An ecosystem engineer is any species that creates, significantly modifies, maintains or destroys a habitat. These organisms can have a large impact on species richness and landscape-level heterogeneity of an area. As a result, ecosystem engineers are important for maintaining the health and stability of the environment they are living in. Since all organisms impact the environment they live in one way or another, it has been proposed that the term "ecosystem engineers" be used only for keystone species whose behavior very strongly affects other organisms.
Beavers are the prototypical ecosystem engineer because of the effects their dams have on channel flow, geomorphology, and ecology.
Kelp are autogenic ecosystem engineers, by building the necessary structure for kelp forests
The Gordon Dam in Tasmania
Beaver dam on Smilga River in Lithuania
Keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. Some keystone species, such as the wolf and lion are also apex predators.
The jaguar: a keystone, flagship, and umbrella species, and an apex predator
The beaver: a keystone species, and habitat creator, responsible for the creation of lakes, canals and wetlands irrigating large forests and creating ecosystems
Ochre seastars (Pisaster ochraceus), a keystone predator
California mussels (Mytilus californianus), the seastar's prey