Discovery of the Northern Map Turtle (Graptemys geographica LeSueur 1817) in Far Northeastern Mississippi
Abstract
The conservation of species at the periphery of their ranges has been gaining increased attention. The northern map turtle (Graptemys geographica) is a wide-ranging species that is considered globally stable. However, some states/provinces may have only peripheral populations of the species with either few existing populations or a small area occupied. In Mississippi, only a small portion of northeastern Mississippi occurs in the Tennessee River drainage, a drainage occupied by G. geographica, and range maps project that the species may occur in a small fraction of the state in Tishomingo County. However, no specimen or photographic vouchers had previously been collected for the species. We conducted canoe and visual point count surveys within creeks of the Tennessee River drainage to determine if G. geographica is present and, if so, to further assess the distribution and abundance of the species in northeastern Mississippi. We discovered the species in 2 confluent creeks in Tishomingo County, but only 1 appears to hold a viable population. The habitat of these creeks, including bedrock or gravel substrate, is similar to localities previously described for the species; the species was absent from areas surveyed dominated by clay- or sandy-bottomed creeks. Peripheral populations of a species are usually the first populations to exhibit declines, and it is therefore important to manage those populations accordingly and protect their genetic integrity. For these reasons, we suggest listing the species as critically imperiled (S1) and endangered in Mississippi. These designations would afford state protection to what is now likely Mississippi's rarest turtle species.
The conservation of species at the periphery of their ranges is gaining increased attention by researchers. Peripheral populations tend to exhibit the overall directional trend of the species status, and populations near the range periphery are usually the first to exhibit either range expansion or contraction (Lesica and Allendorf 1995; Steen and Barrett 2015). Thus, these populations may serve as “canaries in the coal mine” in order for managers and agencies to make appropriate conservation decisions and actions. However, most agencies and states prioritize species with small population ranges, species with declining populations, and/or species with shrinking ranges (Terborgh 1974; Jablonski 1987; Simberloff 1994; Channell and Lomolino 2000; Steen and Barrett 2015). While there is little argument about the importance of this conservation strategy, Steen and Barrett (2015) posed that even species with large ranges should be conserved in states where they may have small peripheral populations for the protection of potentially genetically unique populations or species that serve as a keystone species in a community.
The rivers of the southeastern United States are 1 of 3 major global turtle biodiversity hot spots, and the region boasts 46 of the nation's 59 species (Buhlmann et al. 2009; Rhodin et al. 2017). A large part of this diversity is driven by the map turtles and sawbacks of the genus Graptemys. This highly aquatic, primarily riverine genus is composed of 14 species, 9 of which are endemic to single river drainages (Lindeman 2013). Eight species of Graptemys have been reported from Mississippi's diverse river systems: Graptemys pearlensis and Graptemys oculifera (Pearl River drainage), Graptemys gibbonsi and Graptemys flavimaculata (Pascagoula River drainage), Graptemys pulchra and Graptemys nigrinoda (Tombigbee [Mobile] River drainage), and Graptemys pseudogeographica and Graptemys ouachitensis (Mississippi and Tennessee river drainages). The major threats to most Graptemys species are modifications to stream or river habitat, including impoundments, channelization, and deadwood removal (Lindeman 2013).
Graptemys geographica (the common or northern map turtle) is the most widely distributed member of its genus, occurring in the Mississippi, Tennessee, and Mobile river drainages as well as drainages in and around the Great Lakes (Vogt et al. 2018). It is also the only Graptemys to occur in some Atlantic drainages in the northeastern United States (Lindeman 2013). In Mississippi, only a small portion of the Tennessee River drainage extends into the northeastern corner of the state (1077 km2, or < 1% of the land area of Mississippi). The Tennessee River in Mississippi is impounded to form Pickwick Lake, and only 2 major tributaries occur in Mississippi: Bear and Cedar creeks. Both creeks flow through Mississippi for 35 and 6.1 river km (rkm), respectively, in a south-to-north direction before converging near the Mississippi–Alabama border, where they flow a short distance through Colbert County, Alabama, into a backwater of Pickwick Lake. Both creeks have been altered by headwater dams in Alabama, and Bear Creek has been partially channelized in Mississippi (Fig. 1; Jones et al. 2009). While G. geographica is present in the Mississippi River drainage (which covers a large portion of western Mississippi), in the southern portion of its range, the species is known mostly from small, rocky streams (Mount 1980) and is absent from the muddy and silty rivers of the Mississippi embayment.
