Successional patterns on pen shell communities at local and regional scales

1. I present a successional study of marine organisms on pen shells (Atrina rigida) at different regions of St Joe Bay, Florida. By incorporating measures of relative abundance and assembly time I show how the relationship between local and regional diversity develops through different successional stages. 2. The results showed that, with time, motile species richness increases significantly while evenness indices remain high and constant. Sessile species, on the other hand, increased in both species richness and evenness through time. 3. For the motile species, regions seem to remain different while local saturation is observed. These results suggest that this group is under species-sorting: species are mobile enough that recruits and adult dispersal within a region maintain differences among regions, while local communities are saturated. 4. For the sessile species, the local-regional relationship was unsaturated at all sampling dates with both untransformed and rarefied data. Regions are initially similar in community structure, then differ through time to become similar again at the last sampling date. This may reflect a priority effect: propagules that arrive at a shell may initially exert influence on the species composition on a shell, so that at intermediate sampling times regions differ in community structure. However, at the last sampling there were no differences detected among regions, suggesting that dispersal distances might be larger for this group of species. 5. These results suggest the following. (1) The degree of species saturation will depend on the successional stage of a community. (2) Incorporating species abundances (i.e. through rarefaction or other techniques) demonstrates the role of species commonness or rarity in determining patterns of community diversity at different scales. (3) Depending on the group of species studied, the size of the region will vary and will influence the local-regional dynamics: the perceived region for sessile species may be larger than for motile species.