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Habitat Utilization of Pituophis melanoleucus melanoleucus (Northern Pinesnakes) on Arnold Air Force Base in Middle Tennessee
W. Boyd Blihovde

Southeastern Naturalist, Volume 5, Number 2 (2006): 265–276

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2006 SOUTHEASTERN NATURALIST 5(2):265–276 Terrestrial Movements and Upland Habitat Use of Gopher Frogs in Central Florida W. Boyd Blihovde* Abstract - In recent years, researchers have begun to focus on the upland habitat requirements of pond-breeding amphibians. The increased attention is due to a general lack of knowledge about the terrestrial phase of the life history of most pond-breeding species and a concern over loss of upland habitats. In this study, radio telemetry was used to determine the terrestrial behavior of Rana capito (Gopher Frogs) in central Florida. Frogs were captured at Gopherus polyphemus (Gopher Tortoise) and Geomys pinetis (pocket gopher) burrows. Surgically implanted radio transmitters were used to follow nine Gopher Frogs at various times between September 1999 and May 2000. Radio-located frogs used from one to four terrestrial shelters (Mean ± S.D. = 2.28 ± 1.11). Terrestrial movements ranged in total distance from zero to 35 m (Mean ± S.D. = 15.28 ± 15.29). Mean minimum convex polygons (m2) were calculated for each frog (Mean ± S.D. = 45.29 ± 79.73). Gopher Frogs showed strong site fidelity to both pocket gopher and Gopher Tortoise burrows. Drought conditions could have resulted in an underestimate of movement distance and an overestimate of site fidelity to upland shelters. Upland habitat should be managed to protect all species of terrestrial burrowers; in doing so, Gopher Frogs will be managed. Introduction Amphibians are vulnerable to numerous threats in their environment, and thus may be important indicators of both terrestrial and aquatic habitat degradation (Dodd 1997). Although much research on amphibians has focused on breeding sites, it is now apparent that upland habitats are just as important to the long-term persistence of many species (Semlitsch 1981, Semlitsch and Jensen 2001). Frogs and salamanders are known to migrate considerable distances into upland habitats surrounding breeding sites (Dodd 1996, Dodd and Cade 1998, Franz et al. 1988, Johnson 2003, Muths 2003, Semlitsch 1998). Understanding terrestrial movements of pond-breeding amphibians is necessary to adequately protect the surrounding uplands (Semlitsch 1998, Semlitsch and Jensen 2001). There is increased urgency to obtain data on terrestrial movements of amphibian species that inhabit areas immediately threatened by habitat loss from urbanization. In Florida, increases in the human population have resulted in severe losses of upland habitats (Cerulean 1991). In central Florida, pond-breeding amphibians and their terrestrial retreats are threatened by some of the most rapid urban development in the country (US *Department of Biology, University of Central Florida, 4000 Central Florida Boulevard, Orlando, FL 32816. Current Address - Lake Wales Ridge National Wildlife Refuge, 4177 Ben Durrance Road, Bartow, FL 33830; boyd_blihovde@fws.gov. 266 Southeastern Naturalist Vol. 5, No. 2 Census Bureau 2003). One species in particular, Rana capito LeConte (Gopher Frog), is especially vulnerable to urbanization because individuals breed in small, isolated wetlands, but spend most of their lives in upland habitats. Both Rana capito and Rana sevosa Goin and Netting (Dusky Gopher Frogs) are adapted to living in semi-arid conditions and depend on preexisting burrows and underground retreats in the uplands they inhabit (Franz 1986, Godley 1992, Wright and Wright 1949). Gopher Frogs may spend many months away from breeding sites (i.e., fishless ponds) annually, sometimes moving overland as far as 2 km (Franz 1988, Franz et al. 1988). Outside the breeding season (typically September to March in central Florida), they are most often found in sandhill, scrub, sand pine scrub, and flatwoods habitats (Carr 1940, Godley 1992, Wright and Wright 1949). They have been observed using burrows made by Gopherus polyphemus Daudin (Gopher Tortoises), Peromyscus polionotus Wagner (old field mice), and crayfish (Carr 1940, Gentry and Smith 1968, Lee 1968, Wright and Wright 1949). Gopher Frogs will also use other types of refuges that are associated with stumps, root mounds, dead vegetation, and clumps of grass (Richter et al. 2001, Wright and Wright 1949). Ecological studies of Gopher Frogs have primarily focused on breeding ponds (Bailey 1989, Palis 1998). In one study, Gopher Frogs were tracked