Chelonian Conservation and Biology, 2014, 13(1): 27–34 g 2014 Chelonian Research Foundation
Gopher Tortoise Ecology in Coastal Upland and Beach Dune Habitats in Northeast Florida JAIME C. PAWELEK1,2
AND
MATTHEW E. KIMBALL1,3,*
1
Guana Tolomato Matanzas National Estuarine Research Reserve, Ponte Vedra Beach, Florida 32082 USA; Division of Insect Biology, College of Natural Resources, University of California, Berkeley, California 94720 USA [
[email protected]]; 3 Baruch Marine Field Laboratory, University of South Carolina, Georgetown, South Carolina 29442 USA [
[email protected]; fax: 843-546-1632] *Corresponding author
2
ABSTRACT. – Considered a keystone species in the southeastern US coastal plain, gopher tortoises occupy a variety of upland habitat types. Although upland pine-dominated habitats have received much attention, few studies have examined tortoises in nearby coastal beach dune habitats. To examine the distribution and abundance of gopher tortoises in northeast Florida coastal habitats, comprehensive burrow surveys were conducted 3 times during the last 8 yrs on 2 separate gopher tortoise populations near St. Augustine, Florida. The objectives for these surveys were to 1) examine changes in population size and structure in representative upland and coastal dune habitats and 2) describe the response of gopher tortoises to land management practices (i.e., prescribed fire). Total burrow density within the upland population remained stable over the 3 surveys (0.68 burrows/ha in 2005, 0.66 burrows/ha in 2007, and 0.64 burrows/ha in 2011). Burrow densities in the burned areas of the upland habitat increased in areas that were burned at least once since the time of the first survey in 2005 but decreased in areas that were infrequently burned or remained unburned. Burrow density on the beach dunes, however, greatly increased between the most recent 2 surveys (3.63 burrows/ha in 2007 and 8.95 burrows/ha in 2011), possibly attributable to an increased use of a relatively confined habitat, or sampling biases by surveyors. Three different correction factor methods were used to determine gopher tortoise population estimates for each of the surveys, providing a range of estimates. The results of this study and the few others focused on coastal beach dunes suggest that this vulnerable, yet important, habitat should receive more attention because some of the highest gopher tortoise burrow densities have been observed there. KEY WORDS. – Reptilia; Testudines; gopher tortoise; burrow density; coastal dunes; upland; prescribed fire The gopher tortoise (Gopherus polyphemus), the only land tortoise native to the southeastern coastal plain of North America (Auffenberg and Franz 1982), is state or federally listed throughout its entire range (Diemer 1992a; Mushinsky et al. 2006; Castello´n et al. 2012). Gopher tortoises construct extensive burrows, which they use to protect themselves from adverse weather, temperature extremes, and predators (Diemer 1986). Because these burrows are also used by hundreds of other animals (Jackson and Milstrey 1989), many of which are dependent on them for shelter and even food, gopher tortoises are considered to be a keystone species in coastal plain habitats (Eisenberg 1983). Auffenberg and Franz (1982) estimate that in the last 100 yrs gopher tortoise populations have decreased by about 80%. Population declines throughout their range have been attributed to habitat fragmentation and alteration, disease, harvest by humans, and road mortality (Diemer 1986; McCoy et al. 2006; US Fish and Wildlife Service [USFWS] 2009). Gopher tortoises are found throughout Florida in a variety of upland habitat types including pine flatwoods and xeric hammock, as well as in coastal habitats, all of
which are threatened by land development and urbanization (Auffenberg and Franz 1982; Diemer 1986; Florida Natural Areas Inventory [FNAI] 1990; Johnson and Barbour 1990; Mushinsky and McCoy 1994). They prefer open sandy areas to dig their extensive burrows, where they ultimately spend most of their time. Upland pine-dominated habitats in northern Florida have received much attention (Alford 1980; Means 1982; Diemer 1992a, 1992b; Butler and Hull 1996; Butler and Sowell 1996; Berish et al. 2012), but few studies have examined nearby coastal beach dune habitats (Lau 2011; Lau and Dodd, in press), as much of the coast has been developed and few pristine beach dune habitats remain. The Guana Tolomato Matanzas National Estuarine Research Reserve (GTM NERR), near St. Augustine, Florida, is unique in that it is home to 2 separate gopher tortoise populations residing in upland and coastal habitat types, which have been monitored since 2005. To help open canopy for nesting and basking, and to stimulate regeneration of many forage species, prescribed fire is used to manage GTM NERR’s upland habitats for gopher tortoises. To examine the distribution and abundance of gopher tortoises in northeast Florida coastal habitats, comprehensive
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Figure 1. Burn units (160 ha; 41%) and the total area surveyed (390 ha) on the Guana peninsula.
