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Longevity of Gopher Tortoise Burrows in Sandy Soils
Steven J. Goodman, Jennifer A. Smith, Thomas A. Gorman, and Carola A. Haas

Southeastern Naturalist, Volume 17, Issue 3 (2018): 531–540

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Southeastern Naturalist 531 S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 22001188 SOUTHEASTERN NATURALIST 1V7o(3l.) :1573,1 N–5o4. 03 Longevity of Gopher Tortoise Burrows in Sandy Soils Steven J. Goodman1,*, Jennifer A. Smith1,2, Thomas A. Gorman1,3, and Carola A. Haas1 Abstract - Gopherus polyphemus (Gopher Tortoise) populations historically occurred throughout much of the southeastern Coastal Plain, and burrows created by this species provide refugia for a large suite of commensal species. Our objective was to evaluate the physical degradation of Gopher Tortoise burrows over time. We provide burrow-status information from initial and follow-up surveys of a low-density population of Gopher Tortoises across Eglin Air Force Base (Eglin), FL, a large landscape with very deep, sandy soils. Approximately 63% (n = 79) of active and/or inactive burrows were collapsed, filled in, or substantially degraded after 2 y, 82% (n = 65) after 3 y, and 100% (n = 19) after 5 y. Compared to results from a similar study with different soils, burrows degraded more quickly on Eglin. Our results on burrow longevity can inform interpretation of Gopher Tortoise surveys and aid in predicting temporal availability of burrows for commensals. Introduction Gopherus polyphemus (Daudin) (Gopher Tortoise) inhabits open-upland habitat throughout the southeastern US. In the western portion of its range, the Gopher Tortoise is listed as threatened under the US Endangered Species Act, and it is a candidate species for federal listing in the eastern portion of its range. Both designations are the result of habitat loss and other primarily anthropogenic factors (USFWS 2011). Upper respiratory-tract disease may have also contributed to declines (Berish et al. 2010, USFWS 2011), but further research is needed to assess any long-term effects of this disease on populations (Berish et al. 2010, McCoy et al. 2007). Gopher Tortoises excavate and use multiple burrows (up to 35; Diemer 1992, Eubanks et al. 2003, McRae et al. 1981, Smith et al. 1997) in which they spend >90% of their time (Auffenberg and Iverson 1979). Due to the species’ long lifespan and the lack of long-term tracking studies, it is not known how long a tortoise uses a burrow, or how many burrows a tortoise uses in a lifetime. However, several studies suggest that tortoises readily abandon burrows in poor habitat (Aresco and Guyer 1999, Berish and Leone 2014, Diemer 1992, Guyer and Hermann 1997) and may colonize or increase use of adjacent areas where habitat conditions have improved (Ashton et al. 2008, Berish and Leone 2014, Diemer 1992, Yager et al. 2007). This behavior may be relevant for conservation because their burrows provide shelter and habitat for up to 60 vertebrate and 302 invertebrate commensal species (Jackson and Milstrey 1989, Pike and Mitchell 2013), including several 1Department of Fish and Wildlife Conservation, Virginia Tech, Blacksburg, VA 24061. 2Department of Environmental Science and Ecology, University of Texas at San Antonio, San Antonio, TX 78249. 3Aquatic Resources Division, Washington State Department of Natural Resources, Chehalis, WA 98532. *Corresponding author - sjgood@vt.edu. Manuscript Editor: Cathryn Greenberg Southeastern Naturalist S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 2018 Vol. 17, No. 3 532 imperiled species (e.g., Drymarchon couperi (Holbrook) [Eastern Indigo Snake] and Lithobates capito (LeConte) [Gopher Frog]). The burrows also provide focal points for assessing the presence of Gopher Tortoises and commensals through a variety of monitoring techniques (e.g., burrow-transect surveys, burrow scoping, and camera trapping). However, to understand use and project availability of burrows, it is important to assess their longevity on the landscape. Studies evaluating burrow longevity and degradation rates are scarce and have focused on a limited number of sites with narrow ranges of soil types. To our knowledge, only Guyer and Hermann (1997) specifically studied this topic by comparing burrow longevity across different soil types, though Diemer (1992) reported burrow-retention rates as a part of a larger study. Here, we report on burrow longevity for a population of Gopher Tortoises in Pinus palustris Mill. (Longleaf Pine) sandhills on Eglin Air Force Base, FL (hereafter, Eglin). Upland soils on Eglin are very deep, excessively drained, and