Cross-boundary effects of human impacts on animal assemblages in the coastal zone

Human impacts on ecosystems often transcend ecosystem boundaries and environmental realms, complicating ecosystem assessment, conservation, and management. Whether and how different impacts affect ecosystems in distinct but adjacent domains remains untested in many settings, and is rarely tested concurrently at the same spatial scales. In this study, we quantified the effects of coastal urbanisation on the structure of terrestrial coastal vertebrate assemblages (including birds, reptiles and mammals using baited trail cameras) and marine surf zone fish assemblages (using baited underwater videography) at 100 sites along 50 km of beach in eastern Australia. Sites occurred along a gradient of intensities of urban land conversion in the hind dunes. While the effects of urbanisation on the species sampled were pervasive across both ecosystems, the area of urbanised land mattered more for the coastal vertebrates observed, while proximity to urbanised land was more important for the surf zone fish observed. Here, fewer individuals and species of coastal vertebrates were found at sites with a greater extent of urbanised land within 5 km. Conversely, fish assemblages were more diverse in the surf zones abutting urban areas and more abundant at sites approximately 150 m from urbanised land. The spatial properties of these landscapes, including proximity to headlands for coastal vertebrates and reefs for fish, modified the effects of urbanisation. Our findings suggest that urbanisation can have ecologically nuanced effects that are detectable across the land-sea boundary, and stress the importance of landscape context when assessing and conserving animal assemblages in and around coastal cities.