Although some crayfish can travel overland, many species are fully aquatic. Some have been documented moving long distances within streams and all probably depend on smaller scale movements to maintain continuous and interconnected populations. In headwater stream systems of the Ozarks and southern Appalachians, crayfish are dominant components of these ecosystems, rivaling aquatic insects in importance. Many headwater populations have been isolated long enough (due to natural conditions) to become separate species. In these regions of the U.S., headwater streams support many rare crayfish with very limited distribution. Further population fragmentation could imperil entire species of crayfish.
As a group, the most vulnerable animal species in the U.S. are freshwater mussels. Over 70 percent of the 297 species native to the U.S. and Canada are endangered, threatened, or of special concern (Williams, et al. 1993, Fisheries 18(9):6-22). Although adult mussels have a very limited capacity for movement, dispersal typically occurs when larvae (glochidia) attach themselves to host fish or salamanders. Therefore, survival and persistence of freshwater mussel populations is dependent on the capacity of host fish to move through river and stream systems. Many endangered mussels depend on small, sedentary host fish that are typically weak swimmers and therefore highly vulnerable to movement barriers.
River and stream ecosystems contain many other species for which we know little except that they probably have limited capacities for movement. These include worms, flatworms, leeches, mites, amphipods, isopods, and snails. Collectively, these often overlooked taxa account for a significant amount of the biomass and diversity of river and stream ecosystems. For most, swimming ability is less relevant than the ability to move through streambed substrates. Although large numbers of invertebrates can often be supported in relatively small areas, appropriate habitats may be patchy and dynamic. In these situations, a regional population is generally maintained through cycles of local extinction and colonization in response to changes in habitat conditions. Scour and deposition related to flooding or changes in stream hydraulics (e.g. debris dams and deflectors) may destroy habitat in some areas while creating suitable habitat in others. It is unclear how these organisms move upstream any significant distance. It is safe to assume that some mechanism must exist or else populations would continually shift downstream as upstream populations are lost to local extinctions. One possible mechanism for such movements is when small organisms or eggs are transported by larger animals, perhaps in association with adhered sediment or debris.