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Follow-up Demographic Survey of a Florida Gopher Tortoise Population
Joan E. Diemer Berish and Erin Hoerl Leone

Southeastern Naturalist, Volume 13, Issue 4 (2014): 639–648

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Southeastern Naturalist 639 J.E. Diemer Berish and E. Hoerl Leone 22001144 SOUTHEASTERN NATURALIST 1V3o(4l.) :1633,9 N–6o4. 84 Follow-up Demographic Survey of a Florida Gopher Tortoise Population Joan E. Diemer Berish1,2,* and Erin Hoerl Leone1 Abstract - In 1995, we surveyed a previously studied (1982–1986) northern Florida population of Gopherus polyphemus (Gopher Tortoise) to document demographic changes that may have occurred over time. The sandhill study site had been unburned for approximately 8 years, resulting in increased woody midstory and decreased herbaceous groundcover. We captured 88 Gopher Tortoises in pitfall traps during May–June 1995. Eighteen (20%) of the tortoises had been previously marked; only 11% of 169 marked tortoises were recaptured. Gopher Tortoise distribution appeared to be more clumped in 1995, and density had declined by about half, likely due to habitat degradation associated with fire exclusion. Size- and sex-class distribution and clutch size were not significantly different between the two study periods. In 1995, the smallest female with detected shelled eggs had 11 plastral annuli and a carapace length of 225 mm. Habitat degradation, whether on private or public lands, is an ongoing problem for this species. Introduction Documenting and understanding changes in populations of Gopherus polyphemus Daudin (Gopher Tortoise) over multiple decades has proved challenging (Berish et al. 2012, Diemer 1992a, Diemer and Moore 1993). This burrowing keystone species is not as sedentary as was once thought and exhibits considerable individual variation in home range, movements, and burrow use (Diemer 1992b, McRae et al. 1981a, Mitchell 2005, Smith et al. 1997b). The species’ behavioral plasticity and response to changes in habitat quality result in dynamic and shifting populations over time (Berish et al. 2012). Other North American tortoise researchers have noted the preponderance of short-term studies, the difficulty in determining demographic patterns of these long-lived species, and the need for multiyear monitoring (Averill-Murray et al. 2002, Kazmaier et al. 2001, Medica et al. 2012). The small size and cryptic nature of juvenile tortoises and annual variations in juvenile/adult capture ratios further complicate understanding of tortoise-population dynamics (Averill-Murray et al. 2002, Berish et al. 2012, Kazmaier et al. 2001). One of the authors studied the demography of a Gopher Tortoise population in minimally altered sandhill habitat in northern Florida from 1982 through 1986 (Diemer 1992a). A total of 178 tortoises was captured over that 5-yr period; 10 tortoises were removed as part of another study; and 1 male (of 8 individuals translocated 1.3 km southwest of the study area) returned to the site. During that study, researchers gathered data on Gopher Tortoise density, distribution, tortoise/burrow ratio, 1Florida Fish and Wildlife Conservation Commission, 1105 SW Williston Road, Gainesville, FL 32601. 2Current address - 20 Kiva Court, Sandia Park, NM 87047. *Corresponding author - Manuscript Editor: Max Nickerson Southeastern Naturalist J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 640 recapture rate, sex ratio, population structure, growth rate, clutch size, and age to sexual maturity (Diemer 1992a, Diemer and Moore 1994). In 1995, our objective was to determine whether density, distribution, population structure, and fecundity had changed over time in this previously studied Gopher Tortoise population. Field-site Description The study site was 27 ha within a larger (~160 ha), privately owned sandhill habitat (Roberts Ranch; Diemer 1992a) located about 15 km west of Palatka, Putnam County, FL. The original study area was 10.8 ha, but we expanded it for the 1995 survey after preliminary reconnaissance revealed degraded habitat conditions, widely scattered burrows, and the possibility that some Gopher Tortoises may have emigrated. The site was burned in winter 1983 and again in 1987 after the original study was completed. In 1995, the habitat had been unburned for approximately 8 years, apparently resulting in increased woody vegetation cover and decreased herbaceous groundcover (Fig. 1). The excessively drained, sandy soil (Diemer 1992a) supported an overstory of Pinus palustris Mill. (Longleaf Pine) and mature Quercus laevis Walter (Turkey Oak); a thick midstory of Turkey Oak, Q. geminata Small (Sand Live Oak), and Q. margarettae (Ashe) Small (Sand Post Oak); and a groundcover of Aristida stricta Michx. (Pineland Threeawn), various composites (Asteraceae), and legumes (Fabaceae) (Diemer 1992a). Methods Gopher Tortoise trapping In late April and early May 1995, we conducted burrow surveys by walking transects across the expanded study site. We flagged, numbered, and classified all burrows as