burrow surveys were conducted 3 times during the last 8 yrs. The main objectives for these surveys were to 1) examine changes in gopher tortoise population size and structure in representative upland and coastal dune habitats and 2) describe the response of gopher tortoises to commonly used land management practices (i.e., prescribed fire). METHODS The GTM NERR is composed of 2 separate areas located just north and south of the city of St. Augustine (St. John’s and Flagler counties) in northeast Florida, encompassing roughly 29,685 ha of upland and aquatic habitat. Several state parks, state forests, and other conservation lands fall within the GTM NERR boundaries and are managed by various agencies. The GTM NERR mostly consists of beach dunes, tidal salt marsh, coastal strand, and more upland habitats including maritime hammock, shell mound, mesic flatwoods, and xeric hammock. The specific study sites within GTM NERR include the Guana peninsula and the undeveloped beach dunes, which were surveyed in 2005 (Guana peninsula only), 2007, and 2011. The Guana peninsula is roughly 390 ha of upland habitat, consisting primarily of xeric hammock (300 ha), along with small pockets of mixed habitats including shell mound (35 ha), freshwater marsh (35 ha),
Figure 2. Beach dune burrow locations observed in 2011 with a close-up view of tortoise burrows and habitat north of the north beach crosswalk, showing increased burrow density and open habitat characteristics. The north, middle, and south beach crosswalks, as well as emergency access point over the dunes, are indicated in the larger map. The emergency access point is the division between the north and south sections of the beach.
mesic flatwoods (10 ha), scrub (7 ha), and ruderal (3 ha) habitats. The peninsula is bordered on the east and south by the Guana River and on the west by the Tolomato River (Fig. 1). Thick stands of saw palmetto (Serenoa repens) made up the dominant understory vegetation with various oaks (Quercus virginiana, Quercus laurifolia), pignut hickory (Carya glabra), magnolia (Magnolia grandliflora), and redbay (Persea borbonia) comprising the over-story (Frazel 2009; FNAI 2010). The undeveloped beach dunes are located about 5 km north of the Guana peninsula on the east side of A1A within the reserve (Fig. 2). Extending along the coast for approximately 6.5 km (100 m at its widest point), this undeveloped beach dune habitat includes about 47 ha of vegetated coastal habitat (beach dune and coastal strand). The dunes are some of the highest in the state, ranging up to 10 m high, and are bordered to the north and south by private residences. The area accessible for surveying gopher tortoise burrows was approximately 26 ha. Three boardwalks allowing public access by foot were located within this stretch of undeveloped beach dunes. This 26ha area is roughly divided into two equal beach sections, termed the northern and southern sections, by an
PAWELEK AND KIMBALL — Gopher Tortoise Ecology emergency access road through the dunes. The primary vegetation consisted of sea oats (Uniola paniculata), saltmeadow cordgrass (Spartina patens), blanketflower (Gaillardia pulchella), camphorweed (Heterotheca subaxillaris), railroad vine (Ipomea pes-caprae), and spotted beebalm (Monarda punctata) (Frazel 2009; FNAI 2010). The GTM NERR has a prescribed fire history dating back to 1988. The Guana peninsula includes 4 burn units (1, 2, 3, and 26), separated by sandy trails, which make up 160 ha, roughly 41% of the upland habitat surveyed (Fig. 1). The area outside the burn units will be referred to as ‘‘unburned areas.’’ Unit 1 consists of roughly equalsized areas of mesic flatwoods, scrub, and xeric hammock. Unit 2 is primarily freshwater marsh surrounded by xeric hammock. Unit 3, the largest, is mainly xeric hammock with small fingers of freshwater marsh extending down into the northern part of the unit from Unit 2. The smallest, Unit 26, is all ruderal habitat of mostly grasses, bordered to the north and south by Guana Lake and the Guana River. The beach dunes are divided into seven burn units, but to date no prescribed burns have occurred. Comprehensive burrow surveys examining each habitat section were conducted in 2005 (peninsula only), 2007 (peninsula and dune), and 2011 (peninsula and dune) by teams of GTM NERR biologists and trained volunteers. Surveyors lined up next to each other, roughly 5–10 m apart, and walked