In the ecological context, disturbance is regarded as an event of intense environmental stress occurring over a relatively short period of time and causing large changes in the affected ecosystem. Disturbance can result from natural causes or from the activities of humans.
Disturbance can be caused by physical stressors such as volcanic eruptions, hurricanes, tornadoes, earthquakes, and over geological time, glacial advance, and retreat. Humans can also cause physical disturbances, for example, through construction activities. Wildfire is a type of chemical disturbance caused by the rapid combustion of much of the biomass of an ecosystem and often causing mortality of the dominant species of the community such as trees in the case of a forest fire. Wildfires can ignite naturally, usually through a lightning strike, or humans can start the blaze. Sometimes fires are set deliberately as a management activity in forestry or agriculture. Events of unusually severe pollution by toxic chemicals, nutrients, or heat may also be regarded as a type of disturbance if they are severe enough to result in substantial ecological damages. Disturbance can also be biological, as when a severe infestation of defoliating insects causes substantial mortality of trees in a forest, or of crops in agriculture. The harvesting of forests and other ecosystems by humans is another type of biological disturbance.
Ecologic disturbance can occur at a variety of spatial scales. The most extensive disturbances involve landscape-scale events, such as glaciation, which can affect entire continents. Tornadoes, hurricanes and wildfires can also affect very large areas; sometimes wildfires extend over millions of acres.
Some disturbances, however, are much more local in their effects. For example, the primary disturbance regime in old-growth forests is associated with the death of individual, large trees caused by disease, insect attack, or a lightning strike. This sort of microdisturbance event results in a gap in the otherwise closed forest canopy, which lets direct light reach the forest floor. This encourages the development of a different set of plant and animal communities than those usually found on the dark, moist forest floor. Further ecological changes occur when the dead tree falls to the ground and slowly rots. Diverse processes of ecological recovery occur in response to the within-stand patch dynamics associated with the deaths of large trees in old-growth forests.
Whenever an ecosystem is affected by a substantial disturbance event, individuals and even entire species may be weakened or killed off. Other ecological damages can also occur, such as changes in hydrologic processes or soil contamination. However, once the actual disturbance event is finished, a process known as succession begins, which may eventually produce a similar ecosystem to the one that existed prior to the disturbance.
In a number of regions around the world, human activities are producing dramatic ecological disturbances. Clear-cutting of tropical rainforests, the damming or polluting of rivers and streams, the introduction of various chemicals and particulates into the atmosphere from industrial facilities are all human processes that have major effects on many ecosystems. In cases where the ecological disturbance is ongoing, succession is forestalled, and the damaged ecosystems may fail to recover their complex and sophisticated functions.
See also Stress, ecological.