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 s eagrass bed (Gazi Bay, Kenya) with a clear zonation of different seagrass s pecies. The application of an appropriate sampling strategy made the interp retation of different spatial diversity levels possible. Alpha diversity wa s defined as the diversity of harpacticoid copepods associated with 1 seagr ass species or 1 subhabitat (roots or leaves). Beta diversity was interpret ed as changes in diversity between both subhabitats of 1 seagrass species a nd 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 (1 0.4 %) were only recovered from leaf samples. Higher diversity was recorded for the deeper seagrass species (Syringodium isoetifolium, Halophila stipu lacea). Copepod communities associated with Halophila ovalis and H, stipula cea (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 sampl es was clearly different from the typical right-skewed curves reported in m ost terrestrial studies. This may provide evidence for fundamental differen ces between marine species and terrestrial ones in their range size distrib ution.