Unraveling the role of environmental and anthropogenic drivers in shaping global patterns in mammal diversity

Patterns in functional and phylogenetic diversity reflect ecological and evolutionary relationships among taxa, and thus can offer key insights into the mechanisms underlying species distributions. However, disentangling the relative influence of proximate environmental drivers versus biogeographic evolutionary history can be a challenge. Moreover, human activities have enormously impacted the global distribution of mammals over the past millennia, potentially skewing our understanding of the underlying processes influencing biodiversity accumulation and community structure. Here, we investigated how the environment shapes global patterns in terrestrial mammal diversity, and how anthropogenic impacts have altered our understanding of these mechanisms. To distinguish aspects of mammal diversity most directly influenced by proximate environmental conditions, we employed novel metrics representing the deviation between functional and phylogenetic diversity. We calculated these residual functional diversity values using both current mammal distributions and estimated distributions in the absence of human impacts to characterize the effect of anthropogenic diversity loss. Each dataset was then modeled separately as a function of key environmental drivers and compared. We found remarkable variation in residual functional diversity across terrestrial communities, suggesting the environment strongly mediates the relationship between functional and phylogenetic diversity. Specifically, temperature seasonality and evapotranspiration play key roles in shaping global patterns in mammal functional diversity. Critically, the strength of these relationships is dampened by anthropogenic biodiversity loss, which has homogenized functional and phylogenetic community structure across environmental gradients. By disentangling the role of human impacts on both patterns and purported mechanisms of mammal diversity, our results provide a more accurate depiction of the fundamental relationships underlying mammal communities.