Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
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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).
FIGURE 3.4. The approximate locations of the Cordilleran, Laurentide, and Innuitian ice sheets of the late Wisconsinan glaciation. Based on Clark et al. (1996), Ehlers (1996), Lowe and Walker (1997), and Dyke et al. (2002).
Advances of the glaciers had direct impacts on fishes as the landscape became covered with ice (Crossman and McAllister 1986; McPhail and Lindsey 1986; Matthews 1998). Ice dams caused changes in stream patterns and directions of flow and sometimes created large lakes. Because of the amount of water contained in the glaciers, sea level was lowered so that streams that now enter the sea separately may have been joined. Glacial scour altered the land, and the formation of terminal moraines created new lake habitats. Streams pouring off edges of the ice created plunge pools, and the melting of large blocks of ice formed kettle lakes. Several examples will help to illustrate general Pleistocene effects.
Changes in Drainage Patterns and Stream Connections in Eastern North America
Pleistocene events resulted in substantial changes to earlier drainages, although understanding the details of how glacial advances altered pre-Pleistocene drainage patterns is complex. The ongoing efforts to understand these events have involved geological research as well as biogeographic studies of fishes. An excellent example of drainage changes, as well as complexity, is the Central Highlands region of eastern North America, comprising the Eastern, Ozark, and Ouachita subregions (Figure 3.5A). Fishes endemic to the Ozark Highlands tend to show their closest relationships with fishes in the Ouachita Highlands, and fishes endemic to these two regions tend to have their closest relationships with fishes in the Eastern Highlands (Mayden 1985, 1987b, 1988). Such relationships among freshwater fishes strongly suggests that the three highland areas once shared common drainage connections, even though the three regions are now isolated by intervening lowlands.
FIGURE 3.5. A. The Central Highlands region of eastern North America in relation to current drainage patterns.
B. The formation of the modern Red River from Pre-Pleistocene drainages. Pre-Pleistocene drainages are shown in black: 1 = Plains Stream; 2 = Old Ouachita River; 3 = Old Red River; 4 = Old Mississippi River. The black dot shows the collecting site on the modern Ouachita River. Based on Mayden (1987a, 1987b, 1988).
The Highlands are remnants of an ancient topography that dates to the uplift of the Appalachian Mountains (Chapter 2; Wiley and Mayden 1985; Mayden 1988). The major vicariant events dividing the Central Highlands into eastern and western regions included the southward movement of Pleistocene glacial advances; in fact, the region of the central lowlands (Figure 3.5) was a highland area prior to the intrusion of massive ice sheets. The glacial advance was ultimately followed by the penetration of the lowland area connecting the Eastern and Interior Highlands by the Mississippi River, enlarged because of southward deflection and increased flows of streams that once drained into Hudson Bay (Missouri River) or the Laurentian stream system and the Atlantic Ocean (Ohio River) (Pflieger 1971; Mayden 1985; Wiley and Mayden 1985). The Interior Highlands area was separated into the Ozark and Ouachita highlands by the westward penetration and development of the Arkansas River (Mayden 1987b). Post-Pleistocene dispersal of fishes into some of the formerly glaciated regions from the unglaciated Central Highland areas was also important and adds to the complexity in understanding fish distributions (Berendzen et al. 2003; Near and Keck 2005).
Although the Highlands region is characterized by high fish diversity, the reasons for this diversity are still being debated. The Pleistocene dispersal hypothesis states that the Eastern Highlands represented a center of origin for lineages that subsequently dispersed along glacial fronts during the Pleistocene to streams of the Interior (Ozark and Ouachita) Highlands (Mayden 1987b; Strange and Burr 1997). As such, species in the Interior Highlands should be no older than the Pleistocene. Alternatively, the Central Highlands vicariance hypothesis (CHVH) predicts that the fauna diversified in a widespread and interconnected Highlands region during the Miocene and Pliocene and, after most speciation events had occurred, was fragmented by Pleistocene events into the Ozark and Ouachita Highlands west of the Mississippi River and the Eastern Highlands east of the Mississippi River (Figure 3.5) (Mayden 1988; Near and Keck 2005). Phylogeographic analyses using molecular data do show some support for predictions of the CHVH in divergence times of various lineages. The darter subgenera Litocara (genus Etheostoma) and Odontopholis (genus Percina) have species in the Ozark and Eastern Highlands, and both groups show deep divergences of species between the two regions that likely occurred in the Miocene (Strange and Burr 1997). Four species of the minnow genus Erimystax, which occur in the Ozark, Ouachita, Eastern Highlands, and adjoining areas, also indicate Miocene speciation events (Simons 2004), and divergence within the Hogsuckers (genus Hypentelium) occurred prior to the Pleistocene (Berendzen