THE EFFECTS OF ENERGY INPUT, IMMIGRATION AND HABITAT SIZE ON FOOD-WEBSTRUCTURE - A MICROCOSM EXPERIMENT

It has been hypothesised that larger habitats should support more comp lex food webs. We consider three mechanisms which could lead to this p attern. These are increased immigration rates, increased total product ivity and spatial effects on the persistence of unstable interactions. Experiments designed to discriminate between these mechanisms were ca rried out in laboratory aquatic microcosm communities of protista and bacteria, by independently manipulating habitat size, total productivi ty and immigration rate. Larger habitats supported more complex food w ebs, with more species, more links per species and longer maximum and mean food chains, even in the absence of differences in total energy i nput. Increased immigration rate resulted in more complex food webs, b ut habitats with higher energy input per unit area supported less comp lex food webs. We conclude that spatial effects on the persistence of unstable interactions, and variation in immigration rates, are plausib le mechanisms by which habitat size could affect food web structure. V ariation in total productivity with habitat area seems a less likely e xplanation for variation in food web structure.