Starting points for small mammal population recovery after wildfire: recolonisation or residual populations?

Abstract

Wildfire is a major driver of spatio-temporal variation in terrestrial ecosystems. Large wildfires are predicted to occur more frequently due to climate change. The mechanisms by which post-fire recovery proceeds are influenced by the abundance of survivors, and their distribution in relation to habitat variability and refugia. Thus, characterising early post-fire demographic processes is critical to understanding the demographic and community-level responses of ecosystems to fire. The Black Saturday fires of February 2009 burnt an area of approximately 3500 km2 in Victoria, Australia. We quantified the effects of this high severity forest fire on the habitat, abundance, sex ratio and body mass of two small mammal species, the agile antechinus Antechinus agilis and bush rat Rattus fuscipes. We developed a hypothetical framework to distinguish in situ survival and rapid recolonisation as the processes underlying short-term post-fire distributions. These hypotheses were based on expected patterns of abundance over increasing distances from unburnt sources, and the estimated recolonisation capabilities of each species and sex. The agile antechinus and bush rat were present in burnt sites at 30% and 12% of the density observed in unburnt sites. In situ survival, and not recolonisation, was the most plausible explanation for our findings. Abundance and body mass data indicated a greater effect of fire on the bush rat than the agile antechinus. The bush rat showed a shift in topographic association, whereby drainage lines acted as post-fire refugia. Our findings suggest these species do not depend on recolonisation for recovery, and that the bush rat will follow a nucleated recovery, expanding from topographic refugia. Thus, connectivity-reducing management activities, such as salvage logging and firebreak and road construction, may not affect the early stages of population recovery in remaining stands of burnt forest. Rather, ongoing recovery is likely to be limited by demographic rates and resource availability.