slowly, scanning the ground for evidence of tortoise burrows. The surveyors on each end marked their line with flagging, such that areas surveyed were clearly delineated to ensure complete coverage; thus, the entire area within the peninsula and dune habitats were completely surveyed. Surveys were conducted during fall and spring months, on days when temperatures were warm enough for tortoise activity. Burrow locations were recorded with Garmin global positioning systems (GPSmap 76CSx) and a single lead GTM NERR biologist on each survey assigned an activity status, based on Auffenberg and Franz (1982) criteria (i.e., active, inactive, abandoned). Active burrows were well maintained and showed signs of recent gopher tortoise activity in the form of footprints or plastral slide marks; inactive burrows appeared maintained by tortoises but lacked recent tracks; and abandoned burrows showed no recent signs of activity and may have been filled with leaf litter (Auffenberg and Franz 1982). Gopher tortoise size class, based on burrow width at the opening (i.e., juvenile: , 12 cm, subadult: 12–20 cm, adult: . 20 cm) was also documented (FNAI 2011). Only active and inactive burrows were recorded in 2007. Burrows that were collapsed were not included in any surveys. Gopher tortoise population estimates were derived using multiple correction factors to provide a range of estimates (e.g., conservative to generous). Auffenberg and Franz (1982), proposed a correction factor of 0.614, which is multiplied by the number of active and inactive burrows, to calculate total number of tortoises in a
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population. Amatuli (2012) examined a nearby gopher tortoise population in upland habitat in northeast Florida (approximately 32 km from GTM NERR) and determined a site-specific correction factor of 0.53. A third population estimate was calculated using the least-square regression equation from McCoy and Mushinsky (1992b), which was developed based on 8 other studies on 17 populations, as well as on their own data. They determined that using the number of active burrows to calculate the total number of tortoises resulted in the most accurate population estimates. RESULTS In 2005, 263 burrows were recorded on the Guana peninsula (Table 1). Burrow locations were recorded, as well as the activity status, but for only about half the burrows. No gopher tortoise size-class information was recorded for any burrow in 2005. In 2007, both the Guana peninsula and undeveloped beach dunes were surveyed for a combined total of 351 burrows: 257 on the peninsula and 94 in the beach dunes (Table 1). Burrows were categorized as used by either adult (which combined both adult and subadult size classes) or juvenile gopher tortoises. On the Guana peninsula in 2007, 72% of observed burrows were active (95% adult; 5% juvenile). On the beach dunes, 79% were active and 21% inactive. In 2011, 479 burrows were recorded: 247 on the Guana peninsula, and 232 in the beach dunes (Table 1). On the Guana peninsula, 60% of observed burrows were active (75% adult; 15% subadult; 10% juvenile); 17% were inactive (76% adult; 24% subadult); and 23% were abandoned (81% adult; 19% subadult). On the beach dunes, 83% of burrows observed were active (81% adult; 11% subadult; 8% juvenile), 7% were inactive (63% adult; 19% subadult; 19% juvenile), and 10% were abandoned (62% adult; 13% subadult; 25% juvenile). Burrow density (total burrows observed divided by area surveyed) on the Guana peninsula was consistent for all 3 surveys with 0.68 burrows/ha in 2005, 0.66 burrows/ ha in 2007, and 0.64 burrows/ha in 2011. On the beach dunes, the burrow density was 3.63 burrows/ha in 2007 and 8.95 burrows/ha in 2011. The burrow density was higher in the northern section of the beach dunes, compared with the southern section, in both 2007 and 2011 (Figs. 2 and 3). Considering the 4 individual burn units on the peninsula, burrow density within burned units was slightly higher in both 2005 and 2011 than in the unburned areas (Table 2; Fig. 4). Burrow density increased for all burn units with each successive survey except Unit 1, which steadily decreased with each survey (Table 2). Three of the 4 units have been burned at least 4 times since 1988, and the remaining unit (Unit 26; ruderal habitat) was burned only once in 2010 (Table 2). All but Unit 1 have burned at least once since surveys began in 2005 (Table 2).
