Proposed Zoogeographic History of the Bog Turtle
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Abstract--The bog turtle, Clemmys muhlenbergii, has a complex distributional history. Its sporadic distribution, and its total absence from entire drainage systems and major sections of river basins in which it occurs, can not be explained by availability of habitat. We support Smith's (1957) belief that the bog turtle expanded its range eastward during the Pleistocene, from ancestral habitats in the midwestern prairie. We propose its main eastward dispersal routes were along glacial outlets and through the Pleistocene Laurentian and Teays basins. During glacial periods, expanding ice sheets and the drier conditions south of them, eliminated the original prairie populations. After the Wisconsin glaciation bog turtles in the Susquehanna refugium expanded northward along river flood plains and into taiga communities formed in the wake of the contracting glacier. In the southern Appalachians bog turtles apparently dispersed between river basins through stream capture or via portals formed in the theaters of capture and were thus able to negotiate barriers formed by mountain chains and cross ridges.
Introduction
The distribution of the bog turtle, Clemmys muhlenbergii, because of its fragmented geographical range and its discontinuous, island-like site-specific ecological occurrence, has been a topic discussed by many authors. Landry (1979) provides 158 references in her bibliography on the species; the majority of these reports concern new site records. Despite obvious interest in the distribution of this species there has been little effort directed toward explaining the current overall distribution of the species or its zoogeographical history. Netting (1927) reported the first record of occurrence of this turtle in western Pennsylvania, summarized and mapped all the localities known at that time, and speculated on its biogeographic history. In various popular accounts, authors have discussed in general terms the disjunct distribution of this turtle as a result of Pleistocene events, and Smith (1957) included bog turtles as an example of prairie herpetofauna that migrated into the Eastern United States via a prairie corridor extending through the Appalachians. In the past, however, most attempts to explain modern distribution patterns focused on life zones, physiographic provinces, elevation, rainfall, and other concepts that do not seem to us to be particularly applicable in explaining the occurrence of this species. In this discussion we consider the dispersal history of this turtle and present an explanation for its current distribution.
Current Distribution: Clemmys muhlenbergii has two main areas of occurrence. Its primary center of distribution is in the North Central Atlantic states (eastern New York and western Connecticut, southward through New Jersey, southeastern Pennsylvania, northern Delaware and into the northeastern counties in the Piedmont of Maryland). A separate population occurs in the Southern Appalachian Mountains of Virginia, North Carolina, northern Georgia, northwestern South Carolina, and northeastern Tennessee. There is a 430 km hiatus between these two populations. Additionally, there are three other smaller isolated areas of occurrence - the Piedmont of North Carolina, extreme western Pennsylvania (2 disjunct areas), and the southern shore of Lake Ontario in western New York. A semi-relict population occurs in the upper central Hudson River drainage, and a historic record from Rhode Island (Babcock 1917) probably represents an additional relict population which has been recently extirpated. The overall known historic distribution is delineated in Figure 1.
The bog turtle is not confined to any particular plant community, physiogeographic province or life zone, nor does its distribution appear to be restricted in any degree by elevation. It is found from near sea level to 1373m (Herman and Pharr 1986). Climate may limit the species, but does not fully explain the current distribution. The species is generally confined to areas defined by Hambridge (1941) as north of the 75° F (23.9° C) and south of the 70° F (21.1° C) July isotherms (Fig. 1).
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Pleistocene History
The Quaternary provided a rich period of climatic variation that influenced the present day distribution of reptilian fauna. Auffenberg and Milstead (1965) provide a good overview of the effect of the Pleistocene on reptiles. They state that the reptilian fauna, unlike the terrestrial mammalian fauna, of the entire last half of the Cenozoic was stable in terms of adaptative radiation, speciation and extinction despite major ecological fluctuations. No reptilian species are known to have become extinct during the Pleistocene in northeastern North America or the southern Appalachians. As a rule Pleistocene reptiles were pushed farther south with each succeeding glacial period, and returned less far northward with each interglacial. The modification of distributions as a result of fluctuating climates of this epoch was probably greater in the periglacial zones which were narrower in the east and wider in the Middle West. There was probably little climate change in the southeast during this entire period (Auffenberg 1963). It can be assumed that it is within this climatic framework that C. muhlenbergii achieved its current distribution.
