The effects of between-habitat dispersal rate on protist communities and metacommunities in microcosms at two spatial scales

The effect of manipulation of between-habitat dispersal rates in multiple patch systems was examined experimentally using protist communities in laboratory microcosms. Replicate ''landscapes'' of eight microcosms (patches) at two spatial scales (patch sizes) were inoculated with 13 species of protists. Dispersal was carried out by transferring a small random sample of medium and protists from one randomly selected microcosm to another within a landscape. Four dispersal rates (24, 6, 2 and 0 transfers every 3 days) were used, and the microcosms were sampled 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 slight effect on community and metacommunity diversity after 12 weeks, with 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 individual species results suggest that extinction is selective with respect to both patch size and dispersal rate treatments, and may be influenced by species interactions. It seems likely that in metacommunity systems of this sort, rather than mainland-island systems, the potential effect of between-patch dispersal rate in rescuing and recolonizing where local extinctions occur may be much reduced by the effect of selective extinction, relative to that expected under the assumption of random extinction.