Sustainability science: the emerging paradigm and the ecology of cities

Humans are a true force of nature and human actions have taken their place alongside the biosphere, lithosphere, hydrosphere, and atmosphere as defining processes shaping the global landscape (Ellis & Haff, 2009). Much like forecasting the weather and trends in the economy, the interactions of humans and complex ecosystems (such as a food web in a tropical rainforest) are fraught with complexity, multiscale interactions, unexpected behaviors, nonlinearities, delayed responses, feedback loops, and extensive temporal-spatial heterogeneity (Levin, 1999; Wu & Marceau, 2002). In light of these circumstances, system modelers embrace spatial heterogeneity as a central attribute of ecological systems and hierarchy as a central structural theme of complexity. Accordingly, they suggest that complex ecosystems have both horizontal and vertical structure. As Sterman (2002) comments,