Fossil Record: Although fossil Clemmys date back to the Paleocene, 70,000,000 YBP (Romer 1966), fossil records are scant and provide little insight into the past distribution of the genus. Hay (1908) described Clemmys morrisiae from the Eocene. Based on the four portions of carapace and plastron from which the fossil species was described Hay noted the "close resemblance to the corresponding parts of Clemmys muhlenbergi." How this taxa relates to the evolutionary history of C. muhlenbergii is unclear. We are aware of two fossil records of C. muhlenbergii. One is from the Cumberland Bone Cave, near Cumberland, Allegheny County, Maryland (Holman 1977), and it is only slightly outside the known current distribution limits of the species. The herpetofauna associated with this fossil includes mostly modern species which are present in the general area today, although the presence of one species, the fox snake, Elaphe vulpina, suggests that at least some of the fauna is of mid-western origin. The mammalian fauna of this same deposit has been well studied (Gidley and Gazin 1938, Nicholas 1954) and is represented by midwestern prairie species such as badgers, Taxidea taxus; coyotes, Canis priscolatranus; thirteen-lined squirrels, Spermophilus tridecemlineatus, and pocket gophers, Thomomys potomacensis (Guilday 1971). Also this same site has produced mammals that live in the area today, boreal species and a porcupine, Coendou, of South American origin. This material is from a period that bridges the Kansanan and Illinoian faunas. It dates from about 600,000 YBP. A second fossil had been described from the outer coastal plain of South Carolina (Bentley and Knight 1998). Radiocarbon dating shows it to be 18-19,000 YBP.
Prairie Origin: Smith (1957) presents evidence for a "prairie peninsula corridor" which allowed many prairie species of plants and animals to extend their distributions eastward into the area now occupied by middle latitude deciduous forest. Actually there were probably a number of corridors which formed and reformed during the glacial and interglacial periods. Smith (1957) discussed the biogeography of the prairie peninsula in detail and showed how glacial influence could have affected distributions of various species of reptiles and amphibians. The alternating semi-arid and mesic east-west dispersal corridors brought about an interdigitation and mixing of eastern and western herpetofaunal elements across eastern North America (Auffenburg and Milstead 1965). Glacial retreat opened up unoccupied habitats to the north and glacial gouging created basins for bogs, including some which are extant today. We suggest that the gradual reforestation of glaciated areas eventually fragmented the bog turtle's northern-most populations which formed after glacial retreat. The major portion of the bog turtle’s current range is south of the glacial maximum, and it is likely that during the late Pleistocene they never reoccupied the majority of the interglacial corridor, and this corridor was occupied primarily in earlier interglacial periods. By the end of the Pleistocene the xeric conditions of 4,000 to 2,000 YBP (Smith 1957) may have further restricted their former areas of occupancy.
Zoogeography
An explanation of the broad scale distributional history of bog turtles can be complemented but not fully explained by any number of floral or faunal distribution patterns. We are aware of no other terrestrial organism, other than those confined to the high elevation "islands" of the southern Appalachian, that has such an overall disjunct distribution in the Southeast. Perhaps freshwater fishes give the best zoogeographic illustration of the assumed biogeographic scenario that lead to the current distribution of the bog turtle. Unlike terrestrial animals, fishes must have connecting aquatic corridors for dispersal. While bog turtles are not highly aquatic they occupy habitats on stream terraces and their distribution is thus governed by drainage systems. By tracing the proposed distributional history of the bog turtle through documented aquatic corridors of dispersal one can minimally illustrate the opportunities available for the dispersal of this semi-aquatic turtle.
