Colonization of artificial seagrass versus time and distance from source

Are seagrass-associated communities static? In order to measure the ability of seagrass-associated macrofauna to colonize 'empty' habitat patches, we studied the colonization of artificial seagrass, a natural seagrass mimic. In 2 separate experiments, we measured colonization (1) over a time series of 0.5 to 16 d and (2) over a distance series of 0 to 15 m from the source pool of colonists in a seagrass meadow in the Indian River lagoon, Florida. Colonization over time was rapid. Abundance and species richness peaked in 4 to 8 d, while abundances of individual species continued to fluctuate. Colonization was rapid at all distances from the seagrass bed. Although gastropod abundance decreased with increasing distance, abundance of crustaceans increased with distance. At the margin of the seagrass bed, gastropods and crustaceans colonized in approximately equal abundance, whereas 15 m from the bed, crustaceans colonized 6 to 8 times as abundantly as gastropods. Relative to natural densities, the dominant crustacean Cymadusa compta (Amphipoda) colonized 38 times as abundantly as the dominant gastropod, Bittium varium. We attribute this increased crustacean colonization far from grassbeds to the 'nearest refuge' hypothesis, but also suggest an alternative predation hypothesis. Thus, seagrass-associated communities are not static. The short time scale of these experimental colonizations implies that the maintenance of strong correlations between plant and animal abundances in seagrass meadows requires strong pressures to counterbalance such rapid immigration (e.g. intense predation and/or habitat selection).