Size-mediated competitive interactions between an invasive and an imperiled crayfish may explain extirpation of the imperiled species

Interference competition between native and invasive species can be an important driver of the local extirpation of native species; however, extinctions resulting from competition are rare. This study investigates competitive interactions between an invasive and an imperiled species to assess whether competition is an important mechanism behind this species replacement. Freshwater crayfish are one of the most imperiled taxonomic groups in North America, and nonnative crayfish pose a major threat to native crayfishes. Many crayfish have limited distributions, so merely moving crayfish between adjacent drainages can cause species replacements that threaten native species. Here, we examine competitive interactions between the imperiled Black Creek crayfish (BCC; Procambarus pictus), which is endemic to the lower St. Johns River drainage, Florida, and the white tubercled crayfish (WTC; P. spiculifer), an introduced species from a neighboring drainage. We found that WTC grew more rapidly than BCC in common conditions, and when WTC was larger, this species won aggressive interactions and was dominant in shelter competition with the imperiled species. However, when the species were size matched, BCC was more competitive than WTC. These results highlight the importance of size and growth rate for determining the outcome of interference competition. WTC is replacing BCC throughout a substantial portion of its limited range, and our results suggest that size-mediated competition between these species may be an important mechanism for this species replacement.