It is apparent that a number of fishes of Mississippi Valley affinities, like the prairie biota, had access to Eastern North America. Jenkins et al. (1971) discuss three principle means of dispersal for freshwater fishes between existing drainage systems the last two of which are likely to be responsible for the invasion of bog turtles onto the Atlantic slope. 1) Interconnecting River, where the mouth of the tributary meets another drainage system. These are major dispersal routes but they are essentially unavailable for headwater species such as bog turtles. 2) Stream Capture, where erosion between adjacent systems allows one system to pirate a creek or stream into its own system. Captures of larger drainages are rare (Hack 1969) and most captures involve only small section of the headwaters of larger systems. The associated fauna of one system is thereby captured into the adjacent system. Stream capture is one of the major factors accounting for Mississippi basin fauna in Atlantic slope drainage basins. 3) Glacial Outlets where the actively receding fronts of glaciers create aquatic corridors as a result of melting ice. The water is constantly flowing and series of interconnecting lakes formed by glacial scarring leave a mosaic of interconnected waterways through which aquatic organisms can migrate. While these three zoogeographic avenues of dispersal have been used to explain patterns of dispersal of aquatic organisms they also apply to semi-aquatic species as well. The semi-aquatic habitats of bog turtles and its small size would tend to limit dispersal across major ridges. It seems likely that its past distribution, like its
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In the case of bog turtles, stream capture and glacial outlet are the primary avenues of colonization. For example, not only the creeks and streams are captured when one drainage pirates another, but the entire creek valley and its associated biota become part of the pirate's system. Furthermore, while the stream captures offer a good explanation for dispersal of bog turtles, even if the turtles were not "captured" per se the theaters of capture provides a portal for bog turtles to move between drainage systems. Interconnecting rivers also would have interconnecting river floodplains which could provide corridors between systems for terrestrial and semiaquatic faunal floodplain associates. Finally the major west to east dispersal appears to be along the Pleistocene Laurentian basin where glacial outlets were a major factor connecting drainage basins.
It is known that the bog turtle existed east of the Appalachian divide from at least 600,000 YBP (Holman 1977). Thus, this turtle had the opportunity to achieve its current distribution during many of the periods of climatic shifts in the Pleistocene. It was not until the Sangamonian interglacial period (100,000 YBP) that the drainage pattern that we see today stabilized, so it is likely that the species reached the key refugia responsible for its current distribution prior to the Sangamonian. Based on the studies of Fay (1984) current distribution may have been achieved prior to 30,000 YBP, and the bog turtle almost certainly became restricted to its current assemblage of isolated populations by 4,000 YBP (see below).
Glacial outlet and the Laurentian drainage-
The fossil bog turtle specimen from Cumberland Bone Cave does not fit into the zoogeographic history outlined herein if modern day drainage patterns are used to evaluate the record. However the Pleistocene drainage (see Hocutt et al. 1986) from which this fossil was recovered was not part of the Potomac basin (as today) but in the old Youghiogheny/Casselman drainage that ran northward into the Old Upper Allegheny and Pittsburgh River (Fig 2b). Thus, the record is from a basin that connected to the current eastern Great Lakes region (Lakes Erie and Ontario). Along this same system extant relict populations occur today; Western Pennsylvania - along the Old Upper Allegheny; west central New York - along the Pleistocene Genesee and eastward. These extant populations and the fossil record are all within the former Laurentian basin.
Supporting both the occurrence of mid-western (Prairie) faunal elements in northeastern North America and the potential role of the Laurentian drainage as a west to east dispersal corridor are a number of relict populations of midwestern aquatic and wetland herpetofauna.
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The common map turtle, Graptemys geographia, today occurs throughout the Great Lakes and Saint Lawrence River system east to Vermont. There are scattered isolated populations in Lake Champlain, the central Hudson River, the central and lower Susquehanna River and the central Delaware River. The Blanding's turtle, Emydoidea blandingii, occurs east to eastern Lake Ontario (including northwest Pennsylvania). This species also has relict populations in Nova Scotia, eastern New England, and southeastern New York. The spiny softshell, Apalone spinifera, occurs east to the Great Lakes, western New York and western Pennsylvania. It has disjuncts in Lake Champlain and the Lower Ottawa and the upper Hudson rivers. Groves (1983) and Ernst and Ernst (1971) believed that Chrysemys p. marginata entered Maryland and Pennsylvania via postglacial dispersal routes. These authors believed the midland painted turtles used both the Youghiogheny River drainage to enter the western portions of these states and the Laurentian drainage to enter the Greater Susquahana. The massassauga, Sistrurus catenatus, in the east is a wetland species that occurs in western Pennsylvania and the Finger Lakes region of western New York. Collectively not only do these relict and near-relict reptiles exhibit similar distributional patterns to the bog turtle in northeastern North America, but many currently occur in the same small geographic areas. To this discussion should be added Butler's garter snake, Thamnophis butleri and Kirtland's water snake Clonophis kirtlandii. Both are endemic to the Prairie Penninsula (Smith 1957) but have been regarded as relicts that immigrated from the west, persisted in the Prairie Peninsula, and, like the bog turtle, were eliminated from their original range in the xerothermic period or by glaciers. In the east these two snakes survived the glacial advance somewhere close to the ice front and re-entered glaciated areas subsequently. The shortheaded garter snake, Thamnophis brachystoma, based on its distribution and habitat probably had a similar history.
