THE EFFECTS OF HABITAT SIZE AND PRODUCTIVITY ON FOOD-WEB STRUCTURE INSMALL AQUATIC MICROCOSMS

We discuss two mechanisms by which habitat size could affect food web structure: productive space and spatial effects on the persistence of unstable interactions. We assembled communities of bacteria, protozoa, rotifers, microcrustacea and Hydra in laboratory microcosms, and inde pendently manipulated habitat size and total energy input, in order to distinguish between these mechanisms. After 163 d, larger habitats su pported food webs with more species, more links per species and longer food chains, even in the absence of differences in total energy input . There were no significant differences in food web structure between energy treatments. Some species' relative abundances were affected by habitat size or energy, but there was no consistent overall pattern of responses. These results do not support the productive space hypothes is, but suggest that spatial effects on the persistence of unstable fo od webs may be important, although differences in disturbance between different sizes of microcosm may also have been a factor.