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 nonliving materials from one physical state to another, via mechanical or other means. Here we define and explain engineering, and provide a classification and some general, conceptual models of the processes involved. We show how organismal engineering is related to human engineering and to ecological concepts such as keystone species. We then identify the factors scaling the impact of engineers. 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. We argue that all habitats on earth support, and are influenced to some degree, by ecosystem engineers, and we raise some general questions about organismal engineering that are worth pursuing |
