Parapatric Crayfishes in Southern Ohio: Evidence of Competitive Exclusion?

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Abstract
A survey of lotic crayfish distributions in the Brush Creek drainage basin of southern Ohio was undertaken to test the hypothesis that narrowly overlapping distribution patterns of Orconectes rusticus and O. sanbornii sanbornii within streams are determined by substrate lithology (Rhoades, 1962); we found no such relationship. Rather, an essentially parapatric distribution was observed along gradients from headwaters to major streams. In areas of parapatry, O. rusticus occurred in lower reaches of tributaries and in major trunk streams, whereas O. s. sanbornii was restricted to isolated populations inhabiting headwaters. In other parts of the basin, O. rusticus alone occupied habitats ranging from headwaters to high-order streams. We postulate that in the absence of O. rusticus, O. s. sanbornii would likewise occupy the full continuum of lotic habitats (i.e., ecological release). In light of this and recent evidence implicating invasions by O. rusticus in apparent species displacements, we propose that competitive exclusion is an important factor influencing the distributions of these species in southern Ohio.