Soil fertility and anthropogenic disturbances drive mammal species richness and assemblage composition on tropical fluvial islands

Floodplain areas comprise some 30% of the area in the Amazon, but are currently under severe anthropogenic threat. Across the Amazon Basin, forest-dwelling non-volant mammals play crucial roles in maintaining the integrity of forest functionality, yet have been poorly studied in fluvial island forests. Mammal assemblages may be affected by edaphic characteristics that operate indirectly via food nutritional quality, by patch attributes, and/or can be modulated by anthropogenic disturbances. Here, we conducted systematic and quantitative mammal surveys across fluvial islands of an Amazonian archipelago, to assess the influence of edaphic factors (soil fertility), island attributes (island area and degree of isolation) and anthropogenic characteristics (distance from human settlement and logging) on the patterns of mammal species composition and richness. On 28 islands, we conducted spoor surveys and deployed 49 camera traps (total effort of 2940 camera trap-days). Subsequently, we performed multiple regression analysis to investigate the influence of environmental and anthropogenic predictors on mammal species richness, while dbRDA (distance-based redundancy analysis) was used for species composition. We found that mammal species richness was positively correlated with soil fertility, and in combination with anthropogenic characteristics, both variables affected the species assemblage composition. In particular, smaller species were found across a variety of levels of soil fertility and anthropogenic disturbances, while larger mammals were mostly recorded at sites with higher soil fertility and low levels of anthropogenic disturbances. Understanding the contribution of environmental and anthropogenic characteristics to the observed mammalian species richness and assemblage composition patterns will help optimise management and conservation efforts on Amazonian fluvial islands. In particular, we suggest enforcing hunting and logging restrictions within fluvial islands through surveillance activities, especially in more fertile islands.