discharge, primarily because of winter snowmelt, but far fewer species than the two eastern streams.
Lentic versus Lotic Systems
Somewhat akin to the near-equal split between freshwater and marine fishes, despite the overwhelming preponderance of marine habitats, the North American freshwater fish fauna is primarily a fauna of lotic (flowing water) rather than lentic (standing water) systems despite the much greater volume of lentic habitats (Figure 1.1). On a worldwide basis, inland lentic habitats make up 98.88% of liquid water available for aquatic organisms, compared to 1.12% for lotic systems (data from Shiklomanov 1993). Species richness in lakes is indeed higher in some parts of the world, and certain genera or families of fishes have radiated extensively to form “species flocks”—groups of closely related species found in a restricted geographical area. Examples of species flocks include the cichlid species of African Lakes and the sculpin (Cottidae) fauna of Lake Baikal, Russia (Fryer and Iles 1972; Echelle and Kornfield 1984; G. R. Smith and Todd 1984). However, in contemporary North America, the number of species unique to lakes is much lower, primarily because of the young age of large North American lakes (G. R. Smith 1981). The silversides (genus Menidia) from Mexico’s largest natural lake, Lake Chapala on the Mexican Plateau, provide an example of a small North American species flock with 12 species either restricted to the lake or also occurring in the surrounding streams (Barbour 1973; Miller 2005).
TABLE 1.1 Families Composing 95% of North American Freshwater Fish Species Ranked by the Number of Native Species
FIGURE 1.4. North American topography illustrating the tectonically active regions of the west; the erosional areas to the east of the western mountains; and the lower, depositional areas primarily in eastern North America. Colors indicate topographic elevation, from white at the highest elevations grading through gray, tan, yellow, and green for progressively lower elevations. The image of North America was generated with data from the Shuttle Radar Topography Mission (SRTM). Courtesy of NASA/JPL-Caltech.
FIGURE 1.5. The number of fish species in equal-sized grids across North America, and the limit of Pleistocene glaciations (solid blue line). Reproduced with permission from G. R. Smith.
FIGURE 1.6. West to east contrasts in stream discharge and fish species diversity.
A. Locations of four streams along an east to west pattern in North America.
B. Average annual discharge (m3/s) and fish species richness. Average annual discharge in m3/s from USGS (2007) as determined from the USGS gauge. Species richness is for the entire river. Sources: Hillsborough River (fish diversity: unpublished data, B. S. Barnett 1972); Okatoma Creek (fish diversity: Ross et al. 1992a); San Juan River (fish diversity: Propst and Gido 2004); South Fork Eel River (fish diversity: Brown and Moyle 1997).
The Laurentian Great Lakes also support (or supported, because some taxa are now extinct or restricted to only a fraction of their former range) an incipient species flock of perhaps 8–9 species/morphotypes of ciscoes (family Salmonidae, trouts and salmons, subfamily Coregoninae) (G. R. Smith and Todd 1984; Underhill 1986; Turgeon et al. 1999; Cudmore-Vokey and Crossman 2000; Etnier and Skelton 2003). Overall, the known native fish fauna of the Laurentian Great Lakes, excluding the St. Lawrence River and tributaries and including extirpated or extinct taxa, has comprised 126 species, but only 5% are endemic (primarily ciscoes, Coregonus spp.) (Cudmore-Vokey and Crossman 2000). In contrast, 17.9% of the rich lotic Tennessee River fish fauna (approximately 229 species and subspecies) is endemic (Etnier and Starnes 1993; Warren et al. 2000). The Great Lakes fauna is primarily derived from the upper Mississippi River drainage, streams of the Atlantic coastal plain, and the Beringian Refugium of the Yukon Valley, following retreat of the Pleistocene glaciers (Underhill 1986; see also Chapter 3).
This is not to say that North America has never supported large lacustrine species flocks, but only that such faunas are presently uncommon. For instance, the fossil sculpin fauna (Myoxocephalus and Kerocottus) of the Pliocene Glenn’s Ferry Formation in southwest Idaho provides one of the better examples of a North American lacustrine species flock (G. R. Smith 1981), as does the rich fossil semionotid gar fauna from the Mesozoic Newark lakes of eastern North America (McCune et al. 1984).
Because of the influence on lentic assemblages by species from lotic environments, Kitchell et al. (1977) proposed the term “River Analogy” to explain the distribution of large percid fishes. They argued that most North American and European lakes were of recent origin (i.e., Pleistocene or later), that lake-inhabiting fishes had a riverine ancestry, and that pool habitats in low-gradient rivers (sloughs, oxbows) were analogous to littoral lake habitats. Ross and Matthews (in press) suggested, as did Kitchell et al. (1977), that the river analogy applies to many other groups of lake-inhabiting fishes in North America. Furthermore, fishes of large rivers may use habitats in new lakes (or impoundments) similarly to their use of unimpounded, large-volume habitats, for example, Blue Catfish (Ictalurus furcatus) versus Channel Catfish (I. punctatus) in Lake Texoma (Edds et al. 2002). Thus, although habitats occupied by North American fishes can, at first glance, be separated into lentic and lotic categories, with a few exceptions, this may not be the most meaningful ecological axis along which to consider fish assemblages. In many cases, a more meaningful axis would be upland versus lowland fishes or fishes occupying habitats with lower to higher water retention times (Part 5; Ross and Matthews, in press).
SUMMARY
Fishes are the most diverse group of craniate organisms with nearly 28,000 species distributed among five classes. Freshwater fishes make up nearly half of all fish species (some 43%), yet liquid fresh water accounts for only 0.01% of all water on our planet. The greatest diversity of freshwater fishes is found in the New and Old World tropics, but North America harbors the richest temperate freshwater fish fauna with some 1,061 species. Within North America, more than half of all species occur in the east. The North American fish fauna is primarily a fauna of flowing water, with relatively few contemporary species unique to lakes.
SUPPLEMENTAL READING
Dudgeon, D., A. H. Arthington, M. O. Gessner, Z.-I. Kawabata, D. J. Knowler, C. Lévêque, R. J. Naiman, A.-H. Prieur-Richard, D. Soto, M. L. J. Stiassny, and C. A. Sullivan. 2006. Freshwater biodiversity: Importance, threats, status and conservation challenges. Biological Reviews 81:163–82. An overview of challenges facing aquatic organisms and those working to protect those organisms and their ecosystems.
Smith, G. R., C. Badgley, T. P. Eiting, and P. S. Larson. 2010. Species diversity gradients in relation to geological history in North American freshwater fishes. Evolutionary Ecology Research 12:693–726. A recent and thorough treatment of the interplay between geological processes and fish distributions and diversity.
Stiassny, M. L. J. 1996. An overview of freshwater biodiversity: With some lessons from African Fishes. Fisheries 21(9):7–13. Documents the interplay between demands for water and the protection of aquatic biodiversity.
Warren, M. L., Jr., B. M. Burr, S. J. Walsh, H. L. Bart, Jr., R. C. Cashner, D. A. Etnier, B. J. Freeman, B. R. Kuhajda, R. L. Mayden, H. W. Robison, S. T. Ross, and W. C. Starnes. 2000. Diversity, distribution, and conservation status of the native freshwater fishes of the southern United States. Fisheries 25:7–31. The southeastern United States contains the richest North American fish fauna and also contains the greatest number of species at risk.
WEB SOURCES
