Regional Parapatry of the Congeneric Cyprinids Lythrurus snelsoni and L. umbratilis: Species Replacement along a Complex Environmental Gradient

Here we describe and attempt to explain a parapatric distributional pattern involving two congeneric cyprinid species, Lythrurus snelsoni and Lythrurus umbratilis, in the Little River system of southeastern Oklahoma and southwestern Arkansas. Relationships among species within the genus Lythrurus are largely unresolved. Lythrurus snelsoni is a regional endemic that is limited to upland streams of the Little River system. Lythrurus umbratilis is much more widely distributed, occupying upland and lowland stream habitats throughout the midwest and southeast. We document species replacement at a regional scale and ask whether the pattern can be explained based on measured environmental features. Answering such a question is a first and important step in determining what ecological factors contribute to species borders.