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

Abstract

Grazing 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 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. 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.

The cascading impacts of livestock grazing in upland ecosystems: a 10‐year experiment

“…Vole density and cycle amplitude.-First, to test for a long-term response of vole density to grazing intensity, the proportion of positive VSI was modelled as a function of treatment, with year as an additive explanatory covariate in order to account for some degree of background synchrony in peaks and troughs across treatments detected in previous analyses (Villar et al 2013a), and season to account for seasonality in vole density. Second, to test the effect of grazing intensity on the amplitude of vole density fluctuations, we first derived cycle amplitude as the 10th to 90th inter-quantile range of time series of VSI scores (VSI inter-quantile; Cornulier et al 2013) for each experimental plot separately.…”

“…In our experimental system, vole populations exhibit cycles with periodic multi-annual peaks and troughs in abundance, which are affected by grazing (Villar et al 2013a). It is thought that both mean abundance and amplitude in rodent cycles are critical factors for sustaining populations of endangered carnivore predators in similar grassland systems at higher latitudes, leading to cyclical pulses in survival and reproduction in such predators (Sundell et al 2004, Henden et al 2008.…”

Effects of grazing management on biodiversity across trophic levels–The importance of livestock species and stocking density in salt marshes

“…A1). Each plot was scanned for signs indicating the presence of voles: runways, fresh plant fragments, or faecal pellets (see Villar et al, 2014). For analysis, the proportion of subplots with vole presence was calculated per paddock (N = 3 replicates per treatment).…”

“…Grazing is often beneficial to plant species richness (Olff and Ritchie, 1998), but detrimental to species richness of many arthropod groups (Foster et al, 2014;, molluscs (Boschi and Baur, 2007) and small mammals (e.g. Villar et al, 2014), while mixed effects are reported for species richness of meadow birds (e.g. Atkinson et al, 2005;WallisDeVries et al, 2007).…”

“…Such a short and open vegetation canopy will be beneficial for spring staging geese, as these small herbivores require high-quality forage (Mandema et al, 2014b). By contrast, protective cover for voles will decrease under high stocking densities (Villar et al, 2014), hence, voles are hypothesised to be more abundant under low stocking densities. Also pollinators can be expected to be more abundant under low stocking densities, as their most important floral resource, Aster tripolium, is sensitive to grazing (Kiehl et al, 1996;Nolte et al, 2013).…”