Understanding the importance of predator-prey interactions for species diversity is a central theme in ecology, with fundamental consequences for predicting the responses of ecosystems to land use and climate change. We assessed the relative support for different mechanistic drivers of mammal species richness at macro-scales for two trophic levels: predators and prey. To disentangle biotic (i.e. functional predator-prey interactions) from abiotic (i.e. environmental) and bottom-up from top-down determinants we considered three hypotheses: 1) environmental factors that determine ecosystem productivity drive prey and predator richness (the productivity hypothesis, abiotic, bottom-up), 2) consumer richness is driven by resource diversity (the resource diversity hypothesis, biotic, bottom-up) and 3) consumers drive richness of their prey (the top-down hypothesis, biotic, top-down). We gathered distributional range, mass and diet data for 4,091 terrestrial mammal species, excluding bats. Species richness maps were created for predators and prey and structural equation modelling was used to test the three hypotheses at continental and global scales. We also explored the importance of functional trait composition by analyzing richness of large and small mass categories for prey (division at 10 kg) and predators (division at 21.5 kg).
Mammal species richness increased from the poles to the equator, supporting the classic latitudinal richness gradient. However, only prey was significantly positively correlated with NDVI (p < 0.01) indicating that productivity is a driver of prey but not predator richness. This NDVI-prey correlation was not maintained within the six continental bio-geographical regions, possibly indicating that productivity is a coarse driver of prey richness. Our results strongly supported the resource richness hypothesis as predator richness was significantly correlated with prey richness at the global scale (p < 0.01) and deviated from a null expectation in three bio-geographic regions. Dividing predators and prey into large and small mass categories revealed strong correlations within the functional mass categories (e.g. large prey to large predators) and weak relationships between them (e.g. small prey to large predators), further supporting the resource diversity hypothesis. Predator richness promoted prey richness at the global scale (1-tailed test; p = 0.035) but not at the continental scale. Our results highlight the importance of biotic interactions in maintaining biodiversity at macro-scales. Protected area situation and management should prioritise regions that can maintain ecosystem integrity in the face of climate and land use change to maximise biodiversity conservation throughout the food-web.