Ecosystem engineers are organisms that directly or indirectly modulate the availability
of resources to other species, by causing physical state changes in biotic or abiotic
materials. In so doing they modify, maintain and create habitats. Autogenic engineers
(e.g. corals, or trees) change the environment via their own physical structures (i.e.
their living and dead tissues). Allogenic engineers (e.g. woodpeckers, beavers) change
the environment by transforming living or non-living materials from one physical state
to another, via mechanical or other means. The direct provision of resources to other
species, in the form of living or dead tissues is not engineering. Organisms act as
engineers when they modulate the supply of a resource or resources other than
themselves. We recognise and define five types of engineering and provide examples.
Humans are allogenic engineers par excellence, and also mimic the behaviour of
autogenic engineers, for example by constructing glasshouses. We explore related
concepts including the notions of extended phenotypes and keystone species. Some
(but not all) products of ecosystem engineering are extended phenotypes. Many
(perhaps most) impacts of keystone species include not only trophic effects, but also
engineers and engineering. Engineers differ in their impacts. The biggest effects are
attributable to species with large per capita impacts, living at high densities, over large
areas for a long time, giving rise to structures that persist for millennia and that
modulate many resource flows (e.g. mima mounds created by fossorial rodents). The
ephemeral nests constructed by small, passerine birds lie at the opposite end of this
continuum. We provide a tentative research agenda for an exploration of the phenomenon
of organisms as ecosystem engineers, and suggest that all habitats on earth
support, and are influenced by, ecosystem engineers