New York, Finger Lakes area-
There are eight known localities for bog turtles in western New York (Collins 1988). This disjunct cluster of sites ranges from Genesee County east to Oswego County and south to Ithaca (Tompkins County), and within this area the distribution appears to be quite fragmented. All sites are in systems that drain northward, some through the Finger Lakes, into Lake Ontario. During the Pleistocene this areas' local drainage pattern was certainly similar to what it is today with north flowing water courses going into the Laurentian basin. At least one bog turtle site is still in the basin occupied by the Pleistocene Genesse basin. It is also likely that a glacial refugium existed in west central New York and the turtles simply moved northward through bog habitats left in the wake of the retreating glacial drift.
Western Pennsylvania-
Netting (1927) reported the first record of the bog turtle in northwestern Pennsylvania. His Crawford County locality was in a tamarack-Sphagnum swamp. Since then other colonies were discovered in Crawford County and in two other western Pennsylvania counties - Mercer and Butler. All localities are in the Ohio Basin and near the southern limit of maximum Pleistocene glacial advance. During the Pleistocene all of these localities were in the Old Upper Allegheny basin. This Pleistocene system ran northward and drained into the Laurentian River.
Susquehanna/Hudson River region-
Bog turtles likely invaded the Susquehanna and Hudson River basins from areas where the headwaters of these systems came in contact with the Laurentian basin -one of the species two major west to east dispersal corridors during the Pleistocene. Many freshwater fishes appear to have had similar eastward dispersal routes. Schmidt (1986) listed 25 "Mississippi" freshwater fish taxa which occur today in the northern Appalachians. These fishes gained access to the Hudson River from the Great Lakes during glacial retreat via the Laurentian corridor. As the glaciers receded a series of glacial lakes formed and developed outlets into the Hudson River. These outlets were available for only a short period 13,000 to 11,500 years ago (Flint 1971, Miller 1914) but enabled many species access to Atlantic slope drainages. The freshwater mussel fauna of the Hudson is also a mixture of Mississippi basin and Atlantic coastal forms (Smith 1982). The native Susquehanna fish fauna includes northern Mississippi basin species that used post-glacial outlets, perhaps from even interglacial periods earlier than the Wisconsinan. Hocutt et al. (1986) list 12 fish taxa that are clearly in this category. By contrast the Delaware River has a much less rich fauna, lacks much of the Mississippi elements, and was not directly linked to Pleistocene glacial outlets.
Bog turtles occurring along the Laurentian drainage basin had direct access to at least the Susquehanna and Hudson Rivers from populations residing in the same area they occur today in the Finger Lakes region. Through subsequent lateral captures (or at least shared areas occupied by tributaries), they had access to still other systems in Maryland, Delaware, New Jersey, and New England. Based on current shared fauna, the Susquehanna River, prior to formation of the Chesapeake Bay (15,000 BP), had all Piedmont streams and rivers south to about the Patapsco River (Maryland) as tributaries.
The hellbender (Cryptrobranchus alleganiensis) and the map turtle (Graptemys geographica) are both Mississippi basin species with disjunct populations in the lower Susquehanna River. This portion of Maryland's drainage systems inhabited by bog turtles also supports 56 native species of fishes (Lee et al. 1980) several of which are of midwestern origin (i.e., stoneroller, Campostoma anomalum; silverjaw minnow, Ericymba buccata; walleye, Stizostedion vitreum; and logperch, Percina caprodes).
