Definitions of rarity for stream fishes and factors associated with rarity

Conserving rare species and protecting biodiversity depend on sound information on rarity; however, rarity in fishes is not well understood.  In addition, site-level rarity has been defined by many different criteria, presenting the need for a quantitative definition of rarity.  We performed non-linear quantile regression on two national databases to determine stable proportional and numerical abundance rarity criteria. A species was considered rare if it met at least two of the following three criteria: (1) numerical abundance <20 individuals, (2) proportional abundance <5%, or (3) proportional abundance <1/species richness.  Thirty three percent of 399 species classified were rare at all sites that they occurred and 10% were common everywhere they occurred.  The remaining species could be used to investigate how environmental factors drive rarity in stream fishes.  We used logistic regressions to examine the relationship between rarity and habitat variables at the landscape, watershed, and reach spatial grains.   We found that landscape and watershed habitat variables best explained rarity and significant explanatory variables varied greatly among species.  Understanding the potential factors driving rarity is important for conserving rare fishes by helping researchers identify areas likely to support viable populations and plan captive breeding and translocation programs effectively.