Population-level manipulations of field vole densities induce subsequent changes in plant quality but no impacts on vole demography

Abstract

Grazing-induced changes in plant quality have been suggested to drive the negativedelayed density dependence exhibited by many herbivore species, but little field evidence exists to support this hypothesis. We tested a key premise of the hypothesisthat reciprocal feedback between vole grazing pressure and the induction of antiherbivore silicon defenses in grasses drives observed population cycles in a largescale field experiment in northern England. We repeatedly reduced populationdensities of field voles (Microtus agrestis) on replicated 1-ha grassland plots at KielderForest, northern England, over a period of 1 year. Subsequently, we tested for theimpact of past density on vole life history traits in spring, and whether these effectswere driven by induced silicon defenses in the voles' major over-winter food, thegrass Deschampsia caespitosa. After several months of density manipulation, leaf silicon concentrations diverged and averaged 22% lower on sites where vole densityhad been reduced, but this difference did not persist beyond the period of the density manipulations. There were no significant effects of our density manipulations onvole body mass, spring population growth rate, or mean date for the onset of springreproduction the following year. These findings show that grazing by field volesdoes induce increased silicon defenses in grasses at a landscape scale. However, atthe vole densities encountered, levels of plant damage appear to be below thoseneeded to induce changes in silicon levels large and persistent enough to affect voleperformance, confirming the threshold effects we have previously observed inlaboratory-based studies. Our findings do not support the plant quality hypothesisfor observed vole population cycles in northern England, at least over the range ofvole densities that now prevail here.