Mechanisms, effects, and scales of dispersal in freshwater zooplankton

The distribution of organisms can be regulated by local environmental factors and regional processes such as dispersal. Here, we review recent work on the role of dispersal for generating population and community structure in freshwater zooplankton. We examine evidence for different mechanisms of dispersal among lakes, for the effects of dispersal limitations on populations and communities, and for the effects of spatial scale on dispersal rates. Zooplankton move via human or animal vectors, flowing surface waters, and wind; the relative importance of the different modes of transport is poorly understood. Several lines of evidence suggest that dispersal among lakes separated over short spatial scales (<10 km) is sufficiently rapid that local interactions should limit species diversity and composition more than the supply of colonists. However, dispersal limitation over broad scales (tens to thousands of kilometers) might constrain geographic ranges and influence community structure. The current explosion in the incidence of exotic species indicates that such global- or continental-scale dispersal was limiting in the past. The spread of exotic species also provides opportunities to study the scale dependence of zooplankton dispersal. We show how patterns of range expansion can be used to estimate the change in invasion likelihood with distance to a source population. Such dispersal functions provide a crucial link between small-scale experimental studies and broad-scale geographic patterns.