How can an organism have positive and negative effects on an ecosystem?

Physical ecosystem engineers are organisms that directly or indirectly control the availability of resources ot other organisms by causing physical state changes in biotic or abiotic materials. Physical ecosystem engineering by organisms is the physical modification, maintenance, or creation of habitats. Ecological effects of engineers on many other species occur in virtually all ecosystems because the physical state changes directly create non-food resources such as living space, directly control abiotic resources, and indirectly modulate abiotic forces that, in turn, affect resource use by other organisms. Trophic interactions and resource competition do not constitute engineering. Engineering can have significant or trivial effects on other species, may involve the physical structure of an organism (like a tree) or structures made by an organism(like a beaver dam), and can but does not invariably, have feedback effects on the engineer. We argue that engineering has both negative and positive effects on species richness and abundances at a small scales, but the net effects are probably positive at larger scales encompassing engineered and non-engineered environments in ecological and evolution space and time. Models of the population dynaimcs of the engineers suggest that the engineer/habitat equilibrium is often, but not always, locally stable and may show long-term cycles, with potential ramifications for community and ecosystem stability. As yet, data adequate to parameterise such a model do not exist for any engineer species. Because engineers control flow of energy and materials but do not have to participate in these flows, energy, mass and stoichiometry do not appear to be useful in predicting which engineers have big effects. Empirical observations suggest some potential generalisations about which species will be important engineers in which ecosystems. We point out some of the obvious, and not so obvious, ways in which engineering and trophic relations interact, and we call for greater research on physical ecosystem engineers, their impacts, and their interface with trophic relations.