rapidly to very rapidly permeable (USDA 2018). We estimated the length of time for Gopher Tortoise burrows to change status from open and available for use by tortoises (i.e., active or inactive, as described by Auffenberg and Franz 1982, Mushinsky and Esman 1994) to collapsed or filled (i.e., abandoned) using data collected during past and ongoing burrow-survey projects (Gorman et al. 2015, Haas et al. 2017). We then compared our results to a subset of data from a previous study by Guyer and Hermann (1997) that compared burrow longevity in 2 sites—one with clay-based soils (Wade Tract, GA) and another with sandier loam-based soils (Conecuh National Forest, AL (hereafter, Conecuh). The former site was located in a Aristida stricta Michx. (Wiregrass)-dominated ancestral forest with an old-growth Longleaf Pine component and an intact understory, and the latter site was located in a managed Pinus elliottii Engelm. (Slash Pine) plantation. Soils at these study sites, like Eglin, are very deep, but less well-drained and less permeable (least at Wade Tract) (USDA 2018). Also, the clay content is considerably higher at both sites (most at Wade Tract), with pronounced argillic horizons within the soil profiles. Whereas the single-grained sand soil structure on Eglin remains consistent throughout the soil profile; loose sands and fine granular structures give way at depths of 13–112 cm to less friable subangular blocky structures at the sites (most at Wade Tract) studied by Guyer and Hermann (1997). Hansen (1963) measured Gopher Tortoise burrow depths of 140–240 cm. These soil factors combined result in a lower soil consistence for Eglin soils compared to the Guyer and Hermann (1997) sites. Soil consistence is the relative strength of a soil, or more specifically, its resistance to rupture, penetration, or compression (USDA 2017). Based on prior observations, we predicted that burrows on Eglin would collapse or fill in at a faster rate compared to sites investigated by Guyer and Hermann (1997). Study Site Our study was conducted on Eglin Air Force Base, a large military installation (188,459 ha) located in the Florida Panhandle (Fig. 1). Eglin is primarily underlain by the Lakeland Soil Series (USDA 2018), characterized by nearly 100% Southeastern Naturalist 533 S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 2018 Vol. 17, No. 3 sand soil-horizons. This series belongs to the Entisols order (great group Quartzipsamments), originating from eolian, marine, or fluvio-marine sands typified by minimal pedogenic-horizon development (USDA 1999, 2018). For older deposits, low levels of horizon development may be attributable to quartz sand’s resistance to weathering (USDA 1999). In contrast, the soils studied in Guyer and Hermann (1997) were Ultisols (great group Kandiudults), which are generally considered to be older, and thus have greater horizon-development (USDA 1999, 2018). The landscape on Eglin primarily consists of Longleaf Pine-dominated sandhills interspersed with large areas (~25–4000 ha) of treeless, open test-ranges (areas used for bombing and artillery practice, as well as their associated safety-buffer areas) and pine plantations. In addition, smaller acreages of mesic upland pine and flatwoods habitats are present. After decades of fire suppression (McCay 2000), Eglin has maintained an active prescribed burning program for the past 20 years, with a current burn goal of >35,000 ha/y (USAF 2017). Other habitat-management activities include removing Pinus clausa (Chapm. ex Engelm.) Vasey ex Sarg. (Sand Pine) and Quercus spp. (oaks) and planting Longleaf Pine. Eglin has a robust fire management program at the landscape scale, but fire intensity, frequency, and effectiveness can vary considerably at finer scales, including areas where small, remnant Gopher Tortoise populations occur. Gopher Tortoises are distributed at low densities across most of Eglin (Gorman et al. 2015, Haas et al. 2017, Printiss and Figure 1. Gopherus polyphemus (Gopher Tortoise) study locations on Eglin Air Force Base, FL: Base-wide occupancy project, Turtle Creek, and Duck Pond. Southeastern Naturalist S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 2018 Vol. 17, No. 3 534 Hipes 1999) when compared to densities found by prior range-wide and Floridawide surveys (Auffenberg and Franz 1982, McCoy and Mushinsky 1995, Smith and Howze 2016) and lower than what is generally considered minimally viable (0.40 individuals/ha; Gopher Tortoise Council 2013, Guyer et al. 2012, USFWS 2011). Current low densities on Eglin are likely primarily caused by past human predation that was prevalent in the Florida Panhandle (Auffenberg and Franz 