wind and the tides are normal daily ‘hazards’ in the life of many organisms. The structure and behaviour of these organisms bear some witness to the frequency and intensity of such hazards in the evolutionary history of their species. Thus, most trees withstand the force of most storms without falling over or losing their living branches. Most limpets, barnacles and kelps hold fast to the rocks through the normal day‐to‐day forces of the waves and tides. We can also recognise a scale of more severely damaging forces (we might call them ‘disasters’) that occur occasionally, but with sufficient frequency to have contributed repeatedly to the forces of natural selection. When such a force recurs it will meet a population that still has a genetic memory of the selection that acted on its ancestors – and may therefore suffer less than they did. In the woodlands and shrub communities of arid zones, fire has this quality, and tolerance of fire damage is a clearly evolved response (see Section 2.3.6).
When disasters strike natural communities it is only rarely that they have been carefully studied before the event. One exception is cyclone ‘Hugo’ which struck the Caribbean island of Guadeloupe in 1994. Detailed accounts of the dense humid forests of the island had been published only recently before (Ducrey & Labbé, 1985, 1986). The cyclone devastated the forests with mean maximum wind velocities of 270 km h−1 and gusts of 320 km h−1. Up to 300 mm of rain fell in 40 h. The early stages of regeneration after the cyclone (Labbé, 1994) typify the responses of long‐established communities on both land and sea to massive forces of destruction. Even in ‘undisturbed’ communities there is a continual creation of gaps as individuals (e.g. trees in a forest, kelps on a seashore) die and the space they occupied is recolonised (see Section 18.6.1).
In contrast to conditions that we have called ‘hazards’ and ‘disasters’ there are natural occurrences that are enormously damaging, yet occur so rarely that they may have no lasting selective effect on the evolution of the species. We might call such events ‘catastrophes’, for example the devastating Japanese tsunami (tidal wave) of 2011, or the volcanic eruptions of Mt St Helens in 1980 or of the island of Krakatau in 1883. The next time that Krakatau erupts there are unlikely to be any genes persisting that were selected for volcano tolerance!
APPLICATION 2.7 Coral reefs and mangrove forests may ameliorate the impact of tsunamis
ecosystem services
Ecosystems often provide valuable ecosystem services (see Section 15.4.1) that people use and enjoy. Provisioning services include wild meat and berries, medicinal herbs, fibre products, fuel and drinking water; cultural services include aesthetic fulfillment, education and recreation; regulating services include the ecosystem’s capacity to ameliorate the effects of pollutants or to moderate disasters (such as tsunamis); finally, supporting services, such as primary production and nutrient cycling, underlie all the others (Townsend, 2008).
The devastating tsunamis of 2004 and 2011, caused by earthquakes off Sumatra (9.3 on the Richter scale) and north‐eastern Japan (9.0), took huge tolls in human lives and livelihoods and, hardly surprisingly, also greatly changed near‐shore and coastal ecosystems, both aquatic and terrestrial (e.g. Urabe et al., 2013). More surprising, perhaps, has been the finding that intact coral reefs can absorb some of the wave’s power (Kunkel et al., 2006). According to the American Geophysical Union, illegal coral mining off the south‐west coast of Sri Lanka allowed far more destruction from the 2004 Pacific‐wide tsunami than occurred in nearby areas where coral reefs were intact. It seems that exploitation of a provisioning service (coral crushed to create road surface) resulted in loss of a regulating service. Moreover, muddy shores with intact mangrove forest also seem to have moderated the devastation caused by the 2004 tsunami, both by reducing human mortality inland and by preventing the inland intrusion of saltwater that, where mangroves had been removed, ruined rice and groundnut crops (Kathiresan & Rajendran, 2005). The conservation and restoration of coral reefs and mangrove forests should help protect against these natural catastrophes.
2.8 Environmental pollution
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).
APPLICATION 2.8 Bioremediation and phytomining
Species may differ greatly in their ability to tolerate pollutants. Some plants (often assisted by microbial symbionts in their rhizosphere) are hyperaccumulators
