. Langendorf, Environmental Studies, University of Colorado, Boulder, Boulder, CO, Lauren M. Porensky, Rangeland Resources and Systems Research Unit, USDA-ARS, Fort Collins, CO, Kari E. Veblen, Dept. of Wildland Resources & Ecology Center, Utah State University, Logan, UT, Harry Wells, Leeds University, Leeds, United Kingdom, Truman Young, Mpala Research Centre, Nanyuki, Kenya and Lauren M. Hallett, Environmental Studies Program and Biology Department, University of Oregon, Eugene, OR

Background/Question/Methods
Understanding the mechanisms that maintain temporal stability is a central question in ecology. Aggregate ecosystem properties, such as primary productivity, can be stabilized by a variety of biotic mechanisms, including compensatory dynamics in which a decline in one species is compensated for by a rise in another, a stable dominant species, and a portfolio effect, in which high species richness causes the aggregate community to be less variable than its individual species. There is accumulating evidence in plant communities that the strength of different biotic mechanisms differs with bottom-up controls such as water and nitrogen availability. However, the role of top-down controls on stability mechanisms remain largely unexplored, especially in terrestrial systems. Here we use an herbivore exclusion experiment in Laikipia, Kenya to test whether six different combinations of domestic and wild large mammalian herbivores influence the temporal stability of herbaceous biomass (calculated as the inverse of the coefficient of variation) and whether the biotic mechanisms influencing stability depend on grazing intensity.
Results/Conclusions
We find that relative stability declines as grazing pressure increases, despite concurrent reductions in absolute variance over time. Compensatory dynamics appear to maintain stability of herbaceous biomass at low and medium grazing pressures, while dominance modestly increases stability especially at medium grazing pressure. Reduced stability under high grazing pressure is associated with an increase in synchrony and evenness of species abundances. This effect is likely attributed to the reduction in palatable dominant species creating space for sub-dominate species to fluctuate synchronously in response to climate variability. Our results demonstrate that top-down control by herbivores decreases the stability of above-ground biomass at higher herbivory levels and influences the relative strength of different stabilizing mechanisms.