Indirect effects of human development along the coast on coral health

Accelerated human development in coastal areas is causing declines in coral reefs worldwide, but the mechanisms by which development leads to reef degradation are often difficult to identify. Here, we use Akaike information criterion model averaging in combination with path analysis to test for the direct and indirect effects of potential environmental stressors on two coral diseases on the west coast of Hawai'i. We quantified human-altered land cover and human population density at two spatial scales: inland area of 1-km radius and watershed. We then tested for the effects of these land cover variables, as well as seawater chlorophyll α concentrations, depth, host coral cover, and proximity to surface water discharge on the density of growth anomalies (GA) and prevalence of trematodiasis (TRE) affecting Porites lobata, the locally dominant reef-building coral species. Our analyses showed that human-altered land cover measured at 1-km scale was a strong indirect predictor of GA. Specifically, we found that human interference adjacent to the coast predicted higher chlorophyll α concentrations, which in turn predicted higher GA density. We also found that chlorophyll α and depth were strong negative predictors of TRE, and host coral cover a positive predictor. Our results indicated that GA are likely regulated by indirect land-based anthropogenic impacts, whereas TRE is mostly affected by host density-dependent forces. Path analysis can serve as a useful tool to rapidly identify the scale and indirect effects of anthropogenic stressors related to coral diseases, allowing for accurate conservation planning in the face of limited resources for tropical conservation.