Under habitat loss and fragmentation, the intensity of local ecological processes involving species interactions changes pervasively, accelerating local species extinctions, and disrupting essential ecosystem functions. We addressed this issue by examining the direct effects of forest area on small mammal ( These three mammal groups composed a tri-trophic foodweb surveyed in 25 forest islands of varying sizes and degrees of isolation within the Balbina Hydroelectric Reservoir in Central Brazilian Amazonia, in addition to three adjacent continuous forest sites. Small mammals (rodents and marsupials) were surveyed using live- and pitfall-trapping and mammal mesopredators and apex predators were surveyed using camera-trapping. As response variables for each mammal group, we selected from one of three options: standardized abundance, biomass and metabolic biomass (biomass0.75), according with their explanation of the data. Each of the above-mentioned hypotheses was tested using a piecewise structural equation model (SEM), which fitting was compared using the Akaike Information Criteria for small sample sizes.

Apex predator biomass increased with forest area, as well as the abundance of mesopredators. Metabolic biomass of small mammals, however, was not affected by forest area. From the three hypothesized SEMs, the best explaining the data was the one considering both direct and indirect effects of forest area. According to that, forest area had a positive effect on both apex and mesopredators, whereas mesopredators had an unexpected positive effect on small mammals. As such, although the best model included both direct and indirect effects, the results of that model support the predominance of bottom-up forces regulating this foodweb. This clear prevalence of such mechanisms renders forest islands more susceptible to other major disturbances detrimentally affecting forest dynamics, including timber extraction and the ravages of climate change. Our findings can be used to inform the long-term viability of forest ecosystems affected by hydropower development in lowland Amazonia.