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ОглавлениеTHREE
Reshaping North American Fish Faunas
THE ROLE OF LATE CENOZOIC CLIMATIC AND TECTONIC EVENTS
CONTENTS
Tertiary and Quaternary Events
Examples from Western North America
Examples from Northern and Eastern North America
Changes in Drainage Patterns and Stream Connections in Eastern North America
Changes in Drainage Patterns and Stream Connections in Northern and Northwestern North America
FISH ASSEMBLAGES AND POPULATIONS are continually challenged by changes in their local and regional environments. These changes could be relatively minor, such as local shifts in stream habitats caused by alterations in pools or riffle structure, or changes in access to habitats caused by shifts in the distribution of large piscivores. More extreme changes might include annual shifts in water level and/or flow rates caused by variation in precipitation. On an even larger scale, changes could reflect long-term climatic shifts, such as the onset of the Pleistocene Ice Ages, or major tectonic events, such as the uplift of the Colorado Plateau in the late Cretaceous that resulted in major alterations of stream connections and drainage patterns (G. R. Smith et al. 2010). Examples of some of the large-scale events that characterized the middle to late Cenozoic and their impacts on fishes are the topic of this chapter.
TERTIARY AND QUATERNARY EVENTS
From the previous chapter it is apparent how North American fish assemblages have been shaped by large-scale geologic and climatic events, with some fish lineages, such as lampreys, having experienced events as far back as the late Paleozoic. Such climatic and geologic events have, over time, shaped the pattern of fish diversity that characterizes North America (Figure 1.5). In addition, fish assemblages, like other biotic assemblages, have undergone continual breakup and rearrangement (Jablonski and Sepkoski 1996). In this section the emphasis is on events occurring during the Cenozoic, particularly the late Tertiary and early Quaternary Periods, while still recognizing that many of the long-term climatic and geological impacts are part of continual processes shaping our planet.
Examples from Western North America
In the early Paleozoic (Cambrian), western regions of North America subsided and were covered by seas that lasted, to varying extents, into the Jurassic (Stokes 1986). The exposed continental margin of North America cut through portions of Alberta, Montana, and Utah, essentially following the Wasatch Line in Utah (Figure 3.1) (Stokes 1986; Aberhan 1999; Dickinson 2004). Furthermore, as the eastern margin of the Pacific Plate collided with the North American Plate, primarily by subduction (sliding beneath), terranes of largely oceanic origin were added to the western margin of North America. (A terrane is a discrete, fault-bounded crustal element that is added to a craton through plate movement—a craton is a continental nucleus.) Consequently, much of the extreme western margin of present-day North America is a collage of crustal fragments that have been tacked on to the North American Craton in a complex series of events extending from the Paleozoic through the Miocene (Schermer et al. 1984; Dickinson 2004)—a process termed the accretion of allochthonous terranes.
Middle to late Tertiary changes in landform and climate were extensive throughout North America, but were particularly so in the West. The subduction of the Pacific Plate resulted in orogeny (mountain building), including the formation of the Cascade and, much later during the middle Miocene, the Sierra Nevada ranges, as well as periods of intense volcanism (Schermer et al. 1984; Dickinson 2004). Because these processes have extended well into the Cenozoic, they are certainly recent enough to have impacted the flora and fauna of western North America (Minckley et al. 1986); if not modern species, then certainly their evolutionary lineages.
FIGURE 3.1. The approximate early Jurassic continental margin of North American. Based on Stokes (1986), Aberhan (1999), and Dickinson (2004)
In addition to the major geomorphic changes of mountain building and volcanism, the composition and distribution of Western fish assemblages have been shaped by a general climatic trend toward increasing aridity, resulting in the drying of large lakes and shrinking or loss of streams present during the Miocene and Pliocene, and the concomitant extinction of many populations (G. R. Smith 1978). The restriction or extirpation of some of the early components of the western fish fauna, such as mooneyes, smelts, pikes, sunfishes, and catfishes, was likely associated with habitat alterations brought on by uplift and climatic shifts that changed the low-gradient, meandering rivers of the Oligocene to higher-gradient streams flowing over diverse landforms, and with altered drainages that characterized the Miocene and later streams (Minckley et al. 1986
The general trend of increasing aridity, coupled with aperiodic severe droughts lasting decades or even hundreds of years, prompted Matthews (1998) to suggest that western fishes must have suffered through periodic extirpations followed by long periods of recolonization. In fact, in reference to the development of the western North American fish fauna, Minckley et al. (1986) commented that “taxa that persist have dealt with far more spectacular geologic and climatic events than their counterparts in other parts of the Continent.” In fact, fishes of western North America have suffered higher extinction rates compared to eastern fishes. As a result, many western species are relics of groups that were once more speciose but which have lost species through extinction over the past 1–3 million years (G. R. Smith et al. 2010).