as they migrated from a breeding pond into the surrounding uplands (Richter et al. 2001). However, no study has focused exclusively on the ecology of Gopher Frogs in the terrestrial environment, and therefore, information is lacking on use of terrestrial habitats by these frogs. For example, there are no data available on the number of or specific types of Gopher Tortoise burrows used by Gopher Frogs, or the amount of time they spend at individual burrows. Additional research on the breeding biology of Gopher Frogs is certainly needed. However, studies of their terrestrial requirements are crucial for land managers attempting to conserve these imperiled species. The purpose of this investigation was to determine the terrestrial locations of Gopher Frogs in central Florida using radio telemetry. This study focused on habitat use, movements, and site fidelity of Gopher Frogs at their terrestrial shelters. Methods I established two study areas in locations known to harbor Gopher Frogs; Chuluota Wilderness Area (CWA) (28°38'10"N, 81°05'30"W) in Seminole County and Rock Springs Run State Reserve (RSR) (28°47'31"N, 81°26'50"W) in Lake and Orange Counties. One study plot was established at CWA (Fig. 1), and two study plots (RSR1, RSR2) were established at RSR (Fig. 2). Prior to the protection of CWA and RSR, they were both used as cattle ranches (among other uses). Chuluota Wilderness Area, a 625-acre site, contains remnant sandhill habitat surrounding the one-hectare study 2006 W.B. Bilhovde 267 Figure 1. Burrows occupied and unoccupied by Gopher Frogs at Chuluota Wilderness Area, FL. Figure 2. Burrows occupied and unoccupied by Gopher Frogs at Rock Springs Run State reserve, FL. 268 Southeastern Naturalist Vol. 5, No. 2 plot that has been overtaken by oak species due to the lack of fire. At CWA, the only potential breeding pond was approximately 460 m from the center of the study plot (Fig. 1). Rock Springs Run State Reserve, 13,850 acres in size, has been intensively managed and contains many acres of restored sandhill communities. At RSR, the closest confirmed breeding pond, on the east side of County Road 433, was 130 m from the center of RSR1 (Fig. 2). The pond on the west side of CR 433 was 218 m from the center of RSR1; Gopher Frogs were heard calling at both sites. During each of fourteen months (April 1999 to May 2000), I conducted a systematic search in each one-hectare study plot to locate Gopher Frogs at vertebrate burrows that could serve as retreat sites. I marked every potential terrestrial retreat found with a surveyor’s flag, and later recorded the specific locations via GPS with a Trimble Pro XR/XRS and a TDC1 data logger. The GPS locations of burrows were overlaid onto aerial photos in ArcView GIS software. Burrows where I observed a Gopher Frog at least once during the study were designated as “occupied burrows.” I never observed frogs at “unoccupied burrows.” I fitted Gopher Frogs with radio transmitters to monitor their movements among terrestrial retreats. Because of potential problems associated with externally attached transmitters (e.g., Rathbun and Murphey 1996), I chose to implant transmitters (see below). Candidate frogs for radio-tracking were found during nocturnal or early-morning surveys of the one-hectare study sites. Every burrow was investigated while I walked transects through the study sites. I used a flashlight to thoroughly scan the burrows before I approached them. I captured frogs by quickly placing a net between frogs and the entrances to burrows they used as terrestrial retreats. I surgically implanted small, 1.5-g radio transmitters (AVM Instrument Company, Ltd, Colfax, CA) into the coelomic cavity of the frogs. Surgical procedures followed those for Bufo marinus (Linneaus) and B. americanus (Holbrook) (Seebacher and Alford 1999, Werner 1991). Frogs were held for 24–72 hours until the incisions were healed and the frogs recovered. Frogs were then toe-clipped, measured (snout-to-vent length), and weighed. All frogs, except #8 and #9 at RSR, were released at their original burrows. I displaced these two individuals to the nearest active tortoise burrow (approx. 