A number of environmental conditions that are, regrettably, becoming increasingly important are due to the accumulation of toxic by-products of human activities. Sulfur dioxide emitted from power stations, and metals like copper, zinc and lead, dumped
Kangaroo Island Middle Beach Edinburgh Port Pirie
Figure 2.20 The response of the marine isopod, Platynympha longicaudata, to pollution around the largest lead smelting operation in the world, Port Pirie, South Australia. (a) Tolerance, both summer and winter, 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 RAPDs (random amplified polymorphic DNA). (After Ross et al., 2002.)
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 (see, for example, Lovett Doust et al., 1994).
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 colonize 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 ofjust a few founders, such populations may exhibit notably low genetic diversity overall (Figure 2.20). Moreover, species themselves may differ greatly in their ability to tolerate pollutants. Some plants, for example, are 'hyperaccumulators' of heavy metals - lead, cadmium and so on - with an ability not only to tolerate but also to accumulate much higher concentrations than the norm (Brooks, 1998). As a result, such plants may have an important role to play in 'bioremedia-tion' (Salt et al., 1998), removing pollutants from the soil so that eventually other, less tolerant plants can grow there too (discussed further in Section 7.2.1).
Thus, in very simple terms, a pollutant has a twofold 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 newly evolved, species-poor communities are now an established part of human environments (Bradshaw, 1987).
Pollution can of course have its effects far from the original source (Figure 2.21). Toxic effluents from a mine or a factory may enter a watercourse and affect its flora and fauna for its whole length downstream. Effluents from large industrial complexes can pollute and change the flora and fauna of many rivers and lakes in a region and cause international disputes.
A striking example is the creation of 'acid rain' - for example that falling in acid rain Ireland and Scandinavia from industrial activities in other countries. Since the Industrial Revolution, the burning of fossil fuels and the consequent emission to the atmosphere of various pollutants, notably sulfur dioxide, has produced a deposition of dry acidic particles and rain that is essentially dilute sulfuric acid. Our knowledge of the pH tolerances of diatom species enables an approximate pH history of a lake to be constructed. The history of the acidification of lakes is often rare tolerators
Figure 2.21 An example of long-distance environmental pollution. The distribution in Great Britain of fallout of radioactive caesium (Bq m-2) from the Chernobyl nuclear accident in the Soviet Union in 1986. The map shows the persistence of the pollutant on acid upland soils where it is recycled through soils, plants and animals. Sheep in the upland areas contained more caesium-137 (137Cs) in 1987 and 1988 (after recycling) than in 1986. 137Cs has a half-life of 30 years! On typical lowland soils it is more quickly immobilized and does not persist in the food chains. (After NERC, 1990.)
recorded in the succession of diatom species accumulated in lake sediments (Flower et al., 1994). Figure 2.22, for example, shows how diatom species composition has changed in Lough Maam, Ireland - far from major industrial sites. The percentage of various diatom species at different depths reflects the flora present at various times in the past (four species are illustrated). The age of layers of sediment can be determined by the radioactive decay of lead-210 (and other elements). We know the pH tolerance of the diatom species from their present distribution and this can be used to reconstruct what the pH of the lake has been in the past. Note how the waters acidified since about 1900. The diatoms Fragilaria virescens and Brachysira vitrea have declined markedly during this period while the acid-tolerant Cymbella perpusilla and Frustulia rhomboides increased after 1900.
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