Small rodent population cycles and plants – after 70 years, where do we go?

Abstract

Small rodent population cycles characterise northern ecosystems, and the cause of these cycles has been a long-lastingcentral topic in ecology, with trophic interactions currently considered the most plausible cause. While some researchershave rejected plant–herbivore interactions as a cause of rodent cycles, others have continued to research their potentialroles. Here, we present an overview of whether plants can cause rodent population cycles, dividing this idea into fourdifferent hypotheses with different pathways of plant impacts and related assumptions. Our systematic review of theexisting literature identified 238 studies from 150 publications. This evidence base covered studies from the temperatebiome to the tundra, but the studies were scattered across study systems and only a few specific topics were addressedin a replicated manner. Quantitative effects of rodents on vegetation was the best studied topic, and our evidence basesuggests such that such effects may be most pronounced in winter. However, the regrowth of vegetation appears to takeplace too rapidly to maintain low rodent population densities over several years. The lack of studies prevented assessmentof time lags in the qualitative responses of vegetation to rodent herbivory. We conclude that the literature is currentlyinsufficient to discard with confidence any of the four potential hypotheses for plant–rodent cycles discussed herein.While new methods allow analyses of plant quality across more herbivore-relevant spatial scales than previously possible,we argue that the best way forward to rejecting any of the rodent–plant hypotheses is testing specific predictions of dietaryvariation. Indeed, all identified hypotheses make explicit assumptions on how rodent diet taxonomic composition andquality will change across the cycle. Passing this bottleneck could help pinpoint where, when, and how plant–herbivoreinteractions have–or do not have–plausible effects on rodent population dynamics.