ecological succession

Ecological succession is the process that describes how the structure of a biological community (that is, an interacting group of various species in a desert, forest, grassland, marine environment, and so on) changes over time. Species that arrive first in a newly created environment (such as an island rising out of the sea) are called pioneer species, and they, through their interactions with one another, build a rather simple initial biological community. The structure of this community becomes more complex as new species arrive on the scene.

At every stage there are certain species that have evolved life histories to exploit the particular conditions of the community. This situation imposes a partially predictable sequence of change in the physical environment and species composition of communities.

Primary succession is ecological succession that begins in essentially lifeless areas, such as regions in which there is no soil or where the soil is incapable of sustaining life (because of recent lava flows, newly formed sand dunes, or rocks left from a retreating glacier). The first species to arrive are fast-growing “weedy species,” such as lichens or small annual plants, which create the first layers of soil as they decompose. These plants also provide habitats for small animals and other forms of life. These plants are replaced by grasses and shrubs, which shade out the first colonizers and alter the soil further, before large trees and more shade-tolerant species replace the community of sun-loving grasses and shrubs. Each community may support different collections of animal species.

Secondary succession occurs in areas where a biological community has already existed but some or all of that community has been removed by small-scale disturbances that did not eliminate all life and nutrients from the environment. Although fire, flooding, and other disturbances may drive out many plants and animals and set back the biological community to an earlier stage, the community does not “start from scratch” as it would during primary succession because the soil, which contains many nutrients provided by the former biological community, remains.

In some environments, succession reaches a climax, which produces a stable community dominated by a small number of prominent species. This state of equilibrium, called the climax community, is thought to result when the web of interactions between the members of the biological community becomes so intricate that no other species can be admitted. Because changes in climate, ecological processes, and evolutionary processes cause changes in the environment over very long periods of time, the climax stage is not completely permanent.