active (recent tortoise sign), inactive (open but without recent tortoise Figure 1. Photographs of the Roberts Ranch sandhill study site showing the relatively open canopy and herbaceous groundcover during the 1980s (A) versus the increasing midstory oak cover in 1995 (B). Southeastern Naturalist 641 J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 sign), or abandoned (partly or completely collapsed). On 10 May 1995, we set 92 pitfall traps at active burrows. Pitfall traps were either 19- or 11-L buckets for adult and larger subadult Gopher Tortoises or 4-L planting containers for juveniles; we drilled holes in the bottoms of all buckets to provide drainage. We sank pitfall traps directly in front of burrow openings, covered them with brown paper, camouflaged them with soil, and shaded them with vegetation. We added 29 traps during May and June when burrows previously classified as inactive or abandoned showed evidence of Gopher Tortoise use or when we found new burrows. We checked traps daily until 15 June; however, we temporarily removed some traps in early June due to predicted heavy rains but replaced them when the burrows showed active tortoise sign. We carefully examined all captured Gopher Tortoises to determine whether they had been marked. If not, we marked them by drilling small holes into the marginal scutes using a sequential numbering system modified from Cagle (1939). We recorded Gopher Tortoise number, carapace length (CL), plastron length (PL), shell width and depth (mm), mass (g), approximate age (equivalent to the number of plastral rings, when visible), sex, and any scute abnormalities. We used tree calipers to measure adult tortoises and dial calipers to measure smaller individuals, and weighed the Gopher Tortoises with spring scales. Sex was determined from shell morphology (McRae et al. 1981b), and maturity was determined by dimorphic characteristics for males and by presence of eggs for females (radiographs). We subjectively based our distinction between juveniles (<130 mm CL) and subadults on shell compressibility and coloration (Landers et al. 1982). We radiographed female Gopher Tortoises at the University of Florida in Gainesville, FL, to determine clutch size (Gibbons and Greene 1979), and returned them to their burrows within several days. Data analysis We used chi-square analyses to compare sex ratios and sex- and size-class distributions (adult males, adult females, subadults, juveniles) between the 1982–1986 and 1995 studies. A t-test was used to compare mean CL between sexes in 1995, and between sexes in 1982–1986 and 1995. We employed an analysis of covariance (ANCOVA) to test for a difference in clutch size between the 1995 and 1982–1986 study periods; study year was included as a categorical variable, whereas CL and its interaction with study year were included as continuous covariates to account for the effect of CL on clutch size. We initially included each female’s identification as a random variable to account for multiple observations of some females, but this term was estimated as zero and we subsequently dropped it from the model. The ANCOVA also included fixed factors—study period and its interaction with CL—to account for any differences between the study periods. All analyses were performed using SAS v9.3 (Cary, NC). Results We located and flagged 153 burrows classified as active or inactive within the 27-ha expanded study site. Burrows were sparse in thickets of Sand Live Oak. Southeastern Naturalist J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 642 Burrow concentrations occurred along a recently cut, unimproved road and an old off-road-vehicle trail that skirted the edge of an adjacent creek swamp. We captured 88 Gopher Tortoises between 10 May and 15 June in the expanded plot; 36 of them were captured within the original 10.8 ha (Table 1). The tortoise/ burrow ratio (commonly known as correction factor or occupancy rate) was 0.58. Overall Gopher Tortoise density (all size classes) was 3.3/ha. The density of adults and subadults (excluding juveniles) was 2.9/ha, and the density of adults alone was 2.1/ha. Overall density within the original 10.8 ha was identical to the density in the expanded study area (27 ha), but was only 45% of the mean 1982–1986 density. Eighteen (20%) of the captured Gopher Tortoises had been previously marked. Nine (50%) of the 18 recaptured tortoises and 27 (36%) of the 70 unmarked tortoises were within the original 10.8-ha plot. Overall recapture rate was relatively low: only 11% of 169 previously marked Gopher Tortoises (1982–1986) were recaptured in 1995. Recapture rates were 3% for formerly immature Gopher Tortoises, 10% for adult females, and 22% for adult males. Of the 18 Gopher Tortoises recaptured in 1995, 4 were captured in all 5 years during the earlier study period, 4 were captured in 4 of 5 years, 2 were captured in 3 of 5 years, 3 were captured in 2 of 5 years, and the remaining 5 tortoises were captured only once during the 1980s. The female:male ratio of adults was 1:2.5 in 1995, which did not differ significantly