An ecosystem engineer is any organism that creates or modifies habitats. Jones et al (1994) identified two different types of ecosystem engineers:
- Allogenic engineers modify the environment by mechanically changing materials from one form to another. Beavers are archetypal ecosystem engineers; in the process of clearcutting and damming, beavers alter their ecosystem extensively. Different types and numbers of other organisms will thrive in the region of a beaver dam than would in a non-dammed region. Caterpillars that create shelters from leaves are also creating shelters for other organisms which may occupy them either simultaneously or subsequently.
- Autogenic engineers modify the environment by modifying themselves. As trees grow, their trunks and branches create habitats for other living things. In the tropics, lianas connect trees, which allow many animals to travel exclusively through the forest canopy.
Introduced species are often ecosystem engineers. Kudzu, a leguminous plant introduced to the southeast U.S., changes the distribution and number of animal and bird species in the areas it invades. It also crowds out native plant species. The zebra mussel is an ecosystem engineer in North America. By providing refuge from predators, it encourages the growth of freshwater invertebrates through increasing microhabitats. Light penetration into infected lakes also improves, resulting in an increase in algae.