Pollution can be defined as unwanted or detrimental changes in a natural system. Usually, pollution is associated with the presence of toxic chemicals 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...