COMMUNITIES CHANGING OVER TIME

Ecosystems gradually change over time, showing slight fluctuations around a state of equilibrium. The structure of an ecosystem is constantly changing in response to changing environmental conditions. The structure of an ecosystem refers to:
1) number of individuals in each species
2) number of different species
3) number of roles that species play in an ecosystem
4) physical appearance of the species in an ecosystem



The gradual change of an ecosytem is called succession. It involves a complex series of species interactions over time. During succession, some species colonize increasing their populations. Other populations may decline. Succession may occur as a result of disturbing a community of organisms--removing species, destroying or altering soil or sediments. Some examples where succession might occur include: burned or cut forests, abandoned farms, or heavily polluted streams.Ecosystems that have been altered by humans (ie. cleared land, lawns) generally lack the diversity of a natural ecosystem.


Humans destroy many of the "natural" plants and animal interactions as they alter an ecosystem so that the plants and animals they value most can grow without interference from the original organisms in the ecosystem. If the cleared land or lawns are abandoned, some of the naturally occurring species may return over time. In most of Michigan, the process leads toward the development of mixed and hardwood or evergreen forests.

Ecosystems that are not disturbed by humans also change over time. Most ponds and lake if left alone, gradually get smaller in size and shallower over time, often drying up competely. The organisms that live in or near these ponds and lakes also change as available resources change.