Ecosystem engineers: feedback and population dynamics.

All organisms alter their abiotic environment, but ecosystem engineers are species with abiotic effects that may have to be explicitly accounted for when making predictions about population and community dynamics. The goal of this analysis is to identify those conditions in which engineering leads to population dynamics that are qualitatively different than one would predict using models that incorporate only biotic interactions. We present a simple model coupling an ecosystem engineer and the abiotic environment. We assume that the engineer alters environmental conditions at a rate dependent on engineer density and that the environment decays back to original conditions at an exponential rate. We determine when the feedback to population dynamics through environmental state can lead to altered equilibrium densities, bistability, or runaway growth of the engineer population. The conditions leading to changes in dynamics, such as susceptibility of a system to engineering or alteration of density-dependent and density-independent controls, define cases in which the engineering concept is essential for ecological understanding.