Grasses bite back! : the role of induced silica defences in vole population cycles

Long-term studies investigating the causes of cyclic population dynamics in microtine rodents have focussed on the role of top-down processes such as predation or parasitism. We propose an alternative, bottom-up mechanism which may be able to drive vole population cycles in some locations, e.g. those with low amplitudes, and suggest that nutritional constraints imposed by plant defences may limit individuals' reproductive potential. Laboratory experiments have shown that grazing by field voles (Microtus agrestis) can increase concentrations of silica in grasses, thereby reducing digestibility and limiting animals' ability to absorb nitrogen, which is essential for growth and reproduction. We tested the hypothesis that past grazing pressure de-stabilises vole population dynamics through delayed density-dependence in silica induction. We carried out experiments in the glasshouse and the field and found that vole grazing is sufficient to induce silica uptake in the grass Deschampsia caespitosa, but only after a threshold vole density has been reached. We also demonstrated that induction and post-grazing relaxation of silica are time-lagged processes. Both of these factors are a necessary requirement for the initiation of population cycles. We then used a manipulative field experiment to test relationships between food availability and quality on the reproductive success of individual field voles in Kielder Forest in northern England. Finally, we applied a simple mechanistic model to our empirical data from the glasshouse and a long-term data set of vole population dynamics in Kielder to demonstrate that delayed density-dependence in silica uptake can generate multi-year population cycles in voles.