1982, Diemer 1986, FWC 2012) along with past habitat degradation (e.g., fire suppression) and other causal factors attributed to Gopher Tortoise declines throughout its range (Auffenberg and Franz 1982, Diemer 1986, FWC 2012, USFWS 2011). Methods Field surveys We surveyed for Gopher Tortoise burrows using a 2-observer, complete-survey method, in which 2 observers walked 10-m-wide adjacent transects covering an entire plot (modified from FWC 2007). Data for this study were collected as part of 3 different surveys on Eglin described as follows during: (1) a base-wide occupancy project (hereafter Occupancy Project) conducted in 2014 (Gorman et al. 2015) with a follow-up visit in 2016; (2) surveys at one of our long-term monitoring sites, Turtle Creek, initially in 2012 with a follow-up visit in 2015; and (3) annual surveys conducted at another long-term monitoring site, Duck Pond, from 2010 to 2016 (Fig. 1; Haas et al. 2017). We conducted all surveys and follow-up visits from April through November. During the surveys, we recorded the location (UTM) of burrows using a Garmin GPSMap78 (Garmin International, Inc., Olathe, KS) and measured burrow-tunnel width at a 50-cm depth (McCoy et al. 2006). We examined burrows with a flashlight or mirror and probed with a meter stick to evaluate the degree of ceiling collapse or soil accumulation due to erosion. We determined burrow condition using the following classes and criteria based on previous studies (Auffenberg and Franz 1982, McCoy and Mushinsky 1995, Mushinsky and Esman 1994, Mushinsky and McCoy 1994, Smith et al. 2005): Active/Inactive - Exhibits signs of recent tortoise activity, including footprints, scat, plastron scraping, and/or recent tortoise digging near entrance, or if no obvious signs of recent activity, has a tortoise-shaped entrance and tunnel, suggesting relatively recent maintenance, though the burrow may be partially occluded by debris. Abandoned - Burrow entrance or upper portion of tunnel substantially degraded due to soil accumulation from erosion, ceiling collapse, or substantial vegetation growing at the entrance. Major burrow modification would be needed to become available for tortoise use. Burrows converted to Canis latrans Say (Coyote) dens or usurped by Dasypus novemcinctus L. (Nine-banded Armadillo), indicated by size and shape of burrow and tracks (Kinlaw 2006, Landers and Speake 1980), were also included in this category. Includes burrows completely filled in with soil, resulting in only the apron being visible. Southeastern Naturalist 535 S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 2018 Vol. 17, No. 3 Data analysis We only included burrows categorized as active/inactive during initial surveys and did not incorporate any new burrows observed during the follow-up work. Therefore, we followed a single cohort of active/inactive burrows for each study area. We calculated the proportion of burrows abandoned for each site: annually for Duck Pond; over a 2-y period for sites used as part of the Occupancy Project; and over a 3-y period for Turtle Creek (Table 1). To allow us to compare our results to a subset of data from Guyer and Herman (1997:table 2), we categorized burrows into 3 size-classes, ≤20 cm, >20 to ≤30 cm, and >30 cm. Furthermore, we combined Guyer and Hermann (1997) burrow-status categories of “abandoned”, “invaded by armadillo,” and “filled with soil” into one category that corresponded with our abandoned designation. We did this because, for abandoned burrows observed during our study, it was hard to classify burrows into one of the categories used by Guyer and Hermann (1997). Most of the abandoned burrows we monitored were essentially or completely filled in or completely obstructed within 1 m of the opening. Once abandoned, burrows on Eglin quickly became degraded, and rarely (1 out of 66 observed over 4–5 years) reverted back to active/inactive status (S.J. Goodman, unpubl. data). We also evaluated the relationship between burrow-status change (over time) and burrow size (largest 2 size-categories) and compared our results to Guyer and Hermann (1997). For all comparisons, we used a contingencytable analysis in JMP Pro 13 (SAS Institute, Inc., Cary, NC) to assess significance of differences. Results and Discussion We recorded initial and follow-up activity status for 125 individual burrows: 60 from the Occupancy Project, 46 at Turtle Creek, and 19 at Duck Pond (Table 1, Fig. 2). The 19 burrows at Duck Pond were re-checked annually and are included in all the following totals. Of the burrows re-checked after 1 y, 16% (3/19) reached abandoned status; of those re-checked after 2 y, 63% (50/79); of those re-checked