ages of the taxa. The fish assemblage includes groups fragmented from a once intact Central Highlands fauna, some more recent taxa endemic to the Ouachita Highlands, and species derived from generally widespread, primarily lowland pre-Pleistocene taxa. Such separate origins have substantial consequences for the interpretation of factors like the coevolution of species’ traits, which are treated in Chapter 13.
Changes in Drainage Patterns and Stream Connections in Northern and Northwestern North America
Farther north and west, portions of the Missouri River originally flowed northward into Hudson Bay, and the Bonneville Basin (discussed previously) was also likely once part of the Hudson Bay drainage during the late Miocene through connections via the Snake River (G. R. Smith 1981; Crossman and McAllister 1986). The past connections are reflected in the current fish faunas. For instance, the Bonneville Basin (located primarily in Utah) contains faunal elements from the north and northeast such as whitefishes, Prosopium spp.; suckers, Catostomus spp.; and the minnows Richardsonius and Rhinichthys (G. R. Smith 1981).
Southward Displacement
Beyond the area of direct glacial impact, cooling associated with the Pleistocene resulted in a general southward displacement of terrestrial plants and animals (Pflieger 1971; Whitehead 1973; Pielou 1991). In river systems that were oriented in a primarily north-south direction, such as the Mississippi River, fishes also responded to glacial advances and dropping temperatures by a general southward displacement (Cross 1970; G. R. Smith 1981; Cross et al. 1986). For instance, species that today have a primarily northeastern or north-central distribution, such as Redbelly Dace (Chrosomus erythrogaster), Northern Studfish (Fundulus catenatus), and Rainbow Darter (Etheostoma caeruleum) have disjunct populations as far south as Mississippi (Ross 2001), and Redbelly Dace and Creek Chub (Semotilus atromaculatus) have disjunct populations in northeastern New Mexico (Pflieger 1971).
AFTER THE ICE
Fishes that survived glacial advances did so in areas that remained ice free—the glacial refugia. As the ice retreated, fishes spread out from the refugia to colonize the newly available habitats. There were at least five major glacial refugia as well as various minor refugia that allowed the survival of organisms displaced by advancing ice. Refugia occurred in the Arctic as well as south of major glacial advances (Figure 3.6) (Flint 1971; Crossman and McAllister 1986; Stamford and Taylor 2004; Cox and Moore 2005). Minor refugia tended to occur along the boundary of the Laurentide and Cordilleran ice sheets or in coastal areas. Because of glacial refugia, repopulation of formerly glaciated habitats occurred both from the northwest (Beringia), as well as from the east, west, and south. Recolonization is a gradual process and is still ongoing so that formation of northern fish assemblages may be even more recent than within the last 10,000–12,000 years (Crossman and McAllister 1986; Lundberg et al. 2000). For example, species richness in formerly glaciated areas, as shown for Ontario, Canada, is related strongly to distance from glacial refugia and the time that recolonization corridors have been free of ice (Mandrak 1995).
In central North America, the majority of reintroductions to once glaciated areas occurred via the Mississippi Refugium (Figure 3.6), contributing species to north-central Canada, the Hudson Bay drainage, and the Arctic Archipelago (Mandrak and Crossman 1992; Matthews 1998). In the Canadian province of Ontario, which was totally covered by the Wisconsinan glacial advance, 77 out of 91 species, for which glacial refugia have been resolved, repopulated the area from the Mississippi Refugium (Mandrak and Crossman 1992). Over the larger area of the Hudson Bay drainage, the Mississippi Refugium again provided the greatest number of species (Crossman and McAllister 1986). For the Ontario fauna, 94% of the species for which refugia could be identified, survived the glacial advance in a single refugium (Mandrak and Crossman 1992). Whether assemblages tended to move as a group or as individuals is unknown, although recolonization likely occurred in waves of immigrants as passageways from various refugia became free of ice. For instance, of the 21 common species limited to the Great Lakes and Nelson River (located to the northwest and draining into Hudson Bay) watersheds, 14 originated from the Mississippi Refugium, one species originated from both the Mississippi and Atlantic refugia, one species originated from the Atlantic Refugium, and one species originated from the Atlantic, Mississippi, and Missouri refugia (Figure 3.6) (Mandrak and Crossman 1992).
