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Resource selection and space use of a native and an invasive crayfish: evidence for competitive exclusion?

Abstract.  Competitive exclusion is a commonly cited mechanism of species replacement in nonindigenous crayfish introductions. However, even when it seems highly plausible, it may not be the driving mechanism. We sought to identify competition-driven habitat shifts in populations of native St Francis River crayfish Orconectes quadruncus and invasive woodland crayfish Orconectes hylas. Crayfish were tagged with passive integrated transponder (PIT) tags at locations where the 2 species were sympatric and where the native was allopatric. Information was collected on crayfish habitat use, habitat availability, and habitat associations during multiple seasons. Discrete choice analysis and model selection results indicated that both species selected large, concealing substrate and slower current velocities. We observed no significant shifts in habitat use by the native species when sympatric with the invasive species. A high degree of space-use overlap between the species and few differences in how species used or selected habitats indicated a high potential for competition and provided no evidence that competitive exclusion had occurred. No solid explanation for species replacement exists, but it appears that competitive exclusion for space and habitat does not occur or occurs slowly at some sympatric localities.