Dispersal within a group of locally connected habitats in a metacommunity has been predicted to increase diversity up to relatively high levels of dispersal, after which diversity may ultimately decline.  Recent theory has suggested that this effect of dispersal will be a function of among-habitat variability in environmental quality:  dispersal will have its greatest effect in increasing diversity at intermediate levels of heterogeneity.  This theory was tested for the first time using the guild of bacterivores (mostly protozoa and rotifers) found in the small natural communities in the water filled leaves of the pitcher plant, Sarracenia purpurea.  These leaves undergo strong successional patterns, with high predation by mosquito larvae and high food availability in young leaves, but low predation and resources as the host leaf ages.  Dispersal was implemented by moving 0, 0.1, or 1.0 ml within suites of six leaves (regions) every two days.  Heterogeneity was varied by using suites of leaves that were either identical in age or a combination of ages.   
Results/Conclusions

Within-leaf (alpha) diversity of bacterivores increased with dispersal, as dispersal-limited species were able to move to other local habitats.   Regional (gamma) diversity also increased with increasing dispersal, as increases in local diversity lead to increases in regional diversity.   Dissimilarity among local communities (beta diversity) decreased with dispersal, but increased with increasing heterogeneity, with an interaction between the effects of dispersal and heterogeneity.   In general, these results confirm the theoretical prediction that dispersal can affect diversity through increasing both local and, thus, regional, diversity.  However, increasing among-local habitat heterogeneity had no effect on local or regional diversity, although it did lead to an increase in beta diversity that may have contributed to the effects of dispersal.  This suggests that the bacterivore metacommunity operates through local processes of colonization and extinction among leaves rather than species-sorting to different leaves.