Pollution
Pollution can be defined as unwanted or detrimental changes in a natural system. Usually, pollution is associated with the presence of toxic substances in some large quantity, but pollution can also be caused by the presence of excess quantities of heat or by excessive fertilization with nutrients.
Because pollution is judged on the basis of degradative changes, there is a strongly anthropocentric bias to its determination. In other words, humans decide whether pollution is occurring and how bad it is. Of course, this bias favors species, communities, and ecological processes that are especially desired or appreciated by humans. In fact, however, some other, less-desirable species, communities, and ecological processes may benefit from what we consider pollution.
An important aspect of the notion of pollution is that ecological change must actually be demonstrated. If some potentially polluting substance is present at a concentration or intensity that is less than the threshold required to cause a demonstrable ecological change, then the situation would be referred to as contamination, rather than pollution.
This aspect of pollution can be illustrated by reference to the stable elements, for example, cadmium, copper, lead, mercury, nickel, selenium, uranium, etc. All of these are consistently present in at least trace concentrations in the environment. Moreover, all of these elements are potentially toxic. However, they generally affect biota and therefore only cause pollution when they are present at water-soluble concentrations of more than about 0.01-1 parts per million (ppm).
Some other elements can be present in very large concentrations, for example, aluminum and iron, which are important constituents of rock and soil. Aluminum constitutes 8-10% of the earth's crust and iron 3-4%. However, almost all of the aluminum and iron present in minerals are insoluble in water and are therefore not readily assimilated by biotic community and cannot cause toxicity. In acidic environments, however, ionic forms of aluminum are solubilized, and these can cause toxicity in concentrations of less than one part per million. Therefore, the bio-availability of a chemical is an important determinant of whether its presence in some concentration will cause pollution.
Most instances of pollution result from the activities of humans. For example, anthropogenic pollution can be caused by:
(1) the emission of sulfur dioxide and metals from a smelter, causing toxicity to vegetation and acidifying surface waters and soil,
(2) the emission of waste heat from an electricity generating station into a river or lake, causing community change through thermal stress, or
(3) the discharge of nutrient-containing sewage wastes into a water body, causing eutrophication.
Most instances of anthropogenic pollution have natural analogues, that is, cases where pollution is not the result of human activities. For example, pollution can be caused by the emission of sulfur dioxide from volcanoes, by the presence of toxic elements in certain types of soil, by thermal springs or vents, and by other natural phenomena. In many cases, natural pollution can cause an intensity of ecological damage that is as severe as anything caused by anthropogenic pollution.
An interesting case of natural air pollution is the Smoking Hills, located in a remote and pristine wilderness in the Canadian Arctic, virtually uninfluenced by humans. However, at a number of places along the 18.63 miles (30 km) of seacoast, bituminous shales in sea cliffs have spontaneously ignited, causing a fumigation of the tundra with sulfur dioxide and other pollutants. The largest concentrations of sulfur dioxide (more than two parts per million) occur closest to the combustions. Further away from the sea cliffs the concentrations of sulfur dioxide decrease rapidly. The most-important chemical effects of the air pollution are acidification of soil and fresh water, which in turn causes a solubilization of toxic metals. Surface soils and pond waters commonly have pHs less than 3, compared with about Ph 7 at non-fumigated places. The only reports of similarly acidic water are for volcanic lakes in Japan, in which natural pHs as acidic as 1 occur, and pH less than 2 in waters affected by drainage from coal mines.
At the Smoking Hills, toxicity by sulfur dioxide, acidity, and water-soluble metals has caused great damage to ecological communities. The most- intensively fumigated terrestrial sites have no vegetation, but further away a few pollution-tolerant species are present. About one kilometer away the toxic stresses are low enough that reference tundra is present. There are a few pollution-tolerant algae in the acidic ponds, with a depauperate community of six species occurring in the most-acidic (pH 1.8) pond in the area.