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Table 1. Gopher tortoise burrow abundance (total number) by burrow status (active, inactive, abandoned, unknown) and tortoise size class (adult, subadult, juvenile) for each habitat (peninsula, beach dune) examined. Activity status is unknown for almost half of the burrows sampled in 2005. Abandoned burrows were not recorded in 2007. Size classes were not recorded for any burrow in 2005, and the subadult size class was included with the adult size class for the 2007 survey. Gopher tortoise population estimates for each habitat and year were calculated using 3 individual correction factors. Burrow status and tortoise size class Active Adult Subadult Juvenile Inactive Adult Subadult Juvenile Abandoned Adult Subadult Juvenile Unknown status Total burrows Total tortoisesa Total tortoisesb Total tortoisesc a b c
2005
2007
2011
Peninsula
Peninsula
Beach dunes
Peninsula
Beach dunes
99 — — — 47 — — — 3 — — — 114 263 49 77 90
184 175 — 9 73 73 — 0 — — — — — 257 91 136 158
74 74 — 0 20 20 — 0 — — — — — 94 37 50 58
148 111 22 15 42 32 10 0 57 46 11 0 — 247 73 101 117
192 155 21 16 16 10 3 3 24 15 3 6 — 232 95 110 128
Based on McCoy and Mushinsky (1992b) least-squares regression equation from active burrows. Based on Amatuli (2012) correction factor of 0.53. Based on Auffenberg and Franz (1982) correction factor of 0.614.
Gopher tortoise population estimates were calculated based on the 3 different methods and produced a range of estimates (Table 1). The Amatuli (2012) and Auffenberg and Franz (1982) correction factors combine both active and inactive burrows for their calculations. The McCoy and Mushinsky (1992b) correction factor used only the number of active burrows, and in 2005 only the number of known active burrows (n 5 99) was used for this calculation (note that 114 burrows remain unclassified and are not included in the population estimate). For all habitats in all years, the range was defined with the McCoy and Mushinsky (1992b) correction providing the most conservative estimates and the Auffenberg and Franz (1982) correction providing the most generous
Figure 3. Burrow density on Guana Tolomato Matanzas National Estuarine Research Reserve coastal beach dunes. South section 5 9.89 ha; north section 5 16.02 ha.
population estimates. The regionally derived correction factor from Amatuli (2012) was consistently in the middle of this range. In 2005, the estimates range 49–90 tortoises on the Guana peninsula. For 2007, the estimates range 91–158 on the Guana peninsula and 37–58 on the beach dunes. In 2011, the population estimates range 73–117 on the Guana peninsula and 95–128 on the beach dunes. DISCUSSION Compared with surveys of other coastal beach tortoise populations (Kushlan and Mazzotti 1984; McCoy and Mushinsky 1992a; Muchinsky and McCoy 1994; Waddle et al. 2006), burrow density in GTM NERR habitats was relatively low (Table 3). These other surveys took place in similar habitats along the coast of Florida at Fort Matanzas National Monument (FMNM), located within GTM NERR boundaries, in southern Florida at Cape Sable, within Everglades National Park, and in other coastal state and federal lands. At both J.N. ‘‘Ding’’ Darling NWR and FMNM complete surveys of the entire area were conducted, but only strip transect surveys were conducted at Cape Sable in southern Florida. While burrow density at GTM NERR was lower than Cape Sable and FMNM, an increase in density at GTM NERR was observed from 2007 to 2011, similar to that recorded at both Cape Sable and FMNM during their survey periods. Although the habitat at FMNM is the most similar to that at GTM NERR, burrow density (for active and inactive burrows only) at FMNM was much higher than at GTM NERR (Table 3). The area surveyed at FMNM was smaller (10.97 ha), but it was much wider for most of its
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Table 2. Prescribed fire history (number of fires since 1988), the date (yr) of the last fire (i.e., DLF) and burrow density (burrows per hectare) for 3 surveys (2005, 2007, 2011) within the 4 burn units on the Guana peninsula. Burn Unit No. (area) 1 2 3 26
(24.7 ha) (18.2 ha) (113.7 ha) (3.2 ha)
No. of fires
DLF (yr)
2005 (burr/ha)
2007 (burr/ha)
2011 (burr/ha)
4 8 7 1
2005 2012 2010 2010
2.67 0.22 0.28 7.5
1.09 0.38 0.30 8.13
0.24 1.15 0.48 10.31
length (160–195 m). The beach dunes of FMNM do not transition into coastal strand; thus, the entire habitat is open, providing more suitable conditions preferred by tortoises (i.e., little to no canopy cover). Although numbers of burrows recorded on the Guana peninsula have remained relatively stable over the 3 surveys, the density of active, inactive, and abandoned burrows has varied. The 2005 survey did not document activity status for almost half of the recorded burrows impeding comparisons over the entire series, but from 2007 to 2011, there was a 24% decrease in the number of active adult and subadult-sized burrows recorded. This decrease may be a reflection of habitat degradation over the years with a general lack of prescribed fire being applied to the landscape, because only 41% of the area surveyed is managed by fire. There was also a 42% decrease in the number of inactive adult and subadultsized burrows from 2007 to 2011; however, 57 abandoned burrows were recorded in 2011, which could account for some of the decline if these burrows changed from inactive to abandoned between the 2007 and 2011 surveys. A study by Mushinsky and Esman (1994), which observed changes in burrow status over just a two-month period, noted that burrow status changes occurred frequently, because most tortoises maintain more than one burrow and move among them often. Burrow densities observed in the beach dunes were quite different between the 2007 and 2011 surveys, likely attributable to survey sampling biases or potentially the
Figure 4. Burrow density on the Guana peninsula in burned versus unburned habitat.