10 m away from their capture locations), to determine if they would move back to their original burrows. Initially, I attempted to locate radio-implanted frogs every 24 hours. However, once it was clear that they were sedentary, I extended the tracking interval to every 72 hours. I made more frequent attempts to track implanted frogs during rainfall events, when I expected them to be more active. Frogs that were radio-located at more than one burrow (e.g., activity center) during the study were included in estimates of upland home range. I used ArcView GIS software and the GPS locations for each frog’s activity center to calculate home ranges. Several frogs made only one documented movement to a new activity center and then back to their original burrow. In 2006 W.B. Bilhovde 269 these cases, the minimum rectangular path was used to represent the polygon the frogs likely traveled. During statistical analysis of the Gopher Frog movements (number of burrows used and home-range size), frogs #4 and #9 were removed. This was done because frog #4 moved to the breeding pond (making much greater movements than frogs that stayed in the uplands) and because #9’s transmitter failed before movements could be observed. Results Burrow surveys In total, 127 burrows were located during surveys to find potential terrestrial retreat sites. These included 66 burrows at CWA, 30 burrows at RSR1, and 31 burrows at RSR2. At CWA, at least 21 (31.8%) burrows were occupied at some point during the study. Burrows were not used as heavily at RSR, where seven (23.3%) burrows were occupied at RSR1, and six (19.4%) burrows were occupied at RSR2 (Fig. 2). The following individuals are included as examples of either extraordinary site fidelity to a single burrow or use of several burrows. Frog #1 at RSR was seen throughout the observation period (e.g., during the 14- monthly burrow survey period). This frog made no documented movements during the survey period. Frog #3 at RSR was found at the same abandoned pocket gopher (Geomys pinetis Rafinesque) burrow, in the same hunting position for 14 months. However, it was documented making a short movement while being radio-located (see below). Frog #7 at CWA was located approximately 33 m from its original burrow. This frog was a male that was located at three different burrows (including its original). Surgery I successfully performed surgery to implant transmitters in nine individuals that were captured at their burrow entrances. To replace failed transmitters, I performed multiple surgeries on some frogs. Surgery was performed four times on frog #1 (two insertions and two removals). It received two separate transmitters because the first transmitter’s battery failed. Frog #2 was implanted with one transmitter, and therefore surgery was performed two times (one insertion and one removal). Surgery was performed three times on frog #3 (two insertions and one removal). This frog’s first transmitter failed immediately after surgery and was replaced with a new transmitter the following day. Frogs #4 and #5 were implanted with transmitters on 6 October 1999 and those transmitters were not removed. Frog #6 was implanted with a transmitter on 5 November 1999 and that transmitter was removed on 3 March 2000. Two surgeries were performed on frog #7 (two transmitters had to be inserted). The first transmitter failed immediately after surgery and was replaced on the following day. The second transmitter was not removed. Frogs #8 and #9 had transmitters inserted on 1 May 2000. These transmitters were not removed from either frog. 270 Southeastern Naturalist Vol. 5, No. 2 No frog died during surgery, and frog #8 was the only individual to show signs of infection. This frog had a slight swelling around the incision, including a slight purple discoloration. Incisions always healed in two to three days, and after several weeks, incisions were no longer evident. Nine frogs were implanted with transmitters however, six of those transmitters failed completely. Those transmitters were replaced with working transmitters, unless adequate movement data had been gathered prior to the transmitter failing. In addition to transmitter failure, transmitter range was lower than expected (approx. 20 m). Movements of individual frogs The length of time a frog stayed at a burrow varied greatly. Frog #1 was radio-located for six weeks. This frog, implanted with a transmitter on 12 September 1999, made no movements during the duration of its transmitter’s life (Table 1, Fig. 3). After the transmitter battery failed, this frog continued to use the same burrow for the duration of the monthly burrow surveys. Frog #2 moved a total of 35 m and used three different burrows during the three months that it was tracked (Fig. 3, Table 1). Although it was a female, it was never documented at the breeding pond, most likely due to unfavorably dry conditions that persisted during the study period. Frog #3 was implanted with its first transmitter on 12 September 1999; its second transmitter was implanted on 13 September 1999. This frog’s only movement was documented during heavy rain on 1 November 1999, during which it moved 10-m from the original burrow (Table 1). The frog was back inside its original burrow the next day. Frog #4, a male, moved the greatest distance (286 m) of all animals in the study. It made a breeding migration of approx. 