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 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 moraines (gravel deposits formed where the glacier front was stationary for a number of years) of different ages.
Because nitrogen is virtually absent from recently exposed moraines, only a few plants can grow. In the case of Glacier Bay, two plants that will grow are a herbaceous plant (Dryas) and alder trees (Alnus), both of which have a nitrogen-fixing bacteria in nodules on their roots.
Nitrogen fixation by these bacteria improves the nutrient properties of the soil. As soil condition improves, alder stands are invaded by spruce.
Spruces then outcompete and displace the alders and Dryas. If the local climate does not change dramatically, a forest community dominated by spruces is likely to persist for many centuries.
By looking at moraines 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.