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 negative delayed density dependence exhibited by many herbivore species, but little field evidence exists to support this hypothesis. We tested a key premise of the hypothesis that reciprocal feedback between vole grazing pressure and the induction of anti‐herbivore silicon defenses in grasses drives observed population cycles in a large‐scale field experiment in northern England. We repeatedly reduced population densities of field voles (Microtus agrestis) on replicated 1‐ha grassland plots at Kielder Forest, northern England, over a period of 1 year. Subsequently, we tested for the impact of past density on vole life history traits in spring, and whether these effects were driven by induced silicon defenses in the voles’ major over‐winter food, the grass Deschampsia caespitosa. After several months of density manipulation, leaf silicon concentrations diverged and averaged 22% lower on sites where vole density had been reduced, but this difference did not persist beyond the period of the density manipulations. There were no significant effects of our density manipulations on vole body mass, spring population growth rate, or mean date for the onset of spring reproduction the following year. These findings show that grazing by field voles does induce increased silicon defenses in grasses at a landscape scale. However, at the vole densities encountered, levels of plant damage appear to be below those needed to induce changes in silicon levels large and persistent enough to affect vole performance, confirming the threshold effects we have previously observed in laboratory‐based studies. Our findings do not support the plant quality hypothesis for observed vole population cycles in northern England, at least over the range of vole densities that now prevail here.

Population cycles in voles and lemmings: state of the science and future directions

“…However, there exists no empirical evidence that grazing‐induced changes in silica (or other measures of food quality) are necessary for, or substantially affect, rodent population cycles (Ruffino et al. ).…”

“…Using coupled grass-vole differential equation models, Reynolds et al (2012Reynolds et al ( , 2013 showed that delayed, silicabased plant defensive responses to grazing intensity can, in theory, generate population cycles. However, there exists no empirical evidence that grazing-induced changes in silica (or other measures of food quality) are necessary for, or substantially affect, rodent population cycles (Ruffino et al 2018).…”

Short‐term resistance that persists: Rapidly induced silicon anti‐herbivore defence affects carbon‐based plant defences

“…In order to better understand the role of Si in mediating plant resistance to herbivory it is important to determine how quickly plants are able to accumulate ecologically relevant amounts of Si. Unlike many inducible defences, Si is considered to be chemically inert in plant systems, and once it is deposited it is unable to be remobilised making its effects on herbivory relatively long‐lasting (Cooke & Leishman, 2011a; Hodson, 1990; Reynolds et al., 2012; Ruffino et al., 2018). Additionally, several studies have shown trade‐offs between Si and carbon‐based defences such as phenolics, which are known to play an important role in plant resistance to herbivory (Baldwin & Schultz, 1983; Cooke & Leishman, 2012; Frew et al., 2016; Johnson & Hartley, 2018; Klotzbücher et al., 2018; Quigley et al., 2020).…”

Plant herbivore interactions: combined effect of ground water level, root vole grazing and sedge silication

“…A similar mechanism of silification of grasses caused by vole grazing was observed in studies conducted both in a laboratory (Reynolds et al, 2012) and also in a landscape scale experiments (Ruffino et al, 2018). The question therefore arises of why so few confirmations of silica-induced defence mechanisms in grasses generated by the herbivorous mammals are detected in the wild, while it is so readily detected in laboratory experiments?…”

“…Quigley et al (2020) demonstrated that Si concentration in grass leaves did not respond to large mammalian grazer exclusion studied in a climatic gradient, but was strongly affected by nutrient availability. In turn, field experiments carried out in Kielder Forest (UK) showed that after several months of density manipulation the level of silicon in wavy hair grass (Deschampsia caespitosa ) leaves decreased by 22% on sites where field vole density had been reduced, but the increase in silicon content did not affect body weight of voles, nor their spring population growth rate or survival, which suggests that plant quality hypothesis is unlikely to explain the observed cyclicity in the Kielder Forest field vole population (Ruffino et al, 2018). Likewise, (Wieczorek et al, 2015a;but see Soninen et al, 2017) showed that vole herbivory elevated silicon levels in sedges, albeit with no detectable effect on the winter survival rates of voles.…”