Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
were published after I started work on this book in 2005.
DISTRIBUTION OF FRESH WATER
Global Patterns
Surprisingly, the distribution of fish species among major habitats is not at all predicted by the habitat area or volume. Even though liquid fresh water makes up only 0.0142% of the water on our planet (Figure 1.1A), there are an estimated 11,952 freshwater species worldwide, which make up 43% of all known fishes (Cohen 1970; Shiklomanov 1993; Nelson 2006). In addition, the proportionately small amount of fresh water (including inland saline lakes) is not distributed equally among habitats but occurs primarily in lakes, something that will be considered in more detail later.
Of course, fishes and other aquatic organisms are not the only organisms dependent on the 0.01% of liquid fresh water—the same limited resource supports most of the world’s human population, including human industry, farming, and ranching. Consequently, it is not surprising that freshwater biodiversity is at risk worldwide—indeed, freshwater ecosystems are among the most endangered ecosystems on our planet (Stiassny 1996; Dudgeon et al. 2006).
Diversity of freshwater fishes varies greatly among major zoogeographic regions of the world (Figure 1.2). Considering the six major zoogeographic realms (reviewed in Berra 2001 and Cox 2001), the greatest freshwater fish diversity occurs in the Neotropical realm (Central and South America and tropical Mexico), with 5,000–8,000 species, followed by the Oriental realm (India and southeast Asia), with approximately 3,000 species; the Ethiopian realm (Africa and southern Arabia), with an estimated 2,850 species; the Nearctic realm (North America except tropical Mexico), with at least 1,116 species; the Palearctic (Europe and Asia north of the Himalayas), with 552 species; and the Australian realm, with 500 species, including those marine fishes that enter fresh water (Burr and Mayden 1992; Matthews 1998; Lundberg et al. 2000; Berra 2001; Moyle and Cech 2004; G. R. Smith et al. 2010).
NORTH AMERICAN FRESHWATER FISHES
Although overshadowed by tropical regions, the Nearctic (North American) realm is by far the most speciose of the temperate zoogeographic regions. There are different views on what constitutes the biogeographic extent of North America. I follow Burr and Mayden (1992) in considering the southern boundary of North America to be 18° N latitude on the Atlantic coast (the Río Papaloapan drainage) and 16° N on the Pacific coast (the Río Verde/Atoyac drainage), although southward penetration of Nearctic fish groups varies among families with two gar species, Atractosteus spatula and A. tropicus, reaching the Rio San Juan drainage of Costa Rica (Minckley et al. 2005). Along the eastern coast of Mexico, the boundary between Nearctic and Neotropical fishes has been further examined. Based on the family Cichlidae, the formation of the Mexican Neovolcanic Plateau, some five million years ago, has served as a barrier separating the Neotropical and Nearcticfish faunas (Hulsey et al. 2004).
Patterns of North American Diversity
North American fish species are included in some 201 genera, 50 families, and 24 orders (Figure 1.3) (Burr and Mayden 1992). However, only about half of the families could be considered as major components based on their number of species and/or their breadth of distribution. Families with the greatest number of species include minnows (Cyprinidae, 297 species), perches (Percidae, 186 species), suckers (Catostomidae, 71 species), livebearers (Poeciliidae, 69 species), and North American catfishes (Ictaluridae, 46 species) (Table 1.1). These 5 families alone make up 62% of the fauna, and along with an additional 10 families (Goodeidae through Clupeidae), they compose 90% of the North American freshwater ichthyofauna.
FIGURE 1.1 A. The distribution of liquid water in the biosphere. Percents of freshwater habitats are too small to show above the x-axis.
B. The distribution of liquid freshwater in inland systems. Percentage data in both panels are based on total liquid water in the biosphere; data are from Shiklomanov (1993).
FIGURE 1.2 A. Zoogeographic realms of the world based on Berra (2001) and Cox (2001).
B. The relative sizes of the zoogeographic realms based on the diversity of fishes.
FIGURE 1.3. Relationships of orders that have species of North American freshwater fishes (bold). Based primarily on Nelson (2006).
Patterns of freshwater fish diversity across North America vary widely, in part as a response to variation in landforms, watershed boundaries, historical and recent climates, and geological activity (Figure 1.4). Diversity is greatest in eastern North America, especially in the southeastern United States, which contains more than half (662 species) of the fauna (Warren et al. 2000; G. R. Smith et al. 2010). In fact, the rich southeastern region has been referred to as a “piscine rainforest” and harbors some 662 native freshwater and diadromous fish species (Warren and Burr 1994; Warren et al. 2000). States with the greatest native fish diversity, in order of number of species, are Tennessee (297), Alabama (295), Kentucky (220), Georgia (219), and Mississippi (212) (Etnier and Starnes 1993; Warren and Burr 1994; Ross 2001 and additional unpublished material; Boschung and Mayden 2004).
Western fish diversity is about one-third that of overall eastern diversity, but endemism tends to be greater (Moyle and Herbold 1987; Burr Myxini—Myxiniformes and Mayden 1992). Considering just the United States and southern Canada, McAllister et al. (1986) determined that geographic grids of one degree latitude and longitude contained on average 10 or fewer species in western areas, with maximum values of 19 in Oregon, 14 in California, and 11 along the Colorado River. In contrast, the same-sized grids in the southeastern United States supported up to 73 species. The more recent treatment of North American diversity patterns that include Mexico (Figure 1.5) further illustrates this pattern (G. R. Smith, in Lundberg et al. 2000; G. R. Smith et al. 2010) and shows that a band of high-diversity grids, located in lower elevation areas, continues south into eastern Mexico to the Yucatan Peninsula. Because of the strong regional differences in species richness, streams of approximately similar sizes located across North America harbor drastically different numbers of native fishes. For example, four streams spanning the temperate region of North America (Figure 1.6) differ greatly in species richness, especially in relation to stream discharge. The four streams were chosen for their geographic locations and also because I have worked in or visited all of them. The western streams tend to have higher average annual 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