Table 1. Before and after burrow-activity status of Gopherus polyphemus (Gopher Tortoise) burrows (all sizes) collected from a base-wide occupancy project and 2 long-term monitoring plots, Turtle Creek and Duck Pond, on Eglin Air Force Base, FL. Duck Pond burrows were tracked annually from 2010 to 2016. Active + inactive Abandoned* Abandoned/ Dataset Period (y) Iinitial Follow-up (follow-up) total Occupancy Project 2 60 19 41 0.68 Turtle Creek 3 46 8 38 0.83 Duck pond 1 19 16 3 0.16 Duck Pond 2 19 10 9 0.47 Duck Pond 3 19 4 15 0.79 Duck Pond 4 19 3 16 0.84 Duck Pond 5 19 0 19 1.00 Duck Pond 6 19 0 19 1.00 *Two burrows were invaded by Nine-banded Armadillos. Southeastern Naturalist S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 2018 Vol. 17, No. 3 536 after 3 y, 82% (53/65); of those re-checked after 4 y, 84% (16/19); and of those rechecked after 5 y, 100% (19/19). Guyer and Hermann (1997:table 2) reported 31% (29/94) and 60% (118/196) of their study burrows (all size classes combined) as abandoned, invaded by Nine-banded Armadillos, or filled in after 3 y at the Wade Tract and Conecuh, respectively. Misclassification of burrow status may result in so-called abandoned burrows actually having Gopher Tortoises inside (Smith et al. 2005, Witz et al. 1991; but see Mushinsky and Esman [1994]). We minimized the potential for this error by thoroughly examining the burrow with a light or mirror and probing inside the burrow with a yard stick. Most, if not all, burrows classified as abandoned had substantial ceiling collapse or were mostly filled in with soil within 1 m of the burrow opening. Furthermore, burrow assessments were primarily conducted during seasons when Gopher Tortoises were known to be active on Eglin, which reduced errors associated with limited Gopher Tortoise burrow-maintenance activity. For the comparison to Guyer and Hermann (1997), we combined medium (>20 to ≤30 cm) and large (>30 cm) burrow categories, both for simplicity and also because longevity was not different between the 2 sizes on Eglin (n = 79: 52 medium and 27 large, P = 0.30). This similarity partially contrasts with Guyer and Hermann (1997) where larger burrows had greater longevity at Conecuh (n = 158: 95 medium and 63 large, P = 0.01), but not at Wade Tract (n = 94: 8 medium and 86 large, P = 0.25). The proportion of burrows >20 cm reaching abandonment status on Eglin (0.71, n = 79) was higher than at the more clay-based site, Wade Tract (0.31, n = 94, P < 0.0001), and at the sandy loam-based site, Conecuh National Forest (0.57, n = 158, P = 0.05; Table 2), during a similar time period. For burrows ≤20 cm, the proportion of abandoned burrows at Eglin (0.83, n = 46, P = 0.42) was marginally, but not significantly, higher than the proportion abandoned on Conecuh (0.74). Due Figure 2. Before and after burrow-activity status of Gopherus polyphemus (Gopher Tortoise) burrows (all sizes) collected from a base-wide occupancy project and 2 long-term monitoring plots on Eglin Air Force Base, FL. This figure shows only the presence of burrows detected initially in Year 0, so each line represents a decline in the cohort of burrows present in Year 0 of each study and does not necessarily indicate a decline in the total active burrows at a site (i.e., burrows that newly appeared in later years were not counted). Southeastern Naturalist 537 S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 2018 Vol. 17, No. 3 to a small sample size of small burrows on the Wade Tract, burrows in this category were not reported for this site (Guyer and Hermann 1997). Our results suggest that burrows on Eglin reach a state of abandonment and subsequent collapse at a faster rate than those at other locations (Guyer and Hermann 1997). The cessation of maintenance by Gopher Tortoises clearly decreases the lifespan of most burrows. For example, during past and current Gopher Tortoise monitoring, we have often observed burrows change status from active to abandoned within a couple of months. In addition, using camera traps at our study site has revealed that the entrances of some active burrows begin to fill in with soil after 1 large rain event, only to resume the characteristic half-moon shape once the Gopher Tortoises have conducted burrow maintenance. Burrow-entrance collapse during rain events has also been reported in the sandy soils of Highland County, FL (Mushinsky and Esman 1994). Physical degradation of burrows may depend on a combination of factors, including modification and disturbance by mammals, erosion from rain and wind, impacts from land-management activities, and/or soil type. Burrows on Eglin may be especially susceptible to physical