Experimental Zoogeography: Colonization of Marine Mini-Islands

Plastic mesh sponges, considered to be islands for a variety of marine invertebrates, are used to explore island colonization in the marine environment. Colonization of small and large sponges, simultaneously submerged either near or far from an algal bed thought to be a potential source of colonists, is compared with expectations derived from the MacArthur-Wilson equilibrium model of island colonization. Conclusions in this study are based on colonists from five phyla comprising approximately 220 species and 26,000 individuals. Inspection of colonization curves (species present vs. time) for several sponge series indicates to varying degrees a stabilization of species numbers over time when time is plotted linearly. This impression is confirmed statistically by showing that a significant decrease in slope occurs in the later portion of the colonization curve. This supports the equilibrium concept of the MacArthur-Wilson model. Data from an identical, but much shorter, set of experiments begun during a slightly warmer time of year show little or no seasonal differences in the shape of the colonization curve. Assuming that equilibrium is being approached, we can draw the following additional conclusions. As the MacArthur-Wilson model predicts, more species are present at equilibrium on larger than on smaller sponge islands equidistant from their source area. On sponges of equivalent age, the average number of individuals is always greater and 57%-92% of the shared species are more abundant on the larger sponge: when species numbers in both sponge sets are similar, there is some evidence supporting MacArthur and Wilson's reason for expecting higher extinction rates on smaller islands. They assume that the probability of extinction is greater if population sizes are smaller. In view of actual differences in total individual abundances throughout the colonization period, large population sizes on larger sponges may also act to raise the extinction rate. The equilibrial number of species on near and far sponges of the same age, as well as their colonization curves, does not differ substantially, possibly because a source area other than the algal bed is involved. Minimal turnover rates for islands at equilibrium are high, between one and two species per day. Large islands have higher turnover rates than small ones the same distance from their source-just the reverse of the MacArthur-Wilson prediction. Variation in trophic structure during colonization is examined for these initially barren islands. Patterns for a few taxa give a rough but general impression that organisms taking small food items from the water column or deposited on the substrate are present earlier than are active searchers. In one particular taxon (polychaetes), sessile families taking smaller food sizes are present before more active families taking larger prey sizes.