Competitive release during fire succession drives ecological turnover in a small mammal community

Abstract

As fire seasons increase in severity and length, understanding the drivers of ecological responses to fire may help us predict community resilience. In western North America, post-fire succession in small-mammal communities is marked by a transition between two species: old-growth forests are predominated by southern red-backed voles (Myodes gapperi) which are largely replaced after wildfire disturbance by North American deermice (Peromyscus maniculatus). While this shift has been frequently documented, the processes that mediate this turnover are debated. One possibility is competitive release, which predicts a reduction in vole competition should result in niche expansion in deermice. Alternatively, optimum foraging theory predicts that turnover in both species is shaped by differences in their preferred resource base. We evaluated these hypotheses using stable isotopes and spatial mark-recapture histories taken from deermice and vole populations prior to and following a fire as part of a longitudinal study in the Greater Yellowstone Ecosystem. Fire disturbance was associated with a 94% decrease in voles and a 102% increase in deermice. Even after accounting for microhabitat, vole and deermice populations were negatively correlated spatially and temporally, and models incorporating competitors were a better fit for pre-fire abundance. Pre-fire, when vole abundance was high, deermouse dietary niche was small and marginal to that of voles. Post-fire, deermouse dietary niche nearly tripled and was enriched in 13C (i.e., more C4 plants), while voles occupied a similar dietary niche throughout. The results suggest deermice are experiencing ecological release due to a reduction in vole competition. This research provides important insights into how small mammals are impacted by fire succession and competition for similar foods.