Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.

      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) glaciations. Glacial advances began in the Miocene and Pliocene of the late Tertiary, followed by numerous cold periods and concomitant glacial advances interspersed with temperate periods and their associated glacial retreats (Ehlers 1996).

      Although there are many direct and indirect approaches to dating these cold and warm periods, one of the most fruitful approaches has been the use of ratios of various elements, and of isotopes of elements, that were incorporated into calcium-carbonate shells and skeletons of marine microorganisms and deposited in the stable environment of the deep sea (Lowe and Walker 1997). In particular, the ratio of two oxygen isotopes, ¹⁶O and ¹⁸O, found in the tests of Foraminifera has led to a much more precise understanding of the timing of cold and temperate periods. The ratio is known to vary with temperature, as the lighter isotope tends to accumulate in glacial ice during cold periods so that there is an enrichment of the heavier isotope in the deep sea (Lowe and Walker 1997).

      Previously there were four major glacial advances recognized within the Quaternary for North America (Nebraskan, Kansan, Illinoian, and Wisconsinan); however, based primarily on information from isotopic ratios, the estimate of the number of glacial advances from the dawn of the Pleistocene, approximately 2 mya, is at least 18–20 for the entire planet and perhaps 13–18 major glacial advances in North America (Davis 1983; Ehlers 1996). The last major advance, the Wisconsinan, began perhaps 80,000 years ago (Ehlers 1996; Lowe and Walker 1997). Thicknesses reached by the ice sheets were impressive, reaching 90 m to several kilometers in some areas, and resulting in depressions of the land by 200–300 m (Lowe and Walker 1997; Lomolino et al. 2006). Even within major glacial advances, there was a strong pattern of major and minor variation. For instance, the Wisconsinan glaciation can be subdivided into three periods of advances, with the last advance, the late Wisconsinan, starting approximately 23,000–25,000 years ago (Ehlers 1996).

      The Wisconsinan glaciation comprised two major ice sheets, the Cordilleran in northwestern North America and the Laurentide in eastern and northeastern North America, and one minor ice sheet, the Innuitian along the Arctic coastline (Figure 3.4). The development of the western Cordilleran ice sheet lagged behind that of the Laurentian and Innuitian ice sheets, with the latter two ice sheets reaching their maxima 20,000–24,000 years ago and remaining near maximum until 17,000 years ago. The Cordilleran did not attain its maximum extent until 14,500 years ago, followed by a rapid decline beginning around 12,000 years ago (Clague and James 2002; Dyke et al. 2002). The Laurentian ice sheet during the Wisconsinan glaciation extended across most of eastern Illinois, Indiana (except the south-central region), and most of Ohio, nearly to the present course of the Ohio River (Frye et al. 1965; Goldthwait et al. 1965; Wayne and Zumberge 1965; Clark et al. 1996; Ehlers 1996; Lowe and Walker 1997). Farther east, ice covered upper Pennsylvania and all of New York and New England (Muller 1965; Schafer and Hartshorn 1965). Except for montane glaciers, glacial penetration was less in western states, covering the upper half of most of Washington, Idaho, Montana, and all but the southwest corner of North Dakota (Figure 3.5) (Flint 1971). Higher elevations along the Rocky Mountains supported extensive glaciers as far south as New Mexico (Richmond 1965), and in California there were large glaciers in the Sierra Nevada range and even in the transverse ranges (the San Bernardino Mountains) of Southern California near Los Angeles (Owen et al. 2003).