result of changes in gopher tortoise demography. No juvenile burrows were observed in 2007, but in 2011, numerous juvenile burrows were recorded. Juvenile burrows are relatively difficult to detect (Pike 2006) and some juvenile tortoises are known to use adult-sized burrows (Diemer 1992a); hence, the lack of juvenile burrows in 2007 is not surprising. If surveyors did not have the correct search image while surveying, juvenile burrows could have been missed or confused with ghost crab holes, however, an increase in the number of juvenile burrows could also suggest that recruitment is occurring in this habitat. Another possibility for the observed differences in burrow density could be difficulty locating burrows in the types of habitat surveyed. Although much of the beach dunes are open and fairly easy to traverse, the tops and backs of the dunes transition into thick coastal strand with dense canopy cover. Burrow aprons were difficult to detect under the thick low-growing brambles that led into the coastal strand, but several burrows were located here in 2011. It is possible that in 2007 this edge habitat was less intensely surveyed because gopher tortoises usually construct their burrows in sandy areas that are more open and where the sun can penetrate to the ground (Mushinsky and McCoy 1994). Gopher tortoises may not be limited to these preferred habitats, however, because quite a few burrows (n 5 35) were observed under saw palmetto (Serenoa repens), red bay (Persea borbonia), and yaupon holly (Ilex vomitoria) during the 2011 survey. Thus, considering the slow pace of population growth for gopher tortoises, sampling differences between the 2 survey years and the awareness of burrows located along the edge of the beach dune and coastal strand habitat were likely the cause of the increased burrow densities observed in 2011. Mushinsky and McCoy (1994) suggest that, as habitat availability shrinks or tortoises are confined to smaller and smaller areas, such as on islands, they become closely packed and construct more and more burrows as they move frequently between suitable patches of habitat. This scenario could be occurring in the beach dune habitat at GTM NERR, as tortoises are confined to the north and south by houses with fences, to the east by the Atlantic Ocean, and to the west by a heavily trafficked road. Although it may appear as though the population is doing well with high densities of active and inactive burrows being reported in 2011, these high numbers may actually
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Table 3. Gopher tortoise burrow density of Guana Tolomato Matanzas National Estuarine Research Reserve (GTM NERR) coastal beach dunes compared with other surveys in coastal areas (islands and mainland) of Florida. Sites are organized from north to south along the Florida coast. Burrow density was calculated as the number of active and inactive burrows (not including abandoned burrows) divided by the total area surveyed. Site GTM NERR GTM NERR Fort Matanzas (FMNM) Fort Matanzas (FMNM) Fort Matanzas (FMNM) J.N. ‘‘Ding’’ Darling NWR Cape Sable (Middle Cape) Cape Sable (Middle Cape)
Year
Area (ha)
Burrow density
2007 2011 1978–1979 1987–1988 2011 1987–1988 1979 1990
25.91 25.91 26 10.97 12 10.11 11.16 11.25
3.63 8.03 4.08 17.5 22.9 5.54 20.7 27.1
be caused by their confinement to a small area, especially since the exclusion of fire from the these areas may be promoting a transformation of beach dune habitat to a habitat more similar to the coastal strand. Within the undeveloped GTM NERR beach dune habitats, the area north of the north beach crosswalk may be particularly suitable habitat for gopher tortoises (Lau 2011; Lau and Dodd, in press). Lau and Dodd (in press) found that land cover type was the most important factor in burrow site selection in the beach dunes at GTM NERR. The dunes are the widest in this area (100 m compared with 55 m in the south end of this section), and their slopes are gentler, which allows for greater access and more available habitat, than in the