from the overall sex ratio (1:2) for 1982–1986 (χ2 = 0.37, P = 0.542). Size- and sex-class distribution of all Gopher Tortoises captured during 1982–1986 also did not differ significantly from that observed in 1995 (χ2 = 6.38, P = 0.095; Table 1, Fig. 2). Mean CL (mean ± SEM) for adult males (235.1 ± 3.0 mm) in 1995 was significantly less than that for adult females (256.3 ± 5.79 mm; t54 = 3.55, P < 0.001). Mean CL in 1995 was almost identical to the result from the earlier study for both adult males (1982–1986: 234.1 ± 2.54, t102 = - 0.24, P = 0.81) and adult females (1982–1986: 258.8 ± 2.87, t46= 0.44, P = 0.66). We radiographed 26 female adult or possibly adult Gopher Tortoises in 1995, 13 of which had eggs, and the mean clutch size was 4.9 (range = 2–8). Estimated mean clutch size, accounting for CL, was 5.0 ± 0.22 for 1982–1986 and 4.08 ± 0.53 for 1995, which did not differ significantly (F1,106 = 2.58, P = 0.11). The most Table 1. Mean annual number of Gopher Tortoises captured during 1982–1986 and number of Gopher Tortoises captured in the original 10.8-ha plot and in the expanded 27-ha plot in 1995 on a sandhill study site in northern Florida. 1995 10.8-ha 27-ha 1982–1986 original plot expanded plot Size/sex class Mean # % Range Mean # % Mean # % Juveniles 15 19 4–20 6 17 12 14 Subadults 17 21 14–19 8 22 20 23 Adult males 29 36 25–37 15 42 40 45 Adult females 19 24 16–22 7 19 16 18 Total 78 65–94 36 88 Southeastern Naturalist 643 J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 common (mode) clutch size was 5.0 eggs in both the 1982–1986 and 1995 studies. The smallest female with detected shelled eggs in 1995 had 11 plastral annuli and a CL of 225 mm, which was 7 mm smaller than the smallest mature female in the 1982–1986 sample. Discussion The 1995 follow-up survey revealed a reduction in Gopher Tortoise density on the original study site and a change in burrow distribution, likely due to the increased woody midstory and reduced herbaceous groundcover typically associated with fire exclusion. Burrows were relatively uniformly distributed throughout the homogeneous and high-quality sandhill habitat during the 1982–1986 study period (Diemer 1992a). Our observations suggest that in 1995, burrow distribution was more clumped and was often associated with openings and ecotones; oak thickets had reduced available foraging area. Additionally, in the absence of fire or tree-thinning, it appeared that previously grass-stage Longleaf Pines had become closely spaced saplings that further shaded out the herbaceous understory. Gopher Tortoise densities and movements are primarily related to herbaceous biomass (Auffenberg and Iverson 1979); this species prefers open, grassy, savanna-like habitats (Auffenberg and Franz 1982) and tends to avoid areas with a dense pine canopy and shrub midstory (Aresco and Guyer 1999, Boglioli et al. 2002, Mushinsky and McCoy 1994). Not only do Gopher Tortoises seek open areas for the desired forage plants, but openings facilitate both thermoregulation and proper egg incubation by allowing sunlight to reach the burrow mounds (Mushinsky and Mc- Coy 1994, Rostal and Jones 2002). Despite the habitat changes likely due to infrequent fire, the size- and sexclass structure and clutch size of captured Gopher Tortoises were not significantly Figure 2. Size-class distribution of Gopher Tortoises captured during 1982–1986 compared with 1995 on a sandhill study site in northern Florida. Southeastern Naturalist J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 644 different between the two studies. As indicated by burrow locations in 1995, tortoises selected the more-open areas of the degraded habitat. Although not necessarily desirable from an aesthetic or habitat-conservation perspective, the unimproved road that had been cut diagonally through the northern portion of the study site had opened up a swath of the forest and provided burrowing and foraging sites for the Gopher Tortoises. An older off-road-vehicle trail along the ecotone between the sandhill habitat and creek swamp also provided suitably open habitat conditions. Although we captured Gopher Tortoises in burrows along the outer periphery of a 1-ha deer food plot, we found few burrows in the plot itself; however, Gopher Tortoises probably foraged in the grassy food plot. The Gopher Tortoise recapture rate in 1995 (11%) was low relative to the rates calculated during 1983–1986 (46–75%; Diemer 1992a). During the 1980s, higher recapture rates on our site, when compared with those on a planted pine study site in northern Florida (30–37%) where Gopher Tortoises were also captured for 5 consecutive years, were attributed to more homogeneous habitat and fewer disturbances (Diemer 1992a). A low Gopher Tortoise-recapture rate (13%) was documented during a follow-up survey in 1992 at the planted pine site after it had been clearcut, and this result was attributed to the length of the recapture interval (5–10.5 years) and to post-clearcutting Gopher Tortoise dispersal (Diemer and Moore 1993). During a second follow-up survey on the planted