degradation based on soil type, as the site is primarily underlain by the Lakeland Soil Series, which is characterized by very loose, sandy soils resulting in a very low soil-consistence (USDA 2017, 2018). However, the ultimate cause of the higher level of burrows reaching abandoned status on Eglin is likely due to a combination of factors, including decreasing Gopher Tortoise densities due to changes in habitat conditions or mortality, anthropogenic impacts from land-management activities, and diminished structural integrity and increased erosion and filling of burrows because of the sandier soils. Whether the higher burrow-abandonment rate on Eglin is indicative of changes in Gopher Tortoise populations is unclear; our long-term monitoring data has indicated burrow density decreases across Eglin (Haas et al. 2017). However, these results are difficult to interpret because our survey-site boundaries are area-constrained within an increasingly open landscape from ongoing habitat management. Certainly, we have observed “a continual cycle of burrow creation and abandonment” across Eglin as similarly reported by Diemer (1992). Land-management disturbances do occur on Eglin within tortoise populations, including mowing operations on test ranges that could increase burrow abandonment. However, this Table 2. Comparison of proportion of Gopher Tortoise burrows abandoned by size class after 2–3 y at sites on Eglin Air Force Base, FL, and at sites in Georgia and Alabama studied by Guyer and Hermann (1997). Size columns ≤20 cm and >20 cm refer to burrow widths. ≤20 cm >20 cm Dataset Soil series Period (yrs.) n Abandoned n Abandoned Eglin Occupancy Project Lakeland 2 19 0.79 41 0.63 Eglin Turtle Creek Lakeland 3 25 0.84 21 0.81 Eglin Duck Pond Lakeland 3 2 1.00 17 0.76 Wade Tract Faceville, Lucy, 3 0 n/a 94 0.31 Norfolk, Orangeburg Conecuh Troup, Fuquay 3 38 0.74 158 0.57 Southeastern Naturalist S.J. Goodman, J.A. Smith, T.A. Gorman, and C.A. Haas 2018 Vol. 17, No. 3 538 study suggests that there was no difference (P = 0.16) in proportion of burrows abandoned between test ranges (0.57, n = 21) and forested sites (0.76, n = 58), suggesting that burrow abandonment is similar across habitat types. In summary, we cannot confidently differentiate between higher rates of burrows becoming vacant and higher rates of structural deterioration of vacant burrows. However, we conclude there is sufficient evidence that burrows on Eglin reach a state of abandonment and subsequent collapse at a faster rate than those at other sites, likely because of the lower consistence of sandy soils. Gaining insight into the longevity of Gopher Tortoise burrows is valuable, regardless of the ultimate cause of abandonment and collapse. As burrows become increasingly important as thermal refugia on the landscape (Pike and Mitchell 2013), estimating their temporal availability for commensal species is essential. Furthermore, land managers may use burrow surveys to determine the potential effects of management activities on Gopher Tortoises and high priority commensals, and/or to determine if available habitat is suitable for Gopher Tortoises (e.g., Wigley et al. 2012). Using burrow surveys in this way may or may not provide reliable information depending on how long burrows persist and how rapidly habitats change. Monitoring burrow longevity in areas with different soil types and with different amounts of disturbance would provide valuable information for future researchers and managers. Acknowledgments We thank J. Preston, J. Johnson, K. Gault, and B. Hagedorn of Jackson Guard, and the Natural Resource Division of Eglin Air Force Base for ongoing funding and logistical support. We also thank the Department of Defense Legacy-Resource Management Program for their financial support. This work was also supported by the US Department of Agriculture National Institute of Food and Agriculture, McIntire Stennis project 1006328. We are indebted to C. Abeles, R. Bilbow, L. Blanc, A. Briant, K. Erwin, A. Hillman, K. Jones, S. Konkolics, W. Moore, J. Newman, J. Newton, S. Piccolomini, V. Porter, B. Rincon, and T. Williams for their outstanding fieldwork and enthusiasm. 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. Ashton, K.G., B.M. Engelhardt, and B.S. Branciforte. 2008. Gopher Tortoise (Gopherus polyphemus) abundance and distribution after prescribed fire reintroduction to Florida scrub and sandhill at Archbold Biological Station. Journal of Herpetology 42:523–529. Auffenberg, W., and R. Franz. 1982. The status and distribution of the Gopher Tortoise (Gopherus polyphemus). Pp 95–126, In R.B. Bury (Ed.). 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