Citation: Chelonian Conservation and Biology: Celebrating 25 Years as the World's Turtle and Tortoise Journal 19, 2; 10.2744/CCB-1416.1
Prior to the present study, a small number of anecdotal reports of G. geographica had come from Bear Creek within Tishomingo State Park, and R.L. Jones (as cited by Lindeman 2013) also suspected that G. geographica occurred in Mississippi. However, to date, there has been no verifiable evidence (e.g., photographs or specimens) to confirm the presence of G. geographica in Mississippi. Therefore, the goal of our study was to determine if G. geographica is part of Mississippi's turtle fauna. If so, we wanted to determine its distribution and abundance within the Tennessee River drainage of northeastern Mississippi. Rocky streams are limited in northeastern Mississippi, and we predicted that, if present, the species' distribution would be very small and peripheral.
METHODS
Graptemys species show a strong propensity to bask, and many species in the genus are trap averse (Boyer 1965; Selman and Qualls 2009; Lindeman 2013, 2014a). Therefore, in order to determine if G. geographica occurs in Mississippi, we used a combination of visual point count surveys from bridges or observation points and canoe floats, similar to other studies on species in this genus (Selman and Qualls 2009; Ilgen et al. 2014; Lindeman 2014b; Lindeman et al. 2020). While canoe surveys provide the ability to determine the number of turtles on average per site, the point count surveys allow a broader geographic area to be covered and capture a snapshot of turtle communities. For point count surveys, we used binoculars, spotting scopes (78-mm, 20×–60×), and a Nikon Coolpix P900 digital camera to survey 31 sites at small and moderately sized creeks of the Tennessee River drainage and also 8 sites along Pickwick Lake, a reservoir of the Tennessee River, for a total of 39 sites. Creeks and drainages surveyed included Bear, Cedar, Cripple Deer, Little Cripple Deer, Pennywinkle, Yellow, Indian, Coke, Chambers, and Sevenmile creeks in Tishomingo and Alcorn counties, Mississippi (Fig. 1). For the canoe surveys, Bear and Cedar creeks were the only navigable waterways within the Tennessee River drainage in Mississippi. We floated 3 sections of Bear Creek in Tishomingo County, including an upper reach (5 rkm), a middle reach (4.2 rkm), and a lower reach (11.4 rkm); because of its small size, we surveyed the entire reach of Cedar Creek in Mississippi (6.1 rkm). We floated Cedar Creek twice in May 2017, and we floated the Bear Creek stretches once each in August 2017.
During canoe surveys, we used binoculars and Nikon CoolPix P900 digital cameras to identify distantly basking turtles to species level, and map turtles were identified to sex when possible. Graptemys geographica demonstrate extreme sexual dimorphism, with females being larger in size and megacephalic and males being much smaller, with a conspicuous, large tail (Lindeman 2013). The digital zoom of the Nikon CoolPix P900 (up to ×80 optical zoom) was particularly useful at photographing far-off and wary turtles before they bailed into the water.
For point count surveys, we visited 39 publicly accessible points (bridge crossings or marinas) in Tishomingo and Alcorn counties, Mississippi, where we used a combination of binoculars, a Nikon CoolPix P900 camera, and spotting scopes to count and identify turtles at each site. Each site was visited once during either May 2017 or August 2018. At each location, we identified all basking turtles to species level, and each map turtle was identified to sex when possible.
During both canoe and point count surveys, we collected environmental data on stream characteristics, such as flow rate (low, medium, fast), stream width (measured in GoogleEarth), lotic habitat types (stream, river, reservoir), relative amount of basking structure (low, medium, high), and substrate composition (bedrock, cobble, gravel, sand, silt).
