What is ecological succession?

Ecological succession is the process in which the structure and composition of a community evolves over time. This process is predictable for the most part.

Species composition, density, and distribution of that community is constantly changing as time progresses.

The first species to arrive either after a disturbance, such as a flood, or when the land is first formed, such as a new volcanic island, are called pioneer species. They are the first colonizers. Grasses, mosses, lichens, and other plants are pioneer species. Pioneer species may even be able to survive without soil. These pioneer species are often quite hardy and are thus able to survive in a harsh environment. Pioneer species also typically have light seeds that disperse easily through wind.

As these pioneer plants live, attract consumers, and die, soil is either formed or improved to the point where other plants will begin to grow. As these new species reach and spread across the environment, it is no longer suitable for pioneer species, which will eventually disappear or compose a minimal part of the community.

Herbs and then shrubs will eventually arrive and finally trees. Below is an example of ecological succession of a boreal forest.

A community may eventually reach its climax, the point where the composition remains mostly stable unless some sort of disturbance occurs or changes (climatic, evolutionary) happen over a very long period of time. The community would then be called a climax community .

Sources:
http://www.field-studies-council.org/urbaneco/urbaneco/introduction/succession.htm
http://www.countrysideinfo.co.uk/successn/intro2.htm