Fig. 1. The regional system and its two interactive components: ecosystem and social system. The characteristics of an ecosystem (represented by a circle) are determined by several relatively independent "external state factors" (global climate, geology, time, etc.) and by interactive controls (regional climate, disturbance regime, human activities, etc.) that both affect, and are affected by, ecosystem processes. Similarly, the characteristics of a social system are affected by both external factors (international markets, state and federal regulations, history, etc.) and by interactive controls (institutions, businesses, environment, etc.). Modified from Chapin et al. (1996).