RESULTS
Visual Point Count Surveys from Bridges and Access Points. — During an opportunistic reconnoiter for G. geographica in the spring of 2017, G.J.B. and L.P. observed only river cooters (Pseudemys concinna) and pond sliders (Trachemys scripta) from 2 bridges that crossed Bear Creek near Tishomingo State Park. However, G.B. and L.P. observed 2 basking female G. geographica in Cedar Creek in Mississippi, ∼ 0.8 km from the Alabama border (photographic vouchers UF 179674 and 179675). During our formalized surveys in 2017 and 2018 of 39 localities in the drainage, we identified 83 of 89 individual turtles we observed as members of 6 species. We encountered only 2 G. geographica during the point count surveys (3%), both of which came from the same bridge over Cedar Creek, where the species had first been documented in 2017 (Table 1). The most frequently sighted species during bridge surveys were T. scripta and the Ouachita map turtle (G. ouachitensis).
Canoe Float Density Surveys. — On our 5 canoe floats, we identified 221 of 250 turtles we observed as members of 4 species. Despite their close proximities and shared confluence, Bear and Cedar creeks differed dramatically in turtle communities. We counted 60 G. geographica on 2 floats in Cedar Creek, including both sexes and all age classes, which was 67% of the 90 turtles identified, yet only 2 G. geographica were sighted across the 3 reaches of Bear Creek, or just 1.5% of the 131 turtles identified. We saw a single male G. geographica in the upper reach and a single adult female in the lower reach (Table 2; Fig. 1). One adult male and 2 yearlings were collected during the May 2017 floats of Cedar Creek as voucher specimens deposited in the Mississippi Museum of Natural Science (MMNS 19575–19577).
DISCUSSION
The confirmation of G. geographica in Mississippi brings the state's total number of Graptemys species to 9, more than any other state, and it brings the state's turtle species count to 31 species. Our surveys indicate that G. geographica is a highly localized and peripheral species in the state of Mississippi, and they likely have the smallest range and population of any turtle species in Mississippi (rivaled only by Pseudemys alabamensis in southern Mississippi; Leary et al. 2003). Their range in the state appears to be confined to 2 streams in Tishomingo County: Cedar Creek and Bear Creek. The only individuals of G. geographica sighted during the visual point counts were at Cedar Creek, which has only 1 bridge crossing in Mississippi. During our canoe floats of Cedar Creek, we observed numerous males, females, and juveniles, and we suggest that these observations indicate that the species is viable within in this 6.1-km stretch of creek, particularly because it is continuous with stretches of similar habitat above and below it in Colbert County, Alabama. This stretch was dominated by swift currents over bedrock and rocky/gravel shoals, similar to descriptions for the species in other parts of their range (Mount 1980; Fuselier and Edds 1994; DonnerWright et al. 1999; Jensen 2008; Lindeman 2015). Therefore, our surveys suggest that the population in Cedar Creek may be the only sustaining population of G. geographica in the state, with 3.8–6.0 turtles/rkm observed basking.
Graptemys geographica was present in Bear Creek but not as abundantly as in Cedar Creek. We did not observe any individuals during point counts at 8 bridge crossings on Bear Creek, and we observed only 2 individuals over 20.6 rkm within Bear Creek. The upper reach of Bear Creek was dominated by slow currents and sandy/silty conditions that are not suitable habitat for the species. However, the male we observed was in a small gravel riffle embedded within an otherwise unsuitable stretch of the creek. The middle and lower stretches were more similar to Cedar Creek with bedrock and rocky/ gravel shoals; thus, their much lower abundance—or near absence—in this stretch of Bear Creek is puzzling. Both Cedar and Bear creeks have reservoirs upstream of our sites in Alabama, so this does not seem to explain the differences observed. However, we suspect that the channelization of Bear Creek by the Tennessee Valley Authority in the 1970s in southeastern Tishomingo County and northwestern Franklin County (Alabama) has affected the creek's flow regime and increased stream siltation (Cooper 1987; McGregor and Cook 2006; Jones et al. 2009), which has likely impacted G. geographica abundance. Similar channelization has not occurred along any reaches of Cedar Creek. Therefore, it would be interesting to see if G. geographica exists in portions of Bear Creek upstream of channelized stretches in Franklin and Marion counties, Alabama.