The western Massachusetts bog turtle records are based on three colonies in Berkshire County in the contact zone of the Hudson and Housatonic River. The Massachusetts records are upstream from the Connecticut sites and all are in the Housatonic River basin. A number of headwater fishes that are common in the Hudson River have distributions in the Housatonic River that would indicate pirating of Hudson drainage streams by the Housatonic (i.e., round whitefish, Prosopium cylindraceum; cutlips minnow, Exoglossum maxillingua; and trout-perch, Percopsis omoscomaycus; Lee, et al. 1980). All these fish, as well as the bog turtle, have achieved only minimal success at colonization of the Housatonic drainage suggesting the corridors of dispersal are limited and recent.
Southern Appalachian region -
The Southern Appalachian population is confined to scattered localities on plateaus, stream terraces, seepage slopes, and isolated mountain fens and bogs in the Blue Ridge Province. The current distribution is discontinuous and appears to be grouped into three separate areas (pers. observations). This population is primarily in the Blue Ridge, while the northern one (Maryland to New York and New England) is east and north of it. It is unlikely that the two populations have been in contact in recent times and probably never were in direct contact. While relict populations of bog turtles seem to represent the norm, there are none to indicate a direct bridge between these two current population centers. No likely aquatic corridors between the two areas exist today, and there is no indication that direct ones existed in the past. The northern populations are almost entirely in Atlantic drainages while the majority of southern populations are in the Mississippi basin. The Southern Appalachians have a much more direct tie to the midwestern prairie system than do sites east of the Appalachians, and even today many prairie relict floral elements are found in the region.
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The Southern Appalachians was clearly a refugium that is maintained by cooler climate and higher annual precipitation. The turtles have remained mostly in the southern Blue Ridge because of the surrounding generally drier conditions and they have not expanded into adjacent provinces to any appreciable extent. The drier Cumberland Plateau and Valley and Ridge provinces occur to the west and a rain shadow exists to the east. Bog turtles in Virginia are largely confined to the Blue Ridge Plateau due to ecological constraints.
It would appear that bog turtles first entered the region via the Teays basin - the precursor of the upper Ohio basin. The present distribution of these turtles in both the New and upper Tennessee drainages does not necessarily indicate two different Pleistocene access routes into the Southern Appalachians. The sites occupied in the Tennessee drainage of the southern Appalachians tend to be much more "island-like" and do not seem to be a series of interconnecting sites as found in the New River drainage. This is largely, if not entirely an artifact of availability of habitat. The Pleistocene route of the Tennessee River makes it highly unlikely that this river was available as a dispersal route into the Appalachians.
Based on current distribution we postulate that the bog turtle gained access to the Southern Appalachians via the New River. The species arrived from its mid-western, upper Mississippi basin, ancestral prairie population via the Pleistocene Teays basin. The New River, part of the former Teays basin, is the second oldest river basin in the world (ca over 100 million YBP) and not only was available for colonization for a long period, but because of its age the basin was well eroded and had many level low- lying sites suitable for occupancy as it does today. The bog turtle’s absence from the James River drainage and the presence of a few records from the Roanoke drainage suggest that either the turtles never inhabited the Pleistocene Fincastle River system (see Ross 1969, 1971), or that they invaded the upper Teays after the Fincastle system was captured by the Roanoke and James Rivers.
Linville Gap (Avery Co., NC) is a likely portal (via the uppermost reaches of the Watauga system) between the New drainage and the northern portion of the French Broad River systems (Tennessee drainage). The Linville River (Catawba drainage) is currently within about 2000 feet of the Watauga River (Tennessee drainage) at the heads of two major valley systems at nearly 4200 feet elevation. The Watauga’s upper portion is situated in a narrow valley (ca 3000 feet elevation) that runs northeast and abuts cliffs and knobs that form the divide between it and the South Fork of the New River. Both Hodges Gap (3200 feet elevation) and Council Gap (2900 feet) are likely portals of southward dispersal. The short portion of the Watauga system that lies between the New and Tennessee drainages has three extant bog turtle sites, otherwise the species is absent from the Watauga. The abrupt change in direction of the Watauga River downstream from Laurel Creek and the well defined valley along Laurel Fork (Watauga) and Hodges Creek (New) [a route followed by SR 105] suggest that the upper portion of the Watauga may have been previously part of the New. This site may be an area of capture and the turtles' presence in the Watauga is an artifact of its original colonization of the New River drainage.