Other cases of natural pollution concern places where certain elements are present in toxic amounts. Surface mineralizations can have toxic metals present in large concentrations, for example copper at 10% in peat at a copper-rich spring in New Brunswick, or surface soil with 3% lead plus zinc on Baffin Island. Soils influenced by nickel-rich serpentine minerals have been well-studied by ecologists. The stress-adapted plants of serpentine habitats form distinct communities, and some plants can have nickel concentrations larger than 10% in their tissues. Similarly, natural soils with large concentrations of selenium support plants that can hyperaccumulate this element to concentrations greater than 1%. These plants are poisonous to livestock, causing a toxic syndrome known as blind staggers.
Of course, there are many well-known cases where pollution is caused by anthropogenic emissions of chemicals. Some examples include:
(1) Emissions of sulfur dioxide and metals from smelters can cause damage to surrounding terrestrial and aquatic ecosystems. The sulfur dioxide and metals are directly toxic. In addition, the deposition of sulfur dioxide can cause an extreme acidification of soil and water, which causes metals to be more bio-available, resulting in important, secondary toxicity. Because smelters are point sources of emission, the spatial pattern of chemical pollution and ecological damage displays an exponentially decreasing intensity with increasing distance from the source.
(2) The use of pesticides in agriculture, forestry, and around homes can result in a non-target exposure of birds and other wildlife to these chemicals. If the non-target biota are vulnerable to the pesticide, then ecological damage will result. For example, during the 1960s urban elm trees in the eastern United States were sprayed with large quantities of the insecticide DDT, in order to kill beetles that were responsible for the transmission of Dutch elm disease, an important pathogen. Because of the very large spray rates, many birds were killed, leading to reduced populations in some areas. (This was the "silent spring" that was referred to by Rachel Carson in her famous book by that title.) Birds and other non-target biota have also been killed by modern insecticide-spray programs in agriculture and in forestry.
(3) The deposition of acidifying substances from the atmosphere, mostly as acidic precipitation and the dry deposition of sulfur dioxide, can cause an acidification of surface waters. The acidity solubilizes metals, most notably aluminum, making them bio-available. The acidity in combination with the metals causes toxicity to the biota, resulting in large changes in ecological communities and processes. Fish, for example, are highly intolerant of acidic waters.
(4) Oil spills from tankers and pipelines can cause great ecological damage. When oil spilled at sea washes up onto coastlines, it destroys seaweeds, invertebrates, and fish, and their communities are changed for many years. Seabirds are very intolerant of oil and can die of hypothermia if even a small area of their feathers is coated by petroleum.
(5) Most of the lead shot fired by hunters and skeet shooters miss their target and are dispersed into the environment. Waterfowl and other avian wildlife actively ingest lead shot because it is similar in size and hardness to the grit that they ingest to aid in the mechanical abrasion of hard seeds in their gizzard. However, the lead shot is toxic to these birds, and each year millions of birds are killed by this source in North America.
Humans can also cause pollution by excessively fertilizing natural ecosystems with nutrients. For example, freshwaters can be made eutrophic by fertilization with phosphorus in the form of phosphate. The most conspicuous symptoms of eutrophication are changes in species composition of the phytoplankton community and, especially, a large increase in algal biomass, known as a bloom. In shallow waterbodies there may also be a vigorous growth of vascular plants. These primary responses are usually accompanied by secondary changes at higher trophic levels, including arthropods, fish, and waterfowl, in response to greater food availability and other habitat changes. However, in the extreme cases of very eutrophic waters, the blooms of algae and other microorganisms can be noxious, producing toxic chemicals and causing periods of oxygen depletion that kill fish and other biota. Extremely eutrophic waterbodies are polluted because they often cannot support a fishery, cannot be used for drinking water, and have few recreational opportunities and poor esthetics.
Pollution, therefore, is associated with ecological degradation, caused by environmental stresses originating with natural phenomena or with human activities. The prevention and management of anthropogenic pollution is one of the greatest challenges facing modern society.
This is the complete article, containing 1,470 words
(approx. 5 pages at 300 words per page).
