Organisms that substantially as well as deliberately modify environments.
The prime example of an ecosystem engineer are , which by building literally create .
Beavers specifically can be described as . Alternatively are which instead of modifying the environments outside of their bodies instead modify environments by virtue of generating their bodies. A good example of the latter are trees. Trees, that is, literally create from their own bodies .
It is important to keep in mind that the idea that these efforts are "deliberate" by ecosystem engineers is not intended to imply that the same are effecting these modifications. Instead, it is meant to imply that the end result is considered to be for the species involved rather than (contrast the consequence of by specific species on community diversity and functioning).
See also the concept of keystone species.