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The bog turtle’s general absence from major drainages of the Southern Appalachians (Cumberland, and Broad and its limited occurrence in others) suggest that the species never had access to the entire Southern Appalachian region. Various ridges and mountain ranges may have prevented, and continue to prevent, dispersal into drainage systems which contain proper habitat and are otherwise in close proximity. For example, its occurrence in four sites in the Savannah drainage (North Carolina and Georgia) is matched by what appears to be multiple captures by the Savannah River from various tributaries of the upper Tennessee (Ramsey 1965, Swift et al. 1986, Cashner et al. 1992). Fish species present in the headwaters of the Savannah River that support the Tennessee capture hypothesis include 10-11 species.
The Tennessee basin’s main areas of bog turtle occurrence are in the Little Tennessee River and Nantahala River systems (Clay and Macon counties NC) and in the lower French Broad system (Translyvania, Henderson and Buncombe counties, NC). The Plott Balsams, Great Balsam Mountains, and Tanase Ridge in this area separate the French Broad from the Tuckasegee River System (Jackson Co.). Bog turtles are not presently known from the Tuckasgee system. The Tuckasegee, in turn, is separated by the Cowee Mountains that run along the Jackson/Macon County line that isolate it from the Little Tennessee system. Generally, the extreme variation in elevation near the southern terminus of the Appalachians provides only small pockets of low to medium gradient habitats, and here the species micro-distribution is highly fragmented.
Bog turtles reach the southern limit of their distribution in Georgia and South Carolina. Sites in these states occur in the Blue Ridge Province and in the upper Piedmont (900-1000 ft). In South Carolina the species is known from only three sites in Greenville and Pickens counties. All three are in the Saluda River watershed (Greater Santee, Atlantic slope drainage). This area is immediately adjacent to various tributaries inhabited by bog turtles near headwaters of the French Broad River. Likely portals of dispersal occur in gaps at about 3000 feet in elevation. Portals such as Gum Gap and Slicking Gap occur along the southern ridge of the Blue Ridge (Tennessee Valley Divide) and within a few miles the South Saluda system enters low gradient areas and elevations of 1200 feet which provide suitable habitat sites for bog turtles. Hocutt et al. (1986) note that the divides of the Santee had been crossed more frequently by freshwater fishes than the Pee Dee.
Bog turtles are known in Georgia from the Little Tennessee system in Rabun County, and the Hiwassee/Ocoee River system in Towns and Union counties. They also occur at three sites in Union County in the Toccoa River (Ocoee River system). The headwaters of the Toccoa River (Canada and Kennedy Creeks) are separated from the Coopers Creek headwaters of the Nottely River (Hiwassee system) by less than 1 km. This area could have provided a dispersal corridor between the two systems. The general absence of the bog turtle from nearly all major tributaries of the Tennessee River makes it likely that the bog turtle's presence in the upper Hiwassee is a result of dispersal between headwater areas and not of a former distribution throughout the Tennessee drainage.
Tennessee’s only area of bog turtle occurrence is in Johnson County in extreme northeast corner of the state (Herman and Warner 1986). It is in the watershed of the Holston River (Tennessee/Mississippi basin), not connected to any present day aquatic corridors that would account for the species' presence, and is disjunct from sites in the New River drainage in North Carolina and Virginia. A gap between the two ranges at Green Cove and Taylor Valley is believed to have served as a portal between Big Horse Creek (New River) and Laurel Creek (Holston River). It is likely that the Shady Valley area where the turtles occur today was once part of the New River basin.
Relict Populations -
Rhode Island--While Babcock's (1917) record from New Port, Rhode Island has been questioned by various recent authors (i.e., Ernst and Bury 1977) it is not necessarily erroneous. From a biogeographic perspective the record is explainable in that it occurs near the southern limit of the last (Wisconsin) glacial advance. The locality's proximity to the Atlantic Ocean would suggest that a thermal buffer could have protected coastal areas, and the lower sea level would have provided a corridor of dispersal from areas to the south where these turtles occur today.
