Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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although the southwestern region has progressively become warmer and drier, the pattern is not one of continuous warming and drying but one of high variability in climatic patterns, including periodic severe droughts.
The impact of post-Pleistocene drought on western fishes is illustrated by work on genetic variability in Flannelmouth Sucker, one of the ancient, endemic species of the Colorado River. Genetic diversity in Flannelmouth Sucker is surprisingly limited for such an ancient species and is consistent with the hypothesis of a major, basin-wide, population crash during a known time (post-Pleistocene) of severe western drought, followed by a period of rapid repopulation growth and range expansion during a wetter period. Populations of Flannelmouth Sucker in the upper regions of the Colorado basin are the result of migration from refugia in the lower part of the system, generally within the last 10,000–11,000 years (Douglas et al. 2003).
Great Basin
The Great Basin comprises a large area of complex geology located northwest of the Colorado Plateau and including large areas of Nevada and Utah and parts of southeastern Oregon, southern Idaho, southwestern Wyoming, eastern California, and northern Mexico (Figure 3.2). As suggested by the term “Great,” the Basin makes up almost 20% of the United States and constitutes the largest inland drainage in North America (Sigler and Sigler 1987). It includes more than 150 smaller drainage basins separated by approximately 160 regularly spaced mountain ranges forming the basin and range topography (G. R. Smith 1978; Sigler and Sigler 1987; Sada and Vinyard 2002). Two large subbasins make up the Great Basin—the Bonneville Basin, occurring primarily in eastern Nevada and Utah, and the Lahontan Basin to the west, occurring mostly in Nevada and parts of eastern California. Most of the topography was formed in the last 20 million years, and many of the genera of fishes have occupied the area since the Pliocene (approximately 5 million years ago) and some since the Miocene (G. R. Smith 1981; Dowling et al. 2002; G. R. Smith et al. 2002). The fish fauna includes at least 102 species and subspecies in 15 genera of generally small-bodied forms, but the unique feature is the high level of endemism of both fishes and invertebrates (Sada and Vinyard 2002; G. R. Smith et al. 2010). At least 66% of the fish species are endemic to the Great Basin, most to specific drainages within the region (G. R. Smith 1978; Sada and Vinyard 2002). During the Pliocene and Pleistocene, repeated periods of high rainfall and changes in drainages due to volcanism resulted in a series of lakes, many quite large, in the Great Basin and created a very different environment compared to the modern-day desert—in fact, what is now Nevada was a land of abundant natural lakes (Figure 3.3). As shown by the green areas in Figure 3.3, some of the highest lake levels occurred in the early-middle Pleistocene, approximately 650,000 years ago (Reheis 1999). Two of the largest lakes were Lake Lahontan to the west and Lake Bonneville to the east. The Bonneville Salt Flats are part of the remains of Pleistocene Lake Bonneville, which is survived by the modernday Great Salt Lake and two smaller freshwater lakes, Bear and Utah, in the northeastern part of the Great Basin (Figs. 3.2 and 3.3) (Mock et al. 2006). The large Pleistocene lakes and interconnecting streams allowed aquatic organisms to colonize many of the lake basins; however, the subsequent increasing aridity during the late Pleistocene and post-Pleistocene, in part caused by the uplift of the Sierra Nevada Mountains, resulted in the isolation of the faunas, contributing to the high level of endemism and also to the frequent loss of species through extinction (Hubbs et al. 1974; Reheis 1999; G. R. Smith et al. 2002). Especially because of competition for the limited water in the now arid region, human impacts on the rate of extinction have also been particularly great (Sada and Vinyard 2002).
FIGURE 3.3. Pleistocene pluvial lakes and rivers of the western Great Basin in what is now Nevada. Dashed lines are state boundaries, blue areas show the maximum extent of late Pleistocene lakes, green areas show the possible maximum extent of early-middle Pleistocene lakes, red lines show the modern drainages of the Lahontan Basin, and black lines indicate the late Pleistocene extent of the Lahontan Basin. Based on Reheis (1999).
