A systems-level model of direct and indirect links between environmental health, socioeconomic factors, and human mortality.

Major efforts are being made to better understand how human health and ecosystem health are influenced by climate and other environmental factors. However, studies that simultaneously address human and ecosystem health within a systems-level framework that accounts for both direct and indirect effects are rare. Using path analysis and a large database of environmental and socioeconomic variables, we create a systems-level model of direct and indirect effects on human and ecosystem health in counties throughout the conterminous United States. As indicators of human and ecosystem health, we use age-adjusted mortality rate and an index of biological integrity in streams and rivers, respectively. We show that: (i) geology and climate set boundary conditions for all other variables in the model; (ii) hydrology and land cover have predictable but distinct effects on human and ecosystem health; and (iii) forest cover is a key link between the environment and the socioeconomic variables that directly influence human health.