Colorado Plateau
Distribution patterns of modern western fish faunas tend to correspond to the continental subplates formed from the accreted terranes, and drainages that extend into adjacent subplates tend to have faunas that have been derived from several sources (Minckley et al. 1986). For example, the Colorado Plateau (Figure 3.2) is both a tectonic and physiographic province that has remained internally stable. At the close of the Cretaceous, the region of the Plateau was near sea level; it then experienced approximately 2 km of uplift during the Cenozoic, especially during the Pliocene. The uplift occurred in two phases, with the second phase taking place only within the last 5 million years (Morgan and Swanberg 1985). The Colorado Plateau is drained by the Colorado River; hence, elevation changes of the Plateau have had major impacts on the directions of flow and drainage connectivity. The uplift resulted in the isolation of the Colorado Plateau as north-flowing streams from central Arizona were interrupted. Also, the uplift of the Wasatch Front, starting in the early Eocene, and the subsequent drop of the Great Basin in late Oligocene isolated the upper Colorado River fauna from that of the Great Basin along its northwestern margin (Figure 3.2).
Origins of the upper Colorado River fish fauna (including watersheds of the Green and Colorado rivers) are ancient, likely having begun in the Oligocene and Miocene in streams draining the uplifted Rocky Mountains and flowing across the Colorado Plateau to interior basins in Colorado and New Mexico (the Miocene Bidahochi Basin) or northwestern Arizona (the Miocene Hualapai Basin) (Figure 3.2) (Oakey et al. 2004). A middle section of the Colorado River, currently comprising the Little Colorado, Virgin, and White rivers, drained to the southwest, while a third section, including the Gila River, was incorporated into the drainage after the retreat of the Bouse Miocene/Early Pliocene Embayment (Minckley et al. 1986)—an embayment that, while first thought to be a marine or estuarine extension of the Gulf of California, now appears to have been a series of lakes, with perhaps the lower being saline (Dillon and Ehlig 1993; Spencer and Patchett 1997; Roskowski et al. 2007). The upper and lower Colorado River systems were joined perhaps 10.6 to 3.3 mya, following headward erosion of streams of the middle and lower Colorado watersheds and through reoccupation and reversal of flow in older channels (Figure 3.2). Prior to this time, the upper Colorado River drainages flowed into a closed basin. It was not until the Pliocene that the Colorado River reached the Gulf of California (Minckley et al. 1986; Powell 2005).
The origins of the dominant components of the mainstem Colorado fish assemblage (Colorado Pikeminnow, Ptychocheilus lucius; Humpback Chub, Gila cypha; Roundtail Chub, G. robusta; Bonytail Chub, G. elegans; Speckled Dace, Rhinichthys osculus; Razorback Sucker, Xyrauchen texanus; Bluehead Sucker, Catostomus discobolus; and Flannelmouth Sucker, C. latipinnis) can be traced to these various geological events (Minckley et al. 1986; G. R. Smith et al. 2002). Most of the species have their closest relationships with populations in the north and west, but some also show relationships to the south. Colorado Pikeminnow, and Roundtail, Humpback, and Bonytail chubs, show relationships to the north and west, including the Sacramento-San Joaquin Basin in what is now California, but also to the Miocene Bidahochi Lake deposits to the southeast (Figure 3.2).
FIGURE 3.2. The Colorado Plateau (dark gray), Colorado River Drainage, and Great Basin (light gray), including other features mentioned in the text. The dotted line indicates the area covered by the map inset, which shows periodic connections of the Bonneville Basin with the upper Snake River. The Bonneville Basin (indicated by the light shading in the inset) reached its maximum extent during the late Pleistocene; dashed lines in the inset show boundaries of the two Utah sucker clades. Based on Minckley et al. (1986), Curry (1990), Spencer and Patchett (1997), Gross et al. (2001), Johnson (2002), Cook et al. (2006), and Desert Fishes Council (2012).