90 m on 10 October 1999. This movement was initiated by heavy rain that prompted several frogs to start calling in the nearest breeding pond. This frog was then located at a new burrow south of the breeding pond the next night (11 October), was located back at the pond by 16 October, and was located at the east side of the pond Table 1. Movement distances for the 9 tracked Rana capito at Chuluota Wilderness Area and Rock Springs Run State Reserve, FL. Frogs #4 and #9 were not included in the burrow and distance analysis. Number of Total distance Frog # Sex Dates tracked SVL (cm) burrows used moved (m) 1 F 13 Sept–27 Oct 1999 8.8 1 0 2 F 15 Sept–29 Nov 1999 8.3 3 35 3 F 15 Sept–26 Nov 1999 8.7 2 20 4 M 7 Oct–15 Nov 1999 7.5 2 286 5 M 7 Oct–15 Nov 1999 8.7 1 0 6 F 5 Nov, 99– 3 Mar 2000 9.7 2 1 7 M 18 Feb–10 Apr 2000 9.0 3 33 8 F 1 May–22 June 2000 9.1 4 18 9 F 1 May–5 May 2000 9.0 1 0 Mean 8.76 2.28 15.28 Standard deviation 0.60 1.11 15.29 2006 W.B. Bilhovde 271 on 19 October (Fig. 3, Table 1). On 5 November, the frog was back in the middle of the pond, and then located at the north end of the pond on 9 November. The transmitter could no longer be heard on 11 November. Frog #5 was tracked for five weeks. It was never located away from its original burrow (Table 1), and thus had only one activity center. Frog #6, which I located via radio transmitter for almost 4 months, made one short movement. This frog was documented using two burrows, which were 1 m apart (Table 1). Frog #7 was released on 21 February 2000 at its original burrow and was located at another burrow 10 m away on 29 February. The frog was located back at its original burrow on 10 March where it stayed until the transmitter failed (Fig. 4, Table 1). This frog was also documented moving during the burrow surveys after the transmitter had been removed. Frog #8 was released at a burrow 10 m from its original burrow. Within three days of its release, frog #8 had returned to its original burrow. On 8 May 2000, it was located at a second burrow, approx. 3 m southeast of the release burrow, at a third burrow on 12 May, and then a new burrow 2 m south of the original burrow on 26 May. This frog was then located back at its original burrow on 22 June (Fig. 3, Table 1). Frog #9 was also released at a different burrow 10 m away from its original burrow. The transmitter in frog #9 worked at the time of release, however three days after its release there was no signal from its transmitter Figure 3. Movement of Gopher Frogs 2, 4, and 8 at Rock Springs Run State Reserve, FL. Movements made by each frog are numbered in the order they were made. 272 Southeastern Naturalist Vol. 5, No. 2 (most likely due to transmitter failure or predation; Table 1). Therefore, frog #9 could not be found and no data were gathered on site fidelity or burrow use. Frogs #2 and #4 at RSR had the largest minimum convex polygon estimates based on their number of upland activity centers. Frog #2 had the largest upland home range, yet it only moved a total of 35 m. Frog #4 had the greatest movement distance by far, and had a home range of approximately 169 m2 (Table 2). Because frogs #3, #6, and #8 had very Figure 4. Movements of frog 7 at Chuluota Wilderness Area, FL. Table 2. Upland home range estimates for Gopher Frogs from Chuluota Wilderness Area and Rock Springs Run State Reserve, FL. Frogs #4 and #9 were not included in the home range analysis. Frog # of activity centers (burrows) Minimum convex polygon (m2) 1 (RSR) 1 0.00 2 (RSR) 3 221.88 3 (RSR) 2 20.59 4 (RSR) 2 169.98 5 (RSR) 1 0.00 6 (CWA) 2 11.15 7 (CWA) 3 50.53 8 (RSR) 4 12.91 9 (RSR) 1 0.00 Mean 45.29 Standard deviation 79.73 2006 W.B. Bilhovde 273 restricted movements, the estimates for their minimum convex polygons were small. Home-range estimates could not be calculated for three frogs (#1, #5, and #9) because they had one activity center (Table 2). Discussion Data from this radio telemetry study yielded important information on the natural history of Gopher Frogs in central Florida. Frogs spent most of their time in subterranean retreats, primarily the burrows of Gopher Tortoises and pocket gophers. Frogs used from one to four retreats, but moved little during the study. However, one frog moved a total of 286 m, which included a breeding migration of approximately 90 m. Research has suggested that Gopher Frogs move