During the Pliocene and Pleistocene, the Bonneville Basin was connected at least twice to the upper Snake River via the upper Bear River (Figure 3.2) (Hart et al. 2004). In the late Pleistocene, when most of the upper Snake River was covered by glaciers, extensive lava flows blocked the connection, diverting the Bear River into the Bonneville Basin and contributing to a rise in water level of Lake Bonneville, at that time a freshwater lake. A second connection occurred 145,500 years later, when Lake Bonneville was at its high stand and a breach occurred along its northern shore. This resulted in a major erosive flood into the Snake River. Once lake levels dropped, the Bonneville Basin was again separated from the Snake River drainage, a situation enhanced by the increasing aridity of the region (Curry 1990; Hart et al. 2004; Mock et al. 2006).
As in other areas of North America, patterns of fish diversification are often more closely related to ancient drainage patterns than to modern-day patterns, and the Bonneville and Snake River basins provide excellent examples of this (G. R. Smith et al. 2002; Mock et al. 2006). For instance, mitochondrial and nuclear sequence data show a strong divergence among morphologically similar populations of the Utah Sucker (Catostomus ardens), a widespread endemic to the Bonneville Basin and the upper Snake River (Mock et al. 2006). The divergence reflects the ancient connection between the Bonneville Basin and the Snake River via the upper Bear River and divides Utah Sucker populations into a southwestern group of the Great Basin, centered around Utah Lake and the Sevier River, and a northeastern group in the Snake River and in the northeastern Bonneville Basin east of the Wasatch Mountains (Figure 3.2). The deep genetic divergence suggests that these two groups were separated 1.6–4.5 million years ago during the Pliocene or early Pleistocene. In addition, there is also genetic separation between the Sevier River populations and those in the Utah Lakes region, reflecting the post-Pleistocene isolation of these areas caused by increasing aridity. Surprisingly, the June Sucker (Chasmistesliorus), endemic to Utah Lake, shows little genetic differentiation from the Utah Sucker but strong morphological differentiation, suggesting strong recent selection for a more planktivorous lifestyle in contrast to the benthic feeding Utah Sucker.
The genetic separation between the northeastern Bonneville/lower Snake River and the southeastern Lake Bonneville is also reflected in other species, including the Leatherside Chub, which is now recognized as comprising two lineages—the Northern Leatherside Chub (Lepidomeda copei) and the Southern Leatherside Chub (L. aliciae) (J. B. Johnson et al. 2004). A similar pattern is shown by the Utah Chub (Gila atraria), which shows deep genetic divergence between a northeastern Bear Lake/Snake River clade and a southwestern Bonneville clade, which is again very like the Utah Sucker (Figure 3.2). Molecular evidence indicates that the division occurred sometime in the Pliocene or early Pleistocene (J. B. Johnson 2002). However, unlike the Utah Sucker, the genetic structure of the Utah Chub also shows a more recent connection between the Bonneville and Snake River basins that relates to the late Pleistocene Bonneville flood. In all examples, the impacts of the geological and climatic history of the region contribute greatly to understanding the ecology of the species and, in particular, to conservation efforts that might include transplanting populations (J. B. Johnson et al. 2004; Mock et al. 2006).
The fish fauna of the Great Basin colonized the region through numerous rivers and lakes present during various late Miocene, Pliocene, and Pleistocene pluvial periods. Some populations, such as Utah Sucker and Leatherside Chub, reflect the earlier Pliocene connections, in contrast to others, such as June Sucker, that show more recent responses to ecological opportunities. Since the Pleistocene, the Great Basin fish fauna has been progressively diminished and fragmented as aquatic habitats have dried and fishes have been isolated in small springs, spring runs, and the remaining lakes and streams (Sada and Vinyard 2002).
Examples from Northern and Eastern North America
Late Tertiary (Miocene and Pliocene) and early Quaternary geologic and climatic events also affected fish assemblages in northern, central, and eastern North America. However, in contrast to the high level of tectonic activity and volcanism of western North America, these regions of North America tended to be geologically more quiescent through most of the Pliocene but with major climatic impacts to fishes and other organisms caused by direct and indirect effects of late Tertiary and Quaternary (Pleistocene)