Spatial distributions of common and rare species in a widespread group of stream insects
Abstract
The phenomenon of rarity is understudied in freshwater systems compared with terrestrial systems. We examined whether spatial distributions of rare species of black flies (Diptera: Simuliidae) differed from those of common species. We used mean pairwise distance between stream sites as a proxy for dispersal ability to describe spatial distributions. From a master database of 1445 North American stream-site collections, in which all immature black flies were identified to species, we created one database of 12 ecoregion-level data sets (476 sites) and a second database of 5 larger area-level data sets (504 sites) to address the following questions: 1) Does the spatial distribution (clumped, random, or overdispersed) differ between common and rare species? 2) Does the mean distance among sites occupied by rare species differ from that for common species? 3) Is species frequency (species occurrences/sites in a data set) related to distance among occupied sites? 4) Does the classification of an individual species as common, rare, or vagrant vary with location? We found that 1) spatial distribution patterns were generally the same for common and rare species, 2) mean distances among stream sites typically did not differ between common and rare species, 3) species frequency was rarely related to distance among occupied sites, and 4) species classified as common, rare, or vagrant in one ecoregion data set were often classified differently in other data sets, indicating the importance of location in determining the status of a species. The results suggest that rare species have dispersal abilities similar to those of common species.