Changes in the distribution of Michigan crayfishes and the influence of invasive rusty crayfish ( Faxonius rusticus ) on native crayfish substrate associations

Invasive crayfishes have a strong negative effect on multiple trophic levels, including other crayfishes. However, documentation of the spread of non-native crayfish species and their impact on native crayfishes could be improved, particularly over large spatial scales in stream ecosystems. We collected crayfish and quantified habitat at 461 stream sites throughout Michigan in 2014–2016 and compared our collections to a historical account of crayfish collections published in 1975. Our objectives were to: (1) quantify the change in non-native rusty crayfish ( Faxonius rusticus ) and native species distributions from 1975 to 2014–2016; (2) quantify how rusty crayfish affect the habitat associations of native species in Michigan streams; and (3) determine the effectiveness of dipnets, our primary sampling method. We found all species in more watersheds compared to 1975, likely due in part to increased sampling. However, we found rusty crayfish in 22 more HUC-8 watersheds than in 1975, a larger increase than all other species. Habitat associations of native species also shift in the presence of rusty crayfish. In instances where native species co-occurred with rusty crayfish, most obligate aquatic native species were found in less-preferred habitat such as sand or macrophytes compared to cobble substrate when the species is in isolation. Our results indicate a broad range expansion by rusty crayfish over the last 40 years, suggesting that surveys of crayfish diversity and habitat occupancy should be more routine to inform management of native crayfish species.