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Environmental Stress

Environmental stress


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 usually brief loadings of kinetic energy, perhaps caused by a windstorm, volcanic eruption, tidal wave, or an explosion.
  • Wildfire is another episodic stress, usually causing a mass mortality of ecosystem dominants such as trees or shrubs and a rapid combustion of much of the biomass of the ecosystem.
  • Pollution occurs when certain chemicals are bio-available in a sufficiently large amount to cause toxicity. Toxic stressors include gaseous air pollutants such as sulfur dioxide and ozone , metals such as lead and mercury , residues of pesticides, and even nutrients that may be beneficial at small rates of supply but damaging at higher rates of loading.
  • Nutrient impoverishment implies an inadequate availability of physiologically essential chemicals, which imposes an oligotrophic constraint upon ecosystem development.
  • Thermal stress occurs when heat energy is released into an ecosystem, perhaps by aquatic discharges of low-grade heat from power plants and other industrial sources.
  • Exploitative stress refers to the selective removal of particular species or size classes. Exploitation by humans includes the harvesting of forests or wild animals, but it can also involve natural herbivory and predation, as with infestations of defoliating insects such as locusts, spruce budworm, or gypsy moth , or irruptions of predators such as crown-of-thorns starfish.
  • Climatic stress is associated with an insufficient or excessive regime of moisture, solar radiation, or temperature. These can act over the shorter term as weather, or over the longer term as climate .

Within most of these contexts, stress can be exerted either chronically or episodically. For example, the toxic gas sulfur dioxide can be present in a chronically elevated concentration in an urbanized region with a large number of point sources of emission . Alternatively, where the emission of sulfur dioxide is dominated by a single, large point source such as a smelter or power plant, the toxic stress associated with this gas occurs as relatively short-term events of fumigation .

Environmental stress can be caused by natural agencies as well as resulting directly or indirectly from the activities of humans. For example, sulfur dioxide can be emitted from smelters, power plants, and homes, but it can also be emitted in large quantities by volcanoes. Similarly, climate change has always occurred naturally, but it may also be forced by human activities that result in emissions of carbon dioxide , methane , and nitrous oxide into the atmosphere .

Over most of Earth's history, natural stressors have been the dominant constraints on ecological development. Increasingly, however, the direct and indirect consequences of human activities are becoming dominant environmental stressors. This is caused by both the increasing human population and by the progressively increasing intensification of the per-capita effect of humans on the environment .

[Bill Freedman Ph.D. ]


RESOURCES

BOOKS

Freedman, B. "Environmental Stress and the Management of Ecological Reserves." In Science and the Management of Protected Areas, edited by J. H. M. Willison, et al. Amsterdam: Elsevier, 1992.

Grime, J. P. Plant Strategies and Vegetation Processes. New York: Wiley, 1979.

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Stress, Ecological

Stress, Ecological

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:

(1) 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.

(2) Wildfire is also a disturbance, during which much of the biomass of an ecosystem is combusted, and the dominant species may be killed.

(3) 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.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

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