The amalgamation of faunas is illustrated by examining the origins of 13 fish species taken in a single winter fish collection from a gravel bar on the Ouachita River, a tributary of the modern Red River (Figure 3.5B) (Table 3.1). Four species (Steelcolor Shiner, Bigeye Shiner, Banded Darter, and Channel Darter) are endemic, or largely so, to all three regions of the Central Highlands, and thus would have had the potential for interaction since the Pliocene or earlier. Four species (Highland Stoneroller, Mountain Madtom, Creole Darter, and Orangebelly Darter) are primarily restricted to the Ouachita Highlands and perhaps had a later origin compared to the previous four species. The remaining five species are widespread, generally lowland forms, some of which are sister species to forms occurring in the Central Highlands. This collection of fishes, comprising a few seine hauls along a single gravel bar, emphasizes that contemporary faunas can have different evolutionary origins, ecological histories, and 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).