Support for the size-mediated sensitivity hypothesis within a diverse carnivore community.

Carnivore community dynamics are governed by a complex set of often interacting biotic, abiotic and anthropogenic factors that are increasingly volatile as a result of global change. Understanding how these changing conditions influence carnivore communities is urgent because of the important role carnivores play within ecosystems at multiple trophic levels, and the conservation threats that many carnivores face globally. While a great deal of research attention has historically been focused on large carnivores within ecosystems, the size-mediated sensitivity hypothesis has recently been proposed where the smallest carnivore in a system is likely to be the most responsive to the diverse suite of ongoing environmental and anthropogenic changes within ecological communities. We deployed camera traps at 197 sites over 4 years to monitor a diverse suite of mammalian carnivores within the Blue Ridge Mountains of western North Carolina and then used a two-step occupancy modelling-structural equation modelling framework to investigate the relative support for four primary hypothesized drivers (interspecific competition/predation, habitat complexity, food availability and anthropogenic disturbance) on carnivore occurrence. We found that each of the 10 carnivores in our system responded differently to conditions associated with each of these four hypothesized drivers, but that small and medium-sized carnivores had a greater number of significant (p < 0.05) pathways by which these conditions were influencing occupancy relative to large carnivores. In particular, the smallest carnivore observed in our study was the only species for which we found support for each of the four hypothesized drivers influencing occupancy. Collectively, our study supports the size-mediated sensitivity hypothesis and suggests that small carnivores are ideal sentinel species for global change. We echo recent calls for adopting a middle-out approach to investigations into carnivore community dynamics by refocusing sustained monitoring and research efforts on smaller carnivores within systems.