THE EFFECTS OF BETWEEN-HABITAT DISPERSAL RATE ON PROTIST COMMUNITIES AND METACOMMUNITIES IN MICROCOSMS AT 2 SPATIAL SCALES

The effect of manipulation of between-habitat dispersal rates in multi ple patch systems was examined experimentally using protist communitie s in laboratory microcosms. Replicate ''landscapes'' of eight microcos ms (patches) at two spatial scales (patch sizes) were inoculated with 13 species of protists. Dispersal was carried out by transferring a sm all random sample of medium and protists from one randomly selected mi crocosm to another within a landscape. Four dispersal rates (24, 6, 2 and 0 transfers every 3 days) were used, and the microcosms were sampl ed after 6 and 12 weeks. Patch size had a consistent effect on within- patch (community) and within-landscape (metacommunity) diversity, both being lower in small patch systems. Higher dispersal rates had a slig ht effect on community and metacommunity diversity after 12 weeks, wit h a tendency for higher dispersal to slightly offset the rate of loss of species. Both dispersal and patch size had effects on the abundance of many individual species, though in a variety of ways. The individu al species results suggest that extinction is selective with respect t o both patch size and dispersal rate treatments, and may be influenced by species interactions. It seems likely that in metacommunity system s of this sort, rather than mainland-island systems, the potential eff ect of between-patch dispersal rate in rescuing and recolonizing where local extinctions occur may be much reduced by the effect of selectiv e extinction, relative to that expected under the assumption of random extinction.