more southern section of the dunes. Also, there are several more beach wash-overs that overtop the dunes and almost reach the road, creating more open habitat where vegetation is not as dense and forage species are more diverse. Thus, the high number of burrow sites in these areas may be potentially attributable to these favorable habitat characteristics. Disruption of natural fire regimes in many upland forested habitats has prompted the use of prescribed fire to help simulate natural processes, restore degraded habitats, and thus permit habitat use by various animals, including gopher tortoises. Most burrows in Unit 1 were observed within the mesic flatwoods habitat (8.23 ha) during the 2005 survey (6.68 burrows/ha), but the number of burrows observed in subsequent surveys was greatly reduced (2007: 1.7 burrows/ha; 2011: 0 burrows/ha). Unit 1 was regularly burned (1988, 1996, and 1998) prior to the last burn in March 2005, making the habitat more easily navigable and burrows more detectable for the survey that same year. The unit has not been burned since and is now so thickly overgrown that access is nearly impossible without habitat manipulation (e.g., roller chopping, mowing), making surveys more difficult and less efficient at detecting burrows. The fire return interval for mesic flatwoods is 1–3 yrs (Huffman 2006), and fire exclusion in this habitat can lead to a loss of species diversity, increased cover, a decrease in both forage quality and quantity, and eventually can lead to succession into a hardwood-dominated forest (USFWS 2012). It is likely that the overgrown habitat and
Source Current study Current study McCoy and Mushinsky 1992a Mushinsky and McCoy 1994 Unpublished data Mushinsky and McCoy 1994 Waddle et al. 2006 Waddle et al. 2006
subsequent increase in canopy cover led to a reduction in ground-level herbaceous growth, forcing any gopher tortoises to abandon their burrows and move into other nearby, more suitable areas (Mushinsky and McCoy 1994; Berish et al. 2012). Burrow density in the scrub and xeric hammock habitats within Unit 1 was fairly low, likely attributable to high amounts of canopy cover, similar to that of Unit 3. Burrow density in Unit 3 was low for all 3 surveys, which was not surprising because this habitat type, xeric hammock, has a mostly closed canopy dominated by oaks (Quercus spp.), some pines (Pinus spp.), and southern magnolia (Magnolia grandiflora) with, in the case of the GTM NERR peninsula, a dense understory of saw palmetto (Serenoa repens), rusty stagger bush (Lyonia ferruginea), and sparkleberry (Vaccinium arboreum). Prescribed fires in Unit 3 in 1990, 1994, and 2000 only covered small sections of the 113.7-ha area, and only in 2010 was the entire unit burned. Most burrows were located in areas where downed trees created a gap in the canopy, along trails, and around the edges of the dried freshwater marsh. The density of vegetation from lack of consistent prescribed fire within this burn unit likely prevents tortoises from colonizing more of this habitat. Likely because of the large area of freshwater marsh habitat, which is generally unsuitable habitat for tortoises (Diemer 1986), burrow density in burn Unit 2 was low. Recent drought conditions over the last few years, however, led to much of the marsh habitat drying out and subsequently the encroachment of tortoises along the edge of the marsh. During the 2011 survey, several burrows were located in areas where none had ever been located before. These new burrows may be short-lived though, because a return to normal seasonal weather conditions (e.g., drought-ending summer rains) shortly after the completion of the 2011 survey led the former aquatic marsh habitat to return, which filled these burrows with water. In other parts of Florida, however, tortoises have been observed continuing to use burrows that became inundated by water, with some tortoises staying in burrows submerged underwater (Means 1982; Castello´n 2012); thus, it is possible that the burrows in burn Unit 2 could persist over time.
