A point source is a situation where large quantities of pollutants are emitted from a single, discrete source, such as a smokestack, a sewage or thermal outfall into a waterbody, or a volcano. If the emissions from a point source are large, the environment will be characterized by strong but continuous gradients of ecological stress, distributed more-or-less concentrically around the source, and diminishing exponentially with increasing distance. The stress results in damages to organisms, but because tolerance differs among species, the net result is a continuous gradient of change in the ecological community and in ecological processes, such as productivity and nutrient cycling.
This ecological phenomenon has been well studied around a number of point sources of ecological stress.
For example, the structure of terrestrial vegetation has been examined along transects originating at a large smelter located at Sudbury, Ontario. This smelter is a point source of great emissions of toxic sulfur dioxide and metals. The immediate vicinity of the smelter is characterized by severe ecological degradation, because only a few species can tolerate the toxic stress. However, at increasing distances from the smelter the intensity of the toxic stresses decreases rapidly. Consequently, there is a progressive survival and/or invasion of sundry plant species at greater distances from the smelter, depending on their specific tolerances of the toxic environment at various distances. Farther than about 18.6 mi (30 km) from the smelter the toxicity associated with its point-source emissions no longer has a measurable influence on the vegetation, and there is a mature forest, characteristic of the regional unpolluted, landscape.
Often, species that are most tolerant of the toxic stresses close to a point source are uncommon or absent in the surrounding, non-polluted habitats. Usually, only a few tolerant species are present close to point sources of intense ecological stress, occurring as a sparse, lowgrowing community. At greater distances shrubs may dominate the plant community, and still further away relatively tolerant species of tree may maintain an open forest. Eventually, beyond the distance of measurable ecological responses to the toxic stress, a reference forest occurs. However, it is important to recognize that these ecological changes are continuous, as are the gradients of environmental stress associated with the point source. This syndrome of degradation of vegetation along transects from smelters and other large point sources has been characterized as a peeling of the vegetation.
In addition to changes in ecological communities along environmental gradients associated with point sources, there are also predictable changes in ecological functions, such as productivity, nutrient cycling, and litter decomposition.