In the ecological context, environmental stress can be considered any environmental influence that causes a discernible ecological change, especially in terms of a constraint on ecosystem development. Stressing agents (or stressors) can be exogenous to the ecosystem, as in the cases of long-range transported acidifying substances, toxic gases, or pesticides. Stress can also cause change as a result of an accentuation of some pre-existing site factor beyond a threshold for biological tolerance, for example thermal loading, nutrient availability, wind, or temperature extremes.

Often implicit within the notion of environmental stress, particularly from the perspective of ecosystem managers, is a judgement about the quality of the ecological change. That is, from the human perspective, whether the effect is "good" or "bad."

Environmental stressors can be divided into several, not necessarily exclusive, classes of causal agencies:

• "Physical stress" refers to episodic events (or disturbance) associated with intense but...