Exotic or introduced species are those deliberately or accidentally introduced into a river corridor by humans. Some exotic species survive but do not reach high population densities or spread across large areas. These species are typically not a problem for native species and ecosystems. Exotic invasive species are those that do so well in their new environment that they reproduce abundantly, spread widely, and displace native species. In addition to affecting native biota, some invasive species alter form and function in river corridors. An example is tamarisk (Tamarix spp.), a woody riparian plant introduced from Eurasia that forms dense thickets along river banks. Tamarisk roots help to stabilize streambanks against erosion (the original reason that these plants were introduced to North America) and the dense stems greatly increase hydraulic resistance during overbank flows, facilitating overbank sedimentation and channel narrowing. Other exotic invasive riparian species include Russian olive (Elaeagnus angustifolia), camphor tree (Cinnamomum camphora), Chinaberry (Melia azedarach), and paper mulberry (Broussonetia papyrifera). Exotic invasive aquatic species include brown trout (Salmo trutta; introduced from Europe), brook trout (Salvelinus fontinalis; native to eastern North America but introduced to western North America), rainbow trout (Oncorhynchus mykiss; native to the west coast, but introduced elsewhere in North America), common carp (Cyprinus carpio), Asian carp (bighead carp, Hypophthalmichthys nobilis; grass carp, Ctenopharyngodon Idella; black carp, Mylopharyngodon piceus; silver carp, Hypophthalmichthys molitrix), the diatom Didymosphenia geminata, rusty crayfish (Orconectes rusticus) native to the Ohio River watershed but introduced elsewhere in the United States, zebra mussels (Dreissena polymorpha), and the semi-aquatic rodent nutria (Myocastor coypus).

Sometimes the effect of an exotic invasive species is visually apparent, as in the case of tamarisk. Other exotic invasives can cause equally substantial effects that are less obvious. The substitution of one species of trout for another, as when brook, brown, and/or rainbow trout displace native cutthroat trout (Oncorhynchus spp.) species in the Rocky Mountains, can alter the stream food web. Brook trout, for example, pick invertebrate prey directly from the streambed, whereas cutthroats mostly capture insects drifting on the water surface. By eating the bottom-dwelling grazing insects, brook trout can increase streambed algae that these insects would otherwise eat. Brook trout feeding can also reduce the number of adult insects emerging from the stream by more than half, which can in turn cause a decrease in riverside spiders that eat emerging insects. This then affects the songbirds that eat spiders.

Bibliography

Baxter, C.V., K.D. Fausch, M. Murakami, and P.L. Chapman. 2004. Fish invasion restructures stream and forest food webs by interrupting reciprocal prey subsidies. Ecology, 85, 2656-2663.

Weber, M.J. and M.L. Brown. 2009. Effects of common carp on aquatic ecosystems 80 years after ‘carp as a dominant’: ecological insights for fisheries management. Reviews in Fisheries Science, 17, 524-537.