When ecological communities are disturbed, they sometimes recover their original characteristics, although not necessarily precisely their original species, through a process called succession. In other cases, termed ecological transitions, disturbance leads instead to the eventual development of a community that is distinctly different from the original community.
The patterns of ecological succession are varied, but the species that colonize a site soon after the disturbance often alter environmental conditions so that they become favorable or unfavorable for other species. A good example is the change in the plant community that followed the retreat of a glacier in Glacier Bay, Alaska, over the last 200 years. No human observer was present to record changes over the 200-year period, but ecologists have inferred the temporal pattern of succession by measuring plant communities on gravel deposits that were exposed by retreating glaciers at different times.
By looking at gravel deposits of different ages, ecologists have been able to deduce the process of succession at Glacier Bay, Alaska. The pattern of succession in this area illustrates how succession is caused in part by changes in the soil brought about by the plants themselves and in part by competitive interactions among the plants.
Textbook Reference: Concept 44.2 Communities Change over Space and Time