Although somewhat uncertain, Speckled Dace may have originated from populations in Tertiary Lake Idaho or ancestral Snake River drainages (Oakey et al. 2004) and thus would have relationships to the north and west. Speckled Dace are characteristic of highergradient, smaller streams and show extensive genetic structure among populations, including those in the Colorado River; populations occupying the upper, middle, and lower Colorado River form three distinct genetic groups (Oakey et al. 2004).
The Flannelmouth Sucker also shows relationships to the north and west, and the Bluehead Sucker shows relationships with forms in the Bonneville Basin to the west. Origins of the distinctive Razorback Sucker are less understood, but the divergence of the Xyrauchen lineage from that of Deltistes and Chasmistes likely occurred in the late Miocene, if not before, and suggests a relationship to the north or northwest (Miller and G. R. Smith 1981; Hoetker and Gobalet 1999).
The available evidence indicates that the Colorado River fish fauna is ancient and many of the changes in species composition predate Pleistocene events. Because of the age and action of climatic and tectonic events, faunal assembly of the main-channel Colorado River fish fauna likely occurred as a series of additions, separated in space and time, so that the modern fauna is a composite of species of different evolutionary origins and ages. However, it is also important to recognize that the post-Pleistocene history of the region is again characterized by physical changes. For instance, although the southwestern region has progressively become warmer and drier, the pattern is not one of continuous warming and drying but one of high variability in climatic patterns, including periodic severe droughts.
The impact of post-Pleistocene drought on western fishes is illustrated by work on genetic variability in Flannelmouth Sucker, one of the ancient, endemic species of the Colorado River. Genetic diversity in Flannelmouth Sucker is surprisingly limited for such an ancient species and is consistent with the hypothesis of a major, basin-wide, population crash during a known time (post-Pleistocene) of severe western drought, followed by a period of rapid repopulation growth and range expansion during a wetter period. Populations of Flannelmouth Sucker in the upper regions of the Colorado basin are the result of migration from refugia in the lower part of the system, generally within the last 10,000–11,000 years (Douglas et al. 2003).
Great Basin
The Great Basin comprises a large area of complex geology located northwest of the Colorado Plateau and including large areas of Nevada and Utah and parts of southeastern Oregon, southern Idaho, southwestern Wyoming, eastern California, and northern Mexico (Figure 3.2). As suggested by the term “Great,” the Basin makes up almost 20% of the United States and constitutes the largest inland drainage in North America (Sigler and Sigler 1987). It includes more than 150 smaller drainage basins separated by approximately 160 regularly spaced mountain ranges forming the basin and range topography (G. R. Smith 1978; Sigler and Sigler 1987; Sada and Vinyard 2002). Two large subbasins make up the Great Basin—the Bonneville Basin, occurring primarily in eastern Nevada and Utah, and the Lahontan Basin to the west, occurring mostly in Nevada and parts of eastern California. Most of the topography was formed in the last 20 million years, and many of the genera of fishes have occupied the area since the Pliocene (approximately 5 million years ago) and some since the Miocene (G. R. Smith 1981; Dowling et al. 2002; G. R. Smith et al. 2002). The fish fauna includes at least 102 species and subspecies in 15 genera of generally small-bodied forms, but the unique feature is the high level of endemism of both fishes and invertebrates (Sada and Vinyard 2002; G. R. Smith et al. 2010). At least 66% of the fish species are endemic to the Great Basin, most to specific drainages within the region (G. R. Smith 1978; Sada and Vinyard 2002). During the Pliocene and Pleistocene, repeated periods of high rainfall and changes in drainages due to volcanism resulted in a series of lakes, many quite large, in the Great Basin and created a very different environment compared to the modern-day desert—in fact, what is now Nevada was a land of abundant natural lakes (Figure 3.3). As shown by the green areas in Figure 3.3, some of the highest lake levels occurred in the early-middle Pleistocene, approximately 650,000 years ago (Reheis 1999). Two of the largest lakes were Lake Lahontan to the west and Lake Bonneville to the east. The Bonneville Salt Flats are part of the remains of Pleistocene Lake Bonneville, which is survived by the modernday Great Salt Lake and two smaller freshwater lakes, Bear and Utah, in the northeastern part of the Great Basin (Figs. 3.2 and 3.3) (Mock et al. 2006). The large Pleistocene lakes and interconnecting streams allowed aquatic organisms to colonize many of the lake basins; however, the subsequent increasing aridity during the late Pleistocene and post-Pleistocene, in part caused by the uplift of the Sierra Nevada Mountains, resulted in the isolation of the faunas, contributing to the high level of endemism and also to the frequent loss of species through extinction (Hubbs et al. 1974; Reheis 1999; G. R. Smith et al. 2002). Especially because of competition for the limited water in the now arid region, human impacts on the rate of extinction have also been particularly great (Sada and Vinyard 2002).
