Alpha and beta diversity of harpacticoid copepods in a tropical seagrass bed: the relation between diversity and species' range size distribution

Alpha and beta diversity of harpacticoid copepods was studied in a Kenyan seagrass bed (Gazi Bay, Kenya) with a clear zonation of different seagrass species. The application of an appropriate sampling strategy made the interpretation of different spatial diversity levels possible. Alpha diversity was defined as the diversity of harpacticoid copepods associated with 1 seagrass species or 1 subhabitat (roots or leaves). Beta diversity was interpreted as changes in diversity between both subhabitats of 1 seagrass species and between different seagrass species along the tidal gradient. A total of 115 harpacticoid copepod species were recorded in the seagrass samples. Of these, 36 species (31.3 %) were restricted to the root subhabitat and 12 (10.4 %) were only recovered from leaf samples. Higher diversity was recorded for the deeper seagrass species (Syringodium isoetifolium, Halophila stipulacea). Copepod communities associated with Halophila ovalis and H, stipulacea (both pioneer seagrass species) were clearly different from one another in terms of diversity. A trend towards more specialized habitat preference (i.e. a lower ecological range size) was found with increasing diversity. The left-skewed species' range size distribution for the more diverse samples was clearly different from the typical right-skewed curves reported in most terrestrial studies. This may provide evidence for fundamental differences between marine species and terrestrial ones in their range size distribution.