pine site in 2009, only 8% of 211 tortoises marked in 1981–1996 were recaptured (Berish et al. 2012). In 2009, recapture rates by sex- and size-class on the planted pine site—4% for formerly immature Gopher Tortoises, 10% for adult males, 21% for adult females—showed similar percentages to those on our sandhill site in 1995, but rates for males and females were nearly reversed. Emigration, immigration, mortality, recruitment, failure to capture marked individuals, possible loss of observable marks in younger tortoises over time, and the length of the recapture intervals all probably contributed to the low recapture rates on both the planted pine site (Berish et al. 2012) and our sandhill site. Degradation of the sandhill habitat was another factor likely affecting the recapture rate. As originally conceived, the 1995 survey was designed to assess long-term Gopher Tortoise demographics in a relatively unchanged, high-quality sandhill habitat. If the habitat had been maintained in its more open state, the overall recapture rate in 1995 might have been higher. Because the study site is part of a considerably larger sandhill system, marked Gopher Tortoises could have dispersed from unsuitable habitat to more desirable areas outside the expanded study plot. Even during the 1980s, individual Gopher Tortoises moved in and out of the study area and were not necessarily captured during all 5 years. Freilich et al. (2000) reported similar findings during annual spring surveys for Gopherus agassizii Cooper (Mojave Desert Tortoise); some individuals simply went unnoticed despite intensive search efforts but then reappeared near their original location a year or more later. Unmarked Gopher Tortoises replaced marked animals on our sandhill site in 1995 and the planted pine site in 1992, accounting for similar population structures between surveys conducted in the 1980s and 1990s (Diemer and Moore 1993). However, a paucity of juvenile Gopher Tortoises on the planted pine site in 2009 Southeastern Naturalist 645 J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 changed the population structure (Berish et al. 2012). Study-site expansion, reproduction, and immigration contributed to the prevalence of unmarked Gopher Tortoises (Berish et al. 2012, Diemer and Moore 1993). The ratio of marked to unmarked Gopher Tortoises can also be influenced by dispersal of subadults (especially males) from natal colonies, long-distance movements associated with mate-seeking or nesting, and emigration prompted by habitat modification or plant succession (Auffenberg and Iverson 1979, Berish et al. 2012, Diemer 1992b, Diemer and Moore 1993, McRae et al. 1981a). In the 1995 survey and during the earlier studies on both our sandhill site and the planted pine site, annual percentages of adult tortoises ranged from 34 to 63% (Diemer 1992a, Diemer and Moore 1993). In 1982 on our sandhill site and 2009 on the planted pine site, however, few juveniles were captured, and adult percentages equaled 71–72% (Berish et al. 2012). These latter percentages are closer to the 75% and 79% reported for 2 Georgia sandhill sites by Rostal and Jones (2002), and to the >90% adults reported in Mississippi and Louisiana surveys by Smith et al. (1997a). These percentages underscore the difficulty in detecting and documenting the presence of hatchlings and other juveniles, and perhaps also the vulnerability of these smaller Gopher Tortoises to predation. Annual percentages of juveniles varied considerably on our sandhill site in the 1980s, partly as a result of detectability (Diemer 1992a). However, variations in number of nesting females, availability and locations of suitable nesting sites, and predation rate on eggs and hatchlings can also contribute to differences in percentage of juveniles over time (Diemer 1992a). Berish et al. (2012) reviewed studies of other North American tortoise species where juvenile percentages had fluctuated; those researchers also noted the potential for misrepresentation of population structures due to the challenges associated with finding cryptic, smaller individuals. In many Gopher Tortoise populations, adult sex ratios are ~1:1 (Berish et al. 2012, Rostal and Jones 2002, Smith et al. 1997a). Balanced sex ratios have also been reported for both species of Desert Tortoises (Averill-Murray et al. 2002, Lovich et al. 2011). On our sandhill site, the reasons for a sustained sex ratio in favor of males are unclear. During the 1980s, the annual sex ratio (F:M) was initially closer to 1:1 but then changed to 1:1.4 and eventually to 1:1.8 over time (Diemer 1992a). Rostal and Jones (2002) commented that the unbalanced sex ratio on our study site during the 1980s may have been related to sampling methods. In 1995, the sex ratio of our captured Gopher Tortoises was further skewed to 1:2.5 in favor of males, which could have been an artifact of the expanded study site (i.e., we captured new males that had been present during previous surveys but were outside the original plot). Another contributing factor may be related to temperature-dependent