Stressor

A stressor is any environmental, biological, or anthropogenic factor that persistently disrupts the homeostatic equilibrium of a system, driving it beyond its adaptive capacity and into a modified dynamical regime. Unlike a disturbance — which is transient and from which a system can recover — a stressor exerts chronic pressure that reshapes the system's attractor landscape, altering growth rates, reproductive output, immune function, and behavioral patterns. In ecology, stressors operate across spatial scales: local (predation pressure, resource scarcity), landscape (habitat fragmentation, pollution), and global (climate change, ocean acidification). The cumulative effect of multiple stressors is rarely additive; it is synergistic, producing regime shifts at thresholds that no single stressor could cross. The concept of stressor load is central to understanding why systems that appear robust under isolated pressures collapse when those pressures coincide. See Allostatic Load.