Ecosystem health is difficult to define. Although we understand the con-cept of health thoroughly as it applies to animals or humans, it becomes ambiguous when applied to ecosystems. Typically, the term health is integrated by specific, well-documented individual physiology (e.g., regulation of temperature and blood pressure). Unlike the homeostasis that defines healthy organisms-such as good nutrition, absence of disease caused by parasites, or freedom from infection-ecosystems contain organisms and processes (e.g., herbivorous insects, parasitic plants, and decomposers of dead material) that we consider symptomatic of disease when they are present at the individual level. Nonetheless, ecosystem health is a useful term precisely because we can relate it to the well-understood concept of being either healthy or unhealthy. For practical purposes, an ecosystem is “healthy” when its structure (i.e., components) and function (i.e., processes) are similar to those of the past and are consistent and self-sustainable over time. In addition, in a healthy ecosystem, abiotic and biotic elements are frequent and abundant, and processes occur at rates and in magnitudes that demonstrate long-term quasiequilibrium.