How a community changes in species composition over time is known as succession. Succession is the result of both abiotic (physical and chemical) and biotic factors.

A disturbance is an abiotic event that physically or chemically injures or even kills some individuals, creating opportunities for other individuals to grow and/or reproduce. Some ecologists also consider biotic events, such as elephants trampling vegetation or beavers felling trees, to be disturbances. The magnitude of a disturbance can vary enormously. Many disturbances are limited to small areas—for example, a log carried by waves may crush algae and animals on a rocky shore. In contrast, hurricanes, forest fires, and volcanic eruptions can affect communities over hundreds or thousands of hectares. Although small-scale disturbances are far more frequent, the occasional large-scale event is often responsible for most of the changes in a community. For example, a single volcanic eruption on Mount St. Helens felled many more trees than multiple storm events ever did, even over decades.

Stress, in contrast to disturbance, occurs when some abiotic factor reduces the growth, reproduction, and/or the survival of some individuals. A common stress on the mountain communities of Mount St. Helens is the effect of temperature on the growth and reproduction of plants. As temperatures decrease with increasing elevation on the mountain, you might expect plants at higher altitudes to experience more stress than those at lower altitudes. But as illustrated in Figure 55.14, the stress that alpine plants experience at higher elevations can be tempered by the presence of neighboring plants. That is, some types of interactions can mitigate the effects of stress.

Biotic factors can influence community change as well. On Mount St. Helens, change might be driven by competition between different tree species, with the most dominant tree species surviving over time (see Figure 56.14). Herbivory by elk is another example of a species interaction that can initiate change. By preferentially browsing on certain species such as willow and aspen, elk can shift the trajectory of plant community succession from forest to meadow, as happened in Yellowstone National Park (see Figure 56.9).