The only Graptemys we encountered in Pickwick Lake were Ouachita map turtles (G. ouachitensis). Graptemys ouachitensis is well known to be both a habitat and a dietary generalist and can be abundant in lentic, reservoir habitats (Webb 1961; Lindeman 1999). Because G. geographica occur in lentic habitats in the northern portion of its range, including the Great Lakes, it is unclear why they are absent from Pickwick Lake. It seems possible that G. geographica was not observed in Pickwick Lake because G. ouachitensis excludes them from lentic habitats, impacting the realized niche of G. geographica.
We also did not observe G. geographica at any of the stream sites (other than Cedar Creek) we surveyed within the Tennessee River system of Mississippi. Although many of these streams were small and excluded G. ouachitensis, most were slow flowing with a sandy or silty substrate. Furthermore, many of these streams have been channelized to improve drainage, including Yellow, Little Yellow, Coke, Chambers, and Sevenmile creeks. However, in Cedar and Bear creeks, where the species was observed, these habitats were dominated by bedrock, rock outcrops, and gravel bars. In these stream reaches, both creeks flow through rock formations of the Chester Group from the Mississippian (359–323 MYA) period. The other streams where G. geographica were not observed had very different underlying geology that is Cretaceous in origin (145–66 MYA; Chapman et al. 2004); thus, we suspect that this geological difference likely has an effect on the distribution of this species. Our observations of habitat use by G. geographica in Mississippi are similar to those by Fuselier and Edds (1994), who found G. geographica in Kansas to exclusively use shady streams with rocky or gravel substrates when they co-occurred with other Graptemys species (G. ouachitensis and G. pseudogeographica).
Even though G. geographica may be abundant in neighboring states such as Tennessee and Alabama, peripheral populations of species like G. geographica in Mississippi warrant protection and conservation. Both Bear and Cedar creeks are bounded by steep relief from the Interior Plateau physiographic province, and their shared confluence into Pickwick Lake isolates both drainages from potential recolonization of nearby populations downstream in Tennessee and Alabama. Additionally, both creeks have upstream impoundments in Alabama, further limiting recolonization ability from populations upstream. The physiographic province where we observed G. geographica already contains several critically imperiled (S1), state-listed taxa, such as green salamanders (Aneides aeneus), spring salamanders (Gyrinophilus porphyriticus), cave salamanders (Eurycea lucifuga), and hellbenders (Cryptobranchus alleganiensis) (Mississippi Museum of Natural Science 2015). We also recommend an S1 (state critically imperiled) ranking for G. geographica due to their restricted range, single apparently viable population, and very limited occurrences outside of Cedar Creek. Further, this status is warranted because peripheral populations like this one have been posited to contain adaptive alleles due to living on the fringes of the species' environmental tolerance (Channell 2004). It is possible that these alleles have evolutionary value for the species, especially when peripherally adapted alleles can be incorporated into the core populations of a species if conditions at the core of the range change naturally or unnaturally (Lesica and Allendorf 1995; Channell 2004; Steen and Barrett 2015). For these reasons, Mississippi's G. geographica should be conserved, and future efforts should be made to periodically monitor populations in both Bear and Cedar creeks.
Map of visual point count survey locations and canoe surveys in Tishomingo and Alcorn counties, Mississippi. Graptemys geographica was observed at only 1 site during visual point count surveys (black circle). Only 2 G. geographica were detected in Bear Creek canoe surveys (stars), whereas the species was found throughout Cedar Creek in Mississippi. The thin black line delineates the Upper Tombigbee watershed (shaded gray) from the Tennessee watershed in the northeastern corner of Mississippi. The black bar represents the Tennessee–Tombigbee Waterway divide.
Contributor Notes
Handling Editor: Peter V. Lindeman
