Ecological and evolutionary impacts of urbanization: applying a causal modeling framework

Urbanization, as the fastest growing land use type, shapes biodiversity through a suite of abiotic and biotic changes driven by the profound alteration of environmental conditions in built environments. Understanding the mechanisms behind biodiversity patterns in urban areas requires disentangling the direct and indirect pathways through which urbanization impacts ecological and evolutionary dynamics. We show how a structural causal modeling framework can be used to provide insight into processes generating biodiversity patterns in urban contexts by building structural equation models to disentangle direct and indirect pathways by which urbanization affects abundance and coat color variation in eastern gray squirrels (Sciurus carolinensis). Integrating camera traps and point count surveys across 49 sites, we observed pronounced urban-rural gradients in squirrel abundance and color variation. Human population density affected abundance of color morphs by mediating resource availability, predator activity, habitat amount, and fragmentation. But markedly different mechanisms drove abundance of color morphs across the landscape. Our findings reveal how explicit modeling of multiple pathways of environmental change can deepen our mechanistic understanding of how urbanization alters biodiversity patterns.