ORGANISMS AS ECOSYSTEM ENGINEERS

Ecosystem engineers are organisms that directly or indirectly modulate the availability of resources to other species, by causing physical s tate changes in biotic or abiotic materials. In so doing they modify, maintain and create habitats. Autogenic engineers (e.g. corals, or tre es) change the environment via their own physical structures (i.e. the ir living and dead tissues). Allogenic engineers (e.g. woodpeckers, be avers) change the environment by transforming living or non-living mat erials from one physical state to another, via mechanical or other mea ns. 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 t hemselves. We recognise and define five types of engineering and provi de examples. Humans are allogenic engineers par excellence, and also m imic the behaviour of autogenic engineers, for example by constructing glasshouses. We explore related concepts including the notions of ext ended phenotypes and keystone species. Some (but not all) products of ecosystem engineering are extended phenotypes. Many (perhaps most) imp acts of keystone species include not only trophic effects, but also en gineers and engineering. Engineers differ in their impacts. The bigges t effects are attributable to species with large per capita impacts, l iving at high densities, over large areas for a long time, giving rise to structures that persist for millennia and that modulate many resou rce flows (e.g. mima mounds created by fossorial rodents). The ephemer al nests constructed by small, passerine birds lie at the opposite end of this continuum. We provide a tentative research agenda for an expl oration of the phenomenon of organisms as ecosystem engineers, and sug gest that all habitats on earth support, and are influenced by, ecosys tem engineers.