Hazards disasters and catastrophes the ecology of extreme events

The wind and the tides are normal daily 'hazards' in the life of many organisms. The structure and behavior 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 recognize 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 & Labbe, 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 (Labbe, 1994) typify the responses of long-established communities on both land or 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 sea shore) die and the space they occupied is recolonized (see Section 16.7). After massive devastation by cyclones or other widespread disasters, recolonization follows much the same course. Species that normally colonize only natural gaps in the vegetation come to dominate a continuous community.

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 volcanic eruption of Mt St Helens or of the island of Krakatau. The next time that Krakatau erupts there are unlikely to be any genes persisting that were selected for volcano tolerance!

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