sex determination in this species (Burke et al. 1996), i.e., the skewed sex ratio over time may have resulted from increased shade in the absence of fire, which caused cooler temperatures at nesting sites, and resulted in the sex ratio shift among of hatchlings produced. Additional sampling and monitoring of the population on this site would help explain the unbalanced sex ratio, but, unfortunately, this privately owned habitat is now closed to further studies. Southeastern Naturalist J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 646 Mean clutch size declined between the 1980s and 1995, but not significantly. However, continued habitat degradation that affects forage quality and nutrition could contribute to a significant change in fecundity over time. Diemer and Moore (1994) and Rostal and Jones (2002) reviewed clutch sizes throughout the species’ range and noted that Gopher Tortoises exhibited considerable geographic, individual, and habitat-related variations in reproduction. In 1995, the smallest female with detected shelled eggs (225 mm CL, 225 PL) was smaller than the smallest mature female previously recorded (232 mm CL, 233 mm PL) on this site (Diemer and Moore 1994), but her size was within the range of other findings regarding size at maturity in northern Florida (Iverson 1980, Taylor 1982). Females mature at larger sizes elsewhere in the Gopher Tortoise’s range: 250–265 mm CL in south Georgia (Landers et al. 1982, Rostal and Jones 2002) and Mississippi and Louisiana (Smith et al. 1997a); 282 mm CL in southwest Florida (McLaughlin 1990); and 242 mm CL in central Florida (Mushinsky et al. 1994). Estimated age at maturity for females also varies geographically: 19–21 years in south Georgia (Landers et al. 1982), 10–15 years in northern Florida (Iverson 1980), 12 years in southwest Florida (McLaughlin 1990), and 9–11 years in central Florida (Mushinsky et al. 1994). During the 1982–1986 surveys on the planted pine study site and our sandhill site, females appeared to reach sexual maturity at 14–18 years of age based on discernable plastral annuli (Diemer and Moore 1994). In 1995, the smallest mature female (11 plastral rings) was apparently younger than previous estimates for this sandhill site but was within the estimates given by Iverson (1980) for age at maturity in northern Florida. The 1995 follow-up survey on our study site revealed changes in Gopher Tortoise density and distribution but not in population structure or fecundity. Gopher Tortoise density on this site will likely continue to decline unless plant succession is set back, which is usually accomplished in sandhill habitats by using fire. Gopher Tortoise fecundity could also be adversely affected by the absence of fire over time. McCoy et al. (2006) noted that the complex relationship among initial habitat structure, degree of change in habitat structure, time interval between surveys, size of habitat involved, and the level of habitat management makes it difficult to identify direct ties between changes in habitat quality and declines in Gopher Tortoise populations. Observed similarities in population structure and clutch size between the two study periods on our site do not imply that infrequent burning is adequate, but may suggest that, in this particular case, the absence of fire was at least temporarily counterbalanced by the creation of open microsites— the roadbeds—within the oak-dominated stands. However, without fire, we suggest that this once high-quality sandhill habitat will degrade further and become less suitable for Gopher Tortoises and other species. Information regarding the land-management activities that have occurred on this site since 1995 is not readily available; however, current Google Earth maps of the area indicate that evergreen cover has increased and there are fewer open areas. This site looks vastly different from its appearance on aerial maps of the 1980s. The continued decline in habitat quality does not bode well for the resident Gopher Tortoise Southeastern Naturalist 647 J.E. Diemer Berish and E. Hoerl Leone 2014 Vol. 13, No. 4 population and the reduced quality suggests that habitat degradation, whether on private or public lands, is an ongoing problem for Gopher Tortoises. Acknowledgments We extend our appreciation to I. Roberts, on whose property this study was conducted. D. Berish, D. Morgan, G. Morgan, C. Newman, and J. Wooding provided field assistance. Funding was provided by the Florida Fish and Wildlife Conservation Commission, Gainesville, FL. Literature Cited Aresco, M.J., and C. Guyer. 1999. Burrow abandonment by Gopher Tortoises in Slash Pine plantations of the Conecuh National Forest. Journal of Wildlife Management 63:26–35. Auffenberg, W., and R. Franz. 1982. The status and distribution of the Gopher Tortoise (Gopherus polyphemus). Pp. 95–126, In R.B. Bury (Ed.). North American Tortoises: Conservation and Ecology. Wildlife Research Report 12. US Fish and Wildlife Service, Washington, DC. 126 pp. Auffenberg, W., and J. Iverson. 1979. 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