Disentangling the Effects of Habitat Fragmentation and Top-Down Trophic Cascades on Small Mammal Assemblages on Amazonian Forest Islands

Habitat loss and fragmentation are widely acknowledged to affect species diversity. However, the pathways through which their effects are propagated through foodwebs, ultimately driving species diversity, are less well understood. We investigated to what extent the effects of habitat loss and fragmentation on small mammals in Central Amazonia are mediated by higher trophic levels, specifically mammal mesopredators and apex predators. We surveyed these three mammal groups across 25 islands surrounded by a landscape-scale gradient of forest cover within the Balbina Hydroelectric Reservoir, in addition to three adjacent continuous forest sites. We then applied Structural Equation Modelling based on apex-predator incidence and either the abundance or biomass of mesopredators and small mammals. Apex-predator incidence was positively affected by landscape forest cover, as well as mesopredators abundance and biomass. Small mammal abundance, but not biomass, was negatively affected by forest cover. Contrary to expectations, the negative effect of forest cover on small mammal abundance was not mediated by a decrease in mesopredator abundance nor incidence of apex-predators. Instead, small mammal increase is apparently favoured by the proliferation of the few persisting smaller-bodied species which can take advantage of augmented resources related to habitat degradation. The further increment in small mammal abundance in the absence or lower abundance of predators might be prevented by the resource availability characterising these evergreen forests. Mammal assemblages seem to be primarily regulated by bottom-up forces. While this suggests a delayed extinction debt in Amazonian forest islands, remaining biodiversity might be poorly resilient to additional habitat disturbances.