Synthesizing the effects of large, wild herbivore exclusion on ecosystem function

Abstract

Wild large herbivores are declining worldwide. Despite extensive use of exclosure experiments to investigate herbivore impacts, there is little consensus on the effects of wild large herbivores on ecosystem function. Of the ecosystem functions likely impacted, we reviewed the five most‐studied in exclosure experiments: ecosystem resilience/resistance to disturbance, nutrient cycling, carbon cycling, plant regeneration, and primary productivity. Experimental data on large wild herbivores' effects on ecosystem functions were predominately derived from temperate grasslands (50% grasslands, 75% temperate zones). Additionally, data were from experiments that may not be of adequate size (median size 400 m2 despite excluding all experiments below 25 m2) or duration (median duration 6 years) to capture ecosystem‐scale responses to these low‐density and wide‐ranging taxa. Wild herbivore removal frequently impacted ecosystem functions; for example, net carbon uptake increased by three times in some instances. However, the magnitude and direction of effects, even within a single function, were highly variable. A focus on carbon cycling highlighted challenges in interpreting effects on a single function. While the effect of large herbivore exclusion on carbon cycling was slightly positive when its components (e.g. pools vs. fluxes of carbon) were aggregated, effects on individual components were variable and sometimes opposed. Given modern declines in large wild herbivores, it is critical to understand their effects on ecosystem function. However, this synthesis highlights strong variability in direction, magnitude, and modifiers of these effects. Some variation is likely due to disparity in what components are used to describe a given function. For example, for the carbon cycle we identified eight distinctly meaningful components, which are not easily combined yet are potentially misrepresentative of the larger cycle when considered alone. However, much of the observed difference in responses likely reflects real ecological variability across complex systems. To move towards a general predictive framework we must identify where variation in effect is due to methodological differences and where due to ecosystem context. Two critical steps forward are (a) additional quantitative synthetic analyses of large herbivores' effects on individual functions, and (b) improved, increased systematic exclosure research focusing on effects of large herbivores' exclusion on functions. A free Plain Language Summary can be found within the Supporting Information of this article.

Grazing exclusion erodes the forbs functional group without altering offspring recruitment composition in a typical steppe

“…This finding is similar to that of Wu et al () and Cheng et al (), where fencing increased grassland coverage and productivity by excluding livestock herbivory. The low soil resources and the short evolutionary histories between herbivores and plants resulted in primary productivity increases (Forbes et al, ; Milchunas & Lauenroth, ). The changes in community‐level plant coverage, height, and aboveground biomass were mainly caused by grasses functional group.…”

“…The low soil resources and the short evolutionary histories between herbivores and plants resulted in primary productivity increases (Forbes et al, 2019;Milchunas & Lauenroth, 1993 (Galvanek & Leps, 2008;Miles & Knops, 2009).…”

Landscapes shaped from the top down: predicting cascading predator effects on spatial biogeochemistry

“…The context-dependency of herbivore effects on ecosystems -and, similarly, of cascading predator effects -has been emphasized in many studies (e.g. Anderson et al 2008;Bai et al 2012;Young et al 2015;Haswell et al 2017;Goheen et al 2018;Forbes et al 2019). Further progress will depend on making sense of this context-dependency based on predator and herbivore functional traits and on ecosystem characteristics.…”

Effects of elk and bison carcasses on soil microbial communities and ecosystem functions in Yellowstone, USA

“…By consuming this plant tissue, herbivores can alter plant species composition, plant physiological properties and the amount of plant litter returned to the soil. These changes directly and indirectly affect the activity and abundance of soil organisms and therefore soil carbon (C) and nutrient cycling (Bakker, Ritchie, Olff, Milchunas, & Knops, 2006;Bardgett & Wardle, 2003;del-Val & Crawley, 2005;Forbes et al, 2019;Sitters & Olde Venterink, 2015;Wardle et al, 2004). Trampling and burrowing activities of herbivores and the deposition of dung and urine can also stimulate the activity of roots, microbes and/or soil arthropods and related soil processes (Barth, Liebi, Hendrickson, Sedivec, & Halvorson, 2014;Forbes et al, 2019;Risch et al, 2015;Schrama et al, 2013;Sitters & Olde Venterink, 2015;Sitters et al, 2020).…”