Culverts as a mechanism for habitat fragmentation in crayfish communities

Habitat fragmentation is a leading cause of species loss globally, but fragmentation effects on riverine biota, particularly invertebrates, are less well documented. Human road building activities can cause stream fragmentation particularly when culverts are used rather than bridges. Little is known about how road culverts affect community composition of benthic organisms (e.g., crayfish) in streams. We investigated the hypothesis that crayfish species differ in their ability to migrate upstream in culverts. To test this hypothesis we compared the upstream movement of Orconectes rusticus, Orconectes virilis, and Orconectes propinquus tested inside a culvert. Using a variable speed, re-circulating flume, we also determined the water velocity that crayfish could no longer move upstream. We found the mean flow velocity faster in culverts than upstream riffles. In culverts, O. rusticus moved upstream faster than O. propinquus or O. virilis. In the lab, O. rusticus tolerated flow velocities nearly 8 cm/s faster than its congeners. During summer low flow conditions in northern Michigan ~20% road crossings (with culverts) showed water velocity sufficient to exclude crayfishes. However, O. rusticus' greater capacity to pass through high velocity culverts could alter upstream crayfish community structure by facilitating its invasion and competitive displacement of native congeners.