FIGURE 3.6. Glacial refugia during the Wisconsinan glacial advance and their contributions to repopulating formerly glaciated areas. Small refugia are indicated by closed circles. Lines with arrows show colonization routes; solid black lines show colonization from Cascadia, Nahanni, and Mississippi refugia; dashed gray lines show colonization from the Beringia Refugium; solid gray lines show colonization from the Atlantic Refugium. Based on data from McPhail and Lindsey (1970, 1986), Crossman and McAllister (1986), Mandrak and Crossman (1992), Matthews (1998), McCusker et al. (2000), C. T. Smith et al. (2001), and Stamford and Taylor (2004).
In western North America, four refugia, (Beringia, Cascadia [Pacifi c], Mississippi, and Missouri) contributed most to the formation of the northwestern Canada and Alaskan fish assemblages (McPhail and Lindsey 1970). The times of egress of fishes from these refugia differed because of the earlier retreat of ice from coastal refugia and from the Missouri Refugium of Great Plains compared to the Mississippi Refugium. What these examples suggest is that the fish assemblages in formerly glaciated regions experienced a steplike increase in potential colonizers over time as passage from the various refugia became possible. In addition, as emphasized by Figure 3.6, regional faunas were established by colonizers from potentially a number of different refugia and thus have experienced different evolutionary histories and faunal associates.
The impact of postglacial dispersal is also illustrated by fishes occupying the Chehalis River valley, a small coastal drainage in western Washington that provided a refugium for lowland fishes of Puget Sound drainages and the Olympic Peninsula (McPhail 1967; McPhail and Taylor 1999). During the last advance of the Wisconsinan glaciation, the Puget Lobe of the Cordilleran ice sheet penetrated south to cover what is now Puget Sound (Figure 3.7) (Porter and Swanson 1998). South of the ice sheet, the Chehalis River valley remained unglaciated over much of its area, as did the larger lower Columbia River farther south (McPhail 1967; Pielou 1991). Although early faunal exchange occurred between the Chehalis and Columbia rivers, during the middle to late Pleistocene these faunas remained distinct. Drainages north of the Chehalis River that now flow into Puget Sound were ice covered for approximately 900–1,000 years (Porter and Swanson 1998). The Puget Lobe reached its maximum southern extent 16,950 years ago and then began receding 16,850 years ago (Porter and Swanson 1998). As it began to recede, flow was to the south into the Chehalis River and thence to the Pacific Ocean. The Chehalis River fish fauna, comprising eight species of primary freshwater fishes, gradually dispersed northward as the ice withdrew, especially species such as the Longnose Dace (Rhinichthys cataractae) that are adapted to swiftly flowing water (McPhail 1967). The farthest northward penetration of fishes from the small Chehalis River Refugium was achieved by the Nooksack Dace (Rhinichthys cataractae ssp.) and the Salish Sucker (Catostomus catostomus ssp.), which reached the Fraser River system of southern British Columbia (Figure 3.7) (McPhail 1997; Pearson 2000; Hutchings and Festa-Bianchet 2009). As the ice sheet receded past what is now the mouth of the Snohomish River (Figure 3.7), sea water from the Strait of Juan de Fuca poured into the large proglacial lakes that had expanded to occupy the Puget Sound Basin, quickly changing the basin from fresh water to sea water and limiting further northward distribution of primary freshwater fishes (McPhail 1967).
FIGURE 3.7. Modern and Pleistocene features of western Washington showing the location of the Chehalis River Refugium, modern-day Puget Sound (medium gray), the maximum southward penetration of the Puget Lobe of the Cordilleran ice sheet (light gray), early proglacial lakes, and other place names mentioned in the text. Based on