PAWELEK AND KIMBALL — Gopher Tortoise Ecology Although Unit 26 was the smallest burn unit, it had the highest burrow density. The habitat consists of mostly bunch grasses, including muhly grass (Muhlenbergia capillaris), with cabbage palms (Sabal palmetto), wax myrtle (Myrica cerifera), and salt bush (Baccharis halimifolia) along the marsh borders. A dirt road leading to the trailhead bisects the unit. Burrows were located in the grassy areas both north and south of the road. Several juvenile burrows were also located in this unit in 2007 (2.19 burrows/ha) and 2011 (3.13 burrows/ha), suggesting recruitment in this habitat is high. Although generally similar to the rest of the peninsula habitat, it is relatively open, compared with the more closed canopy of the interior of the peninsula, which is likely the basis for the high density of burrows in this area. Multiple studies have suggested that using a general correction factor to determine population estimates may not be reliable, because many other factors may contribute to differences in tortoise populations (e.g., habitat type, season of survey, experience of surveyors, and location), thus determining that site-specific correction factors are preferred (Breininger et al. 1991; McCoy and Mushinsky 1992a, 1992b; Mushinsky and McCoy 1994; Smith et al. 1997; Hermann et al. 2002; Berish et al. 2012). Determining site-specific correction factors, however, was not possible for the present study, because correction factors are determined by using the number of burrows as well as the number of actual tortoises at the site. Tortoises need to be trapped to get an accurate count, but this was not possible for this study. Thus, 3 different population estimation methods were used to provide a range of gopher tortoise population estimates for each habitat and survey (Auffenberg and Franz 1982; McCoy and Mushinsky 1992b; Amatuli 2012). Auffenberg and Franz (1982) provided the first, and most widely used, correction factor based on a long-term study of 122 burrows. The correction factor of a nearby study (Amatuli 2012) was used because it represented a locally relevant tortoise population. McCoy and Mushinsky (1992b) used data from their own surveys, as well as others across Florida, to develop an equation that would provide a correlation between the number of active burrows and the number of tortoises in a population. Based on their results, they determined that their method was more accurate at calculating numbers of gopher tortoises than was the correction factor developed by Auffenberg and Franz (1982), which would have overestimated total numbers of tortoises in 85% of the populations examined (McCoy and Mushinsky 1992b). The McCoy and Mushinsky (1992b) method for determining the total number of gopher tortoises was the most conservative, with the Auffenberg and Franz (1982) correction factor being the least conservative of all three methods. The total number of tortoises in 2005 was likely underestimated for all methods, because burrow status was unknown for a large number of burrows (n 5 114). The correction factor from Amatuli (2012) also may be
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partially unreliable because the population in that study was located in sandhill habitat, which has a more open canopy than does the Guana peninsula tortoise population. No studies have determined a correction factor for tortoises residing in coastal beach dune habitats; thus, tortoise population estimates for the beach dune habitat in the present study should be interpreted with caution. All correction factor methods used in the present study demonstrate a decline in the number of tortoises on the Guana peninsula between 2007 and 2011, which also supports the idea that the habitat is becoming less suitable for gopher tortoises through habitat degradation from the lack of prescribed fire. Although burrow density on the Guana peninsula has remained fairly stable across years, the actual number of tortoises estimated has declined because of the variation in active and inactive burrows observed across years. With regard to the Guana peninsula, to help open the canopy and provide more preferable habitat for tortoises to move to (Ashton et al. 2008), more frequent fires and the reintroduction of fire to fire-suppressed areas are recommended. Monitoring of all coastal beach dune tortoise populations should be continued and initiated in additional areas attributable to the high density of burrows located in this vulnerable habitat. The variability in gopher tortoise distribution and abundance observed in this and other similar studies suggest that continuous monitoring of this long-lived species is crucial even on protected reserves (e.g., McCoy and Mushinsky 1992a). ACKNOWLEDGMENTS We thank former GTM NERR biologists M. Love and R. Endicott for their reports on the 2005 and 2007 gopher tortoise surveys. Also, we thank the dozens of dedicated volunteers and reserve staff that fought thick palmetto, ticks, and chiggers to help conduct surveys over the years. Kurt Foote provided 2011 survey data for Fort Matanzas National Monument. Dr. Joe Butler kindly provided helpful comments on an earlier draft of this manuscript. The findings and conclusions presented in this paper are those of the authors and do not necessarily represent the views of the NOAA National Estuarine Research Reserve system. LITERATURE CITED ALFORD, R.A. 1980. Population structure of Gopherus polyphemus in northern Florida. Journal of Herpetology 14(2):177– 182. AMATULI, K. 2012. Population structure and nest success of gopher tortoises (Gopherus polyphemus), and vegetative response to prescribed burning in northeast Florida. Master’s Thesis, University of North Florida, Jacksonville. ASHTON, K.G., ENGELHARDT, B.M., AND BRANCIFORTE, B.S. 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.
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