Environmental stress refers to physical, chemical, and biological constraints on the productivity of species and on the development of ecosystems. When the exposure to environmental stressors increases or decreases in intensity, ecological responses result. Stressors can be natural environmental factors, or they may result from the activities of humans. Some environmental stressors exert a relatively local influence, while others are regional or global in their scope. Stressors are challenges to the integrity of ecosystems and to the quality of the environment.
Species and ecosystems have some capacity to tolerate changes in the intensity of environmental stressors. This is known as resistance, but there are limits to this attribute, which represent thresholds of tolerance. When these thresholds are exceeded by further increases in the intensity of environmental stress, substantial ecological changes are caused.
Environmental stressors can be grouped into the following categories:
- Physical stress refers to brief but intense exposures to kinetic energy. This is a type of ecological disturbance because of its acute, episodic nature. Examples include volcanic eruptions, windstorms, and explosions.
- Wildfire is also a disturbance, during which much of the biomass of an ecosystem is combusted, and the dominant species may be killed.
- Pollution occurs when chemicals are present in concentrations large enough to affect organisms and thereby cause ecological changes. Toxic pollution can be caused by gases such as sulfur dioxide and ozone, by elements such as arsenic, lead, and mercury, and by pesticides such as DDT. Inputs of nutrients such as phosphate and nitrate can influence productivity and other ecological processes, causing a type of pollution known as eutrophication.
- Thermal stress occurs when releases of heat influence ecosystems, as happens in the vicinity of natural hot-water vents on the ocean floor, and near industrial discharges of heated water.
- Radiation stress is associated with excessive loads of ionizing energy. This can occur on mountain tops where there are intense exposures to ultraviolet radiation, and in places where there are exposures to radioactive materials.
- Climatic stress is associated with excessive or insufficient regimes of temperature, moisture, solar radiation, and combinations of these. Tundra and deserts are examples of climatically stressed ecosystems, while tropical rainforests occur under a relatively benign climatic regime.
- Biological stresses are associated with the diverse interactions that occur among organisms of the same or different species. Biological stresses can result from competition, herbivory, predation, parasitism, and disease. The harvesting and management of species and ecosystems by humans is a type of biological stress. The introduction of invasive, non-native species may be regarded as a type of biological pollution.
Various types of ecological responses occur when the intensity of environmental stress causes significant changes. For example, disruption of an ecosystem by an intense disturbance causes mortality of organisms and other ecological damage, followed by recovery through succession.
More permanent ecological adjustments occur in response to longer-term increases in the intensity of environmental stress, associated perhaps with chronic pollution or climate change. The resulting effects can include reductions in the abundance of vulnerable species, their elimination from sites stressed over the longer term, and replacement by species that are more tolerant of the changed environmental conditions. Other commonly observed responses to longer-term increases in stress include a simplification of species richness and decreased rates of productivity, decomposition, and nutrient cycling. In total, these changes represent a longer-term change in the character of the ecosystem, or an ecological conversion.
See also Ecological integrity.