Deconstructing occupancy frequency distributions in stream insects : effects of body size and niche characteristics in different geographical regions

1. Occupancy frequency distributions (OFDs) are one means to study species distribution patterns, allowing the delineation of rare and common species. Very few studies have deconstructed entire assemblages by ecological or biological characteristics and subsequently examined OFDs in subgroups of species. 2. The effect of deconstruction of entire assemblages by niche breadth, niche position or body size classes on OFDs in stream insects in three drainage basins was examined. It was hypothesized that OFDs should not vary between different drainage basins, but they should be affected by deconstruction into different niche breadth, niche position or body size classes. 3. The OFDs were typically strongly right-skewed in all drainage basins. The set of small-sized species was strongly dominated by rare species, whereas the importance of rare species decreased with increasing body size. Further, while the OFDs of sets of species with marginal niche position or small niche breadth were strongly dominated by rare species, those of species with non-marginal niche position or large niche breadth showed highly variable degrees of occupancy. The OFDs of non-marginal species were even uniform in the entire data and one drainage basin, providing partial support to the a priori hypothesis. 4. Niche-based explanations are likely to account for occupancies of marginal and small-niched species, whereas the distributions of non-marginal and broad-niched species may be not only affected by niche-based mechanisms but also by spatial dynamics. Deconstruction of OFDs by ecological and biological traits thus showed that the patterns may vary between different subgroups of species.