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Knowledge of dispersal vectors used by organisms is essential to the understanding of population and community dynamics. We report on ice rafting, a vector by which intertidal benthic invertebrates can be transported well outside their normal dispersal range during winter in temperate climates. We found multiple invertebrate taxa in sediment-laden ice blocks sampled in the intertidal zone. A large proportion of individuals were alive and active when freed from the ice. Using radio tracking, we found that ice blocks can travel over 20 km within a few days. Given the abundance of highly mobile ice blocks carrying viable invertebrates, we conclude that ice-rafting is likely an important dispersal vector, contributing to spatial community dynamics in intertidal systems. This mechanism helps explain observed genetic structure of populations, but it also raises concerns about potential negative impacts of climate change on connectivity between populations.

The understory flora of successional forest in the Delaware/Pennsylvania Piedmont zone (USA) is species poor relative to old regrowth stands in the region. Im- poverishment may reflect (a) limitations on dispersal of potential colonists, or (b) the unsuitability of successional stands for establishment. To examine the importance of dis- persal, herb and shrub distributions were surveyed in successional stands of varying age and spatial arrangement. In successional stands contiguous with species-rich old-regrowth forest, understory species richness declined with distance into the successional stands, implying contagion across the old-regrowth ecotone. Individual species showed evidence of contagion in 67% of site x species combinations. Mean rates of migration ranged from no measurable movement (e.g., Carex laxiflora, Cimicifuga racemosa) to >2 m/yr in some species (e.g., Galium aparine, Potentilla canadensis). Significant differences in rate of migration were observed among seed dispersal modes: ingested > adhesive > wind - ants - none. By contrast, there were no significant differences in rate between clonal and exclusively sexual species. In stands disjunct from old regrowth, the understory was species poor relative to old- regrowth forest due to a lack of species dispersed by ants and spores and those with no obvious dispersal vector. Understory species richness was greater in older disjunct stands and in stands closer to potential sources of propagules than in young and isolated stands, trends that were also noted in distributions of individual species. These results suggest that accessibility to colonists plays an important role in deter- mining understory composition of successional stands. Medium- and long-range migration appears to be an issue of seed and spore dispersal rather than vegetative propagation. The extremely low migration rates of some species threaten their continued existence in the second-growth forest landscape.