without preference through upland habitats (Greenberg 2001), but my study indicates that they can regularly navigate distances of at least 10 m. In addition, one study documented a Rana sevosa individual move 236 m to a breeding pond and then back to the original burrow (Richter et al. 2001). In this study, two frogs released away from the burrows where they were captured moved back to their capture burrows. One returned to its capture burrow the following day. Several frogs in my study used multiple terrestrial retreats and did not appear to move among potential retreats at random. Gopher Frogs do not make many movements outside of the breeding season. Richter et al. (2001) found that Dusky Gopher Frogs exhibited inactivity following breeding migrations. Similarly, frogs in my study were sedentary before and after breeding migrations. Data from my burrow surveys and radio locations of animals in the uplands showed that Gopher Frogs commonly stayed inactive in their upland shelters for months at a time, spending up to 14 months away from breeding ponds. Despite relatively sedentary behavior of the frogs, the mean home-range size for all frogs was 45.29 m2 (not including breeding movements). Drought that occurred during the study period likely contributed to the sedentary nature of the frogs. After successive years of drought, ponds in central Florida (including those at RSR and CWA) held very little water, which may have increased the level of site fidelity frogs exhibited. Rainfall has significant effects on the behavior of this species (Jensen et al. 2003); lack of moisture may therefore have affected my findings. Even though it is the most terrestrial ranid frog in Florida, Gopher Frogs are susceptible to desiccation in the dry, sandy uplands they inhabit. They depend on upland retreats to avoid predators and maintain adequate hydration, especially during drought. Thus, my results underscore the importance of well-managed upland habitats with numerous subterranean shelters for the persistence of Gopher Frogs. Because they require numerous upland shelters, Gopher Frogs also depend on those species that build burrows. The loss of the Gopher Tortoise or the pocket gopher would remove the primary source of their upland shelters. For instance, one such threat (a large Gopher Tortoise die-off) occurred just 274 Southeastern Naturalist Vol. 5, No. 2 after this study near the RSP site (Rabatsky and Blihovde 2002). Increases in tortoise mortality events across Florida may exacerbate the decline of burrow commensals, including Gopher Frogs (Eisenberg 1983, Gates et al. 2002). My data indicate that pocket gophers, via the construction of burrows, are extremely important to Gopher Frogs. Without burrow shelters in the uplands, Gopher Frogs would very likely have a difficult time surviving a drought like the one encountered in this study. Increased scientific focus on pond-breeding amphibians has led to important life history discoveries for many amphibians in the Southeast (Dodd 1997). There are numerous studies that promote the establishment of buffer zones, and perhaps more appropriately, core habitat around breeding ponds (Semlitsch 1998). The upland habitat around isolated ponds in central Florida, including the microhabitats of Gopher Tortoise and pocket gopher burrows, is part of the Gopher Frog’s core habitat. Removal of the upland habitat and loss of the shelters there would be as destructive to Gopher Frogs as removing the essential breeding pond where eggs are deposited. Researchers have recommended a terrestrial buffer surrounding the core habitat to protect upland habitat for semi-aquatic species of amphibians (Semlitsch and Jensen 2001). The aquatic buffer, core habitat, and terrestrial buffer can be combined into a life zone for some species. For instance, the Gopher Frog spends a great deal of time in the terrestrial uplands. The uplands are so important to the Gopher Frog that they should be considered essential portions of their life zone. Acknowledgments I thank L.M. Ehrhart, W.K. Taylor, J.F. Weishampel, P.E. Moler, R. Mulholland, and R.A. Seigel for their critiques of earlier versions of this paper. J. Duby, J. Fillyaw, R. Franz, J. Gibson, E. High, T. Hirama, J. Jensen, B. Kermeen, T. Mallow, J.C. Mixon, D. Nickerson, J. Plitt, S. Richter, F. Seebacher, P. Small, I.J. Stout, and Seminole County Natural Lands and Wetiva River Basin State Parks provided assistance. The Gopher Tortoise Council, Hughes Supply Co., S. Fullerton, L.M. Ehrhart, J.F. Weishampel, and UCF Department of Biology provided crucial funding. 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