Island size predicts mammal diversity in insular environments, except for land-bridge islands

Insular environments are among the most endangered ecosystems as they face a myriad of anthropogenic stressors. Forest mammals perform a wide range of ecological services, with their persistence being vital for ecosystem functionality in both natural and artificial islands. Studies revealed that shrinkage in island size usually leads to the decay of mammal species richness and abundance in patchy landscapes. However, mammal species-area (SARs) and abundance-area (AARs) relationships can differ among insular environments: oceanic, fluvial, artificial, and land-bridge islands (i.e., natural islands connected to the mainland). Large dams create vast insularized landscapes after river impoundment, leading to pervasive habitat loss and potentially causing even worse biodiversity losses than other insular systems. We conducted an extensive literature search and used meta-analysis techniques to quantify the magnitude of SAR and AAR for forest mammals across different archipelago landscapes worldwide. After a screening process, we ended up with 26 studies comprising 55 different effect sizes representing the magnitude of SARs and AARs. Our global analysis unveiled a positive relationship between effect sizes and island area, with mammal species richness and abundance increasing in fluvial, oceanic, and artificial islands accordingly with island area, but not in land-bridge islands. These results demonstrate that, except for land-bridge islands, SAR and AAR are still fair models to predict mammal diversity. These results could improve the prediction of SAR and AAR in insular environments under habitat loss scenarios and propose sound conservation strategies since the rate at which insular communities have been lost is presently unknown.