North Carolina Piedmont -
There are 35 disjunct sites in the upper Piedmont of North Carolina which can be roughly grouped into seven isolated bog turtle populations. Elevations of these localities range from 220 to 583 m above sea level. They are associated with four major Atlantic drainages (Roanoke, Yadkin, Catawba, and Broad) that have headwaters in the Blue Ridge Escarpment. The Yadkin River is part of the Great Pee Dee River and the Catawba is part of the Santee River. These populations are isolated from those in the Blue Ridge by distances of 4.5 to 112.5 km and from each other. Smaller Atlantic drainages that do not originate in the Blue Ridge, do not support bog turtle populations (Neuse, Cape Fear, Tar, etc.). Since there are documented theaters of capture for the Pee Dee and New (Wright 1932), it is possible that C. muhlenbergii gained primary access to the Piedmont of North Carolina via these captures or at least at portals where these respective drainages are in close contact with minimal barriers to dispersal. It is important to note that these turtles are absent from some Piedmont drainage systems that abut the Blue Ridge systems in which the turtles occur and are absent from all the ones that do not come in direct contact with drainage systems occupied with bog turtles. It is not clear if these disjunct populations are simply isolates that resulted from capture and past down stream (perhaps storm-related) movement of turtles, or if they represent relicts from a former period when the species was more widespread on the Carolina Piedmont. The fossil reported by Bentley and Knight (1998) from the outer coastal plain of South Carolina would suggest the latter, but it is likely that there were multiple extinctions in the Piedmont, and that the current distribution is related to more recent invasions.
Summary
We offer the following scenario to summarize the zoogeographic history of the bog turtle. The species evolved in wet prairie habitats west of the Appalachians. During inter-glacial periods it expanded eastward using glacial outlets and the Pleistocene Laurentian and Teays basins as its main routes of dispersal. Once dispersed into the eastern portion of North America during glacial periods bog turtles were confined to refugia primarily in wet meadows of the lower Susquehanna basin, and in the Southern Appalachians in more or less the areas they occupy today. During these glacial periods, most regions of North America were drier than during the interglacials as vast quantities of freshwater were stored in ice (some as thick as 10,000 feet) and snow. Thus, the wet areas occupied by bog turtles south of the glacial advance became fragmented and eventually lost as suitable habitat through drying and forestation. At the same time ancestral populations in the prairie region west and north of the Appalachians also declined and totally disappeared by the Xerothermic period and perhaps long before. In most recent interglacial periods, and probably prior to the end of the last glacial advance, the Southern Appalachian population became, and remained, isolated much as it is today. The species' dispersed southward from its Teays basin refugium (New River basin) into other drainage systems in the Blue Ridge and eventually into adjacent headwaters of several Atlantic slope systems. After the Wisconsin advance bog turtles in the Susquehanna refugium expanded northward along river flood plains and followed a succession of open bog habitats developing in the wake of the retreating glaciers. Others simultaneously expanded north and east, invading, or re-invading adjacent river systems (primarily the Delaware and Hudson Rivers). From the Hudson basin bog turtles, either through stream capture or via limited access portals in the Green Mountains, entered the western portion of the Housatonic basin (Connecticut and Massachusetts). Habitats forming in basins left by glacial scarring (most to later disappear because of natural succession) account for populations in western New York and Pennsylvania, and their locations suggest that they are relicts occupying former Pleistocene basins which were once major corridors of eastward dispersal. Through stream capture and subsequent dispersal along river flood plains, bog turtles occupied portions of the Carolina Piedmont and Coastal Plain perhaps during a period of climatic equability. These populations, while at one time more extensive, were reduced to relicts as modern day climates prevailed or became extinct and reinvaded the Piedmont from adjacent drainages in the Blue Ridge.
Acknowledgments: We thank Arnold Norden, William Palmer, Bern Tryon, and Robert E. Jenkins for reviewing earlier drafts of this manuscript and for their helpful criticisms.
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