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Distribution of Fresh Water: Global Patterns
North American Freshwater Fishes
Patterns of North American Diversity
Lentic versus Lotic Systems
Understanding fish ecology is both exciting and challenging. Fishes have the greatest diversity among the craniate organisms, composing more than half of all extant species, with an estimated 27,977 species of fishes worldwide (Nelson 2006), and with the descriptions of "new" (newly documented) species continuing at approximately 200 species per year (Eschmeyer 1998). As further emphasis of fish diversity, unlike other recognized groups within the phylum Chordata (amphibians, reptiles, birds, and mammals), organisms commonly referred to as fishes comprise five living classes (hagfishes, Myxini; lampreys, Petromyzontidae; sharks and rays, Chondrichthyes; ray-finned fishes, Actinopterygii; and lobe-finned fishes, Sarcopterygii). Three of these groups, petromyzontids, chondrichthyans, and actinopterygians, are represented in North American fresh water. The Myxini are restricted to the marine environment, whereas the fishes included within the class Sarcopterygii occur in the marine environment (Coelacanthimorpha, coelacanths) or fresh waters of Australia, Africa, and South America (Dipnoi, lungfishes). Tetrapods, including, of course, readers of this book, are also included in the Sarcopterygii (Nelson 2006).
Despite the abundance of organisms to study, the aquatic environment provides challenges to work in, making ecological studies difficult or even dangerous. Especially in many North American streams, visual observations are often not possible because of high fl ows or turbidity, necessitating indirect approaches, such as seining, electrofishing, or using electronic tags, to infer information on fish habitat use, behavior, and interactions.
This is nonetheless an exciting, as well as challenging, time to write about North American freshwater fish ecology. Modern computers and the rapidly increasing quality of electronic libraries and data searches have made the literature much more available, and the rate of publications has greatly accelerated. In fact, many of the cited references 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 Nearctic fish 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.
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 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).
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).
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.
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.
Digital image of North America. http:// photojournal.jpl.nasa.gov/catalog/PIA03377.CHAPTER 2
Origin and Derivation of the North American Freshwater Fish Fauna
Assembling a Fauna: Fish Evolution and
A Dynamic Earth
Ages of North American Fish Families
Origins of North American Fish Families
Numerically Dominant Families
Ages and Origins of Major Fish Families
ASSEMBLING A FAUNA: FISH EVOLUTION AND PLATE TECTONICS
Studies of fish distribution and ecology are often initiated by making a series of collections in various aquatic habitats. In doing so, there is a tendency to consider fishes taken in each particular mesohabitat, such as a pond, lake shore, or stream riffle, to be part of a natural assemblage developed as a unit over evolutionary and ecological time through the interaction of local processes. However, even discounting the recent major role of humans in introducing fishes outside of their natural ranges, this assumption that species in the assemblage share a long history of coexistence may not be valid. As pointed out by Brooks and McLennan (1991), Matthews (1998), and others, in addition to local determinism in shaping assemblage composition, contemporary species assemblages may also be due to the association of the species' ancestors in that particular geographic region, or the species may have evolved within different assemblages and one or more entered the modern assemblage through dispersal.
Excerpted from Ecology of North American Freshwater Fishes by Stephen T. Ross. Copyright © 2013 The Regents of the University of California. Excerpted by permission of UNIVERSITY OF CALIFORNIA PRESS.
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