Читать книгу Ecology - Michael Begon - Страница 60

A number of environmental conditions that are, regrettably, becoming increasingly important are due to the accumulation of toxic byproducts of human activities. Sulphur dioxide emitted from power stations, and metals like copper, zinc and lead, dumped around mines or deposited around refineries, are just some of the pollutants that limit distributions, especially of plants. Many such pollutants are present naturally but at low concentrations, and some are indeed essential nutrients for plants. But in polluted areas their concentrations can rise to lethal levels. The loss of species is often the first indication that pollution has occurred, and changes in the species richness of a river, lake or area of land provide bioassays of the extent of their pollution.

rare tolerators

Yet it is rare to find even the most inhospitable polluted areas entirely devoid of species; there are usually at least a few individuals of a few species that can tolerate the conditions. Even natural populations from unpolluted areas often contain a low frequency of individuals that tolerate the pollutant; this is part of the genetic variability present in natural populations. Such individuals may be the only ones to survive or colonise as pollutant levels rise. They may then become the founders of a tolerant population to which they have passed on their ‘tolerance’ genes, and, because they are the descendants of just a few founders, such populations may exhibit notably low genetic diversity overall (Figure 2.24). Thus, in very simple terms, a pollutant has a two‐fold effect. When it is newly arisen or is at extremely high concentrations, there will be few individuals of any species present (the exceptions being naturally tolerant variants or their immediate descendants). Subsequently, however, the polluted area is likely to support a much higher density of individuals, but these will be representatives of a much smaller range of species than would be present in the absence of the pollutant. Such novel, species‐poor communities are now an established part of human environments (Bradshaw, 1987).

Figure 2.24 Individuals of Platynympha longicaudata in a polluted site are more tolerant of pollution and have lower genetic diversity. (a) Tolerance of this marine isopod around Port Pirie, South Australia (the largest lead smelting operation in the world), was significantly higher (P < 0.05) than for animals from a control (unpolluted) site, as measured by the concentration in food of a combination of metals (lead, copper, cadmium, zinc and manganese) required to kill 50% of the population (LC₅₀). (b) Genetic diversity at Port Pirie was significantly lower than at three unpolluted sites, as measured by two indices of diversity based on RAPD (random amplified polymorphic DNA).

Source: After Ross et al. (2002).