Experimental evidence that livestock grazing intensity affects cyclic vole population regulation processes

Nacho Villar,Xavier Lambin,Robin Pakeman,Steve Redpath,Thomas Cornulier,Darren Evans

doi:10.1007/s10144-013-0398-x

Nacho Villar, Xavier Lambin + Show 4 more

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https://doi.org/10.1007/s10144-013-0398-x

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Abstract

AbstractGrazing by domestic ungulates may limit the densities of small herbivorous mammals that act as key prey in ecosystems. Whether this also influences density dependence and the regulation of small herbivore populations, hence their propensity to exhibit multi‐annual population cycles, is unknown. Here, we combine time series analysis with a large‐scale grazing experiment on upland grasslands to examine the effects of livestock grazing intensity on the population dynamics of field voles (Microtus agrestis). Using log‐linear modelling of replicated time series under different grazing treatments, we show that increased sheep densities weaken delayed density dependent regulation of vole population growth, hence reducing the cyclicity in vole population dynamics. While population regulation is commonly attributed to both top‐down and bottom up processes, our results suggest that regulation of cyclic vole populations can be disrupted by the influence of another grazer in the same trophic level. These results support the view that ongoing changes in domestic grazing intensity, by affecting small mammal dynamics, can potentially have cascading impacts on higher trophic levels, and strongly influence the dynamics of upland grassland systems.

Highlights

Grassland ecosystems in many parts of the northern hemisphere are dominated by pulses in the abundance of key herbivores, such as vole population cycles, with large impacts across trophic levels (Ims and Fuglei 2005)
We combine time series analysis with a large-scale grazing experiment on upland grasslands to examine the effects of livestock grazing intensity on the population dynamics of field voles
While population regulation is commonly attributed to both topdown and bottom up processes, our results suggest that regulation of cyclic vole populations can be disrupted by the influence of another grazer in the same trophic level

Summary

Introduction

Grassland ecosystems in many parts of the northern hemisphere are dominated by pulses in the abundance of key herbivores, such as vole population cycles, with large impacts across trophic levels (Ims and Fuglei 2005). Popul Ecol (2014) 56:55–61 example, decreased food and cover, increased exposure to predators and/or decreased nesting and burrowing sites (e.g., Steen et al 2005; Evans et al 2006; Torre et al 2007 and references therein). Whether such changes affect small mammal herbivore population regulation processes and propensity to exhibit multi-annual cycles is not known. Interpretations of geographical gradients in the density dependence structure of vole population cycles are a high profile case where multiple plausible explanations might fit the same observed pattern (e.g., Bjørnstad et al 1995; Erb et al 2000; Saitoh et al 2003). Causal interpretation of variations in the density dependence structure observed over time or space are, necessarily, ambiguous in the absence of experimental validation (Stenseth 1999)

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