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 et al. 2003). However, not all evidence supports Miocene or Pliocene ages of species. In a study of lineage divergences in the 20 species of the darter genus Nothonotus, times ranged from the Miocene (six events), Pliocene (four events), to the Pleistocene (eight events) (Near and Keck 2005). Divergences of subspecies of Studfish (Fundulus catenatus) occurred by dispersal or peripheral isolation in the late Pleistocene or later. Divergence of subspecies of Banded Sculpin (Cottus carolinae), perhaps by peripheral isolation, also occurred within the Pleistocene (Strange and Burr 1997), as did divergence within the Gilt Darter (Percina evides) (Near et al. 2001). Consequently, the rich fish fauna of the Central Highlands seems to be a product of both vicariant and dispersal events, facilitated by the region’s great age and topographic diversity. The high fish diversity in many ways follows predictions of island biogeography theory and species-area relationships (Page 1983; Near and Keck 2005).
There are several consequences of ecological importance that are apparent from these events. First, much of the history of the faunas of the Highlands is pre-Pleistocene so that species or species groups have lineages dating to the middle or even early Cenozoic, and some groups thus have had the potential for long periods of interaction. Second, species have experienced major changes in range size (range being contracted during glacial advances), followed by expansion when habitats again became available as ice sheets retreated. Third, the faunas of present-day rivers may reflect species groups that originally occurred in separate drainages, so that species or lineages in a modern river may or may not share a long history.
For example, the modern fish fauna of the Red River and its tributaries (Figure 3.5B) is thought to comprise faunas from three distinct pre-Pleistocene river systems: the Plains Stream in the headwaters of the Red River, the Old Ouachita River (Little-Kiamichi-Ouachita system), and the Old (lower) Red River (Mayden 1985).
TABLE 3.1 Biogeographic Relationships of Species from a Sample of Fishes from the Ouachita River, Arkansas, at the Confluence with the Little Missouri River (Ross, pers. observ.)
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,
