We have seen how predation, herbivory, and parasitism can reduce the effects of competition. Severe physical impacts on a habitat, such as those caused by storms, earthquakes, or road building, can also dramatically lower the abundances of some species, as for example when an early frost kills mosquitoes and other insects. Such events are known as disturbances, and they have effects on populations of interacting species that are independent of their densities, as we saw in Chapter 46. Disturbances often affect multiple species in the same community and so can exert a strong influence over community composition. We usually think of disturbances as abiotic, but the line is not so clear with road-building by humans, for example, or dam building by beavers. Such changes are not uncommon, but are usually unpredictable.

Severe weather in forests demonstrates how disturbance can affect a community. When fire or a hurricane takes down tall trees in a forest, it also takes away the shade that had prevented some species from establishing new seedlings. Fires are frequent sources of disturbance in regions with a seasonally dry climate, such as parts of western North America and southwestern Australia. Like storms, fires remove existing plants and animals, opening up habitat for recolonization. Some species have adapted to frequent fires. For example, several pine species have evolved thick bark and high branches, traits that help protect them from fire. Others, which have evolved cones that open only in the wake of fire, are adapted to use the habitat space opened up by the fire and the nutrients that dead plant matter releases into the soil.

Noting the influence of storms on community composition in forests and coral reefs, American ecologist Joseph Connell proposed that the diversity of species in a given community reflects, at least in part, the frequency and intensity of disturbance. If disturbance is intense, few species can tolerate the physical conditions of the habitat and diversity will be low. On the other hand, if disturbance is rare or weak, competition may take over and only a few, stronger, competitors may remain. Species diversity tends to be highest when disturbance is frequent or intense enough to inhibit competition, but not so strong as to limit the number of species that can tolerate the environment.