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).
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, 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).
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 McPhail (1967) and Porter and Swanson (1998).
Because glaciers are currently retreating, it is also possible to study postglacial colonization as it is now occurring. Cluster analysis of observed plant and animal taxa from newly developing streams in Glacier Bay, Alaska, results in three categories (Milner 1987). Newly emergent, meltwater streams support a limited biota consisting of algae and insects. Clearwater streams that are supported by runoff and snowmelt from the watershed have a greater diversity of insects compared to meltwater streams and also support a few Pink and Chum salmon (Oncorhynchus gorbuscha and O. keta). Clearwater streams fed from lakes, resulting in increased buffering of water quality, have extensive growth of algae and mosses, as well as a higher diversity of invertebrates and fishes.
SUMMARY
Biotic assemblages in general have undergone a continual cycle of breakup and rearrangement over geological time. As shown by this chapter, it is clear that fishes are no exception to this general pattern. Populations of freshwater fishes in most of North America have, through various means, been subjected to periodic fragmentation and restriction of their ranges. In some cases, as with typically lowland groups such as mooneyes and catfishes in western North America, their extirpation subsequent to the Oligocene was complete throughout the region. In other cases, range restriction and fragmentation was followed by population and range expansion, such as in Flannelmouth Sucker or in numerous species that recolonized northern North America following the retreat of the Pleistocene ice sheets. Recolonization of faunas most likely occurred as a mosaic, with specific faunal elements added over time from specific source regions. Especially in formerly glaciated regions of northern North America, recolonization of postglacial habitats occurred from often multiple refugia, greatly adding to the historical complexity of species and assemblages. Regions that remained free of ice and were otherwise less impacted by tectonic or climatic changes, such as the southeastern United States, support the greatest diversity of freshwater fishes.
The dynamic history of North American fish assemblages also carries an important conservation message. The goal of conservation of fishes should not only be to preserve species and assemblages in a “snapshot” of time but to conserve the full biodiversity of species and assemblages so that they have the potential to respond to natural (as well as anthropogenic) changes in their environments. This is clearly a challenging but critically important objective.
SUPPLEMENTAL READING
Pielou, E. C. 1991. After the Ice Age, the return of life to glaciated North America. University of Chicago Press, Illinois. An important general reference on the recolonization of formerly glaciated areas of North America.
Powell, J. L. 2005. Grand Canyon, solving the earth’s grandest puzzle. Penguin Group, New York, New York. A fascinating account of the untangling of formation of the Grand Canyon, beginning with the work of John Wesley Powell.
Sada, D. W., and G. L. Vinyard. 2002. Anthropogenic changes in biogeography of Great Basin aquatic biota. Smithsonian Contributions to the Earth Sciences 33:277–93. Details the multiple ways that humans (including native hunters and gatherers, the first European settlers, and modern society) have interacted and impacted aquatic faunas in the Great Basin.
Smith, G. R., C. Badgley, T. P. Eiting, and P. S. Larson. 2010. Species diversity gradients in relation to geological history in North American freshwater fishes. Evolutionary Ecology Research 12: 693–726. A recent and thorough synthesis of factors shaping the North American freshwater fish fauna.
WEB SOURCES
Desert Fishes Council. 2012. Species tracking. http://www.desertfishes.org/?page_id=327.
Reheis, M. 1999. Extent of Pleistocene Lakes in the western Great Basin: U.S. Geological Survey Miscellaneous Field Studies Map MF-2323. U.S. Geological Survey, Denver, CO. http://geo-nsdi.er.usgs.gov/metadata/map-mf/2323/metadata.faq.html.
