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2006 SOUTHEASTERN NATURALIST 5(2):277–284
Changes in Abundance of Gopher Tortoise Burrows at
Cape Sable, Florida
J. Hardin Waddle1,*, Frank J. Mazzotti2, and Kenneth G. Rice3
Abstract - The southernmost population of Gopherus polyphemus (Gopher Tortoises)
occurs at Cape Sable, FL. The burrows of this population were surveyed in
1979 using strip transects to estimate Gopher Tortoise burrow density. We present
data from a 1990 strip-transect survey and a 2001 line-transect survey of burrows
for comparison. We found a significantly lower density of active burrows and a
significantly higher density of abandoned burrows in 2001 compared to 1979 and
1990, but the overall density of burrows did not differ significantly over time. The
distribution of burrow widths in 2001 was not significantly different from the 1982
distribution, and the bimodal pattern suggests that reproduction has occurred at the
site. The 76% decline of active burrows at the site suggests that Gopher Tortoises
have been subject to mortality. Reduction of habitat quality and tropical storms are
possible explanations for the apparent decline in the Gopher Tortoise population at
Cape Sable, but more monitoring is required to understand the long-term trends in
this coastal population.
The southern extent of the range of Gopherus polyphemus Daudin (Gopher
Tortoise) is Cape Sable, FL. This population of Gopher Tortoises
occurs in the palm savanna of East and Middle Cape Sable within Everglades
National Park. Kushlan and Mazzotti (1984) surveyed the burrows of
this population in 1979 and reported an estimated density of 11.3 tortoises/
ha using the burrow-to-tortoise conversion factor of 0.614 from Auffenberg
and Franz (1982). McCoy and Mushinsky (1992) compared burrow abundance
estimates at Cape Sable from 1978–79 and 1987–88, and found no
evidence of a change in tortoise density during that time period.
Many estimates of tortoise burrow densities appear in the literature
(Auffenberg and Franz 1982, Breininger et al. 1994, Mushinsky and McCoy
1994, Stewart et al. 1993), but few offer published accounts of long-term
trends of burrow density at a single site. Kushlan and Mazzotti (1984)
suggested that the inherent instability of the coastal habitat and the periodic
occurrence of hurricanes and other tropical storms could lead to fluctuations
in the population size of Gopher Tortoises at Cape Sable. Our objective in
1Florida Cooperative Fish and Wildlife Research Unit, PO Box 110485, University of
Florida, Gainesville, FL 32611. 2Department of Wildlife Ecology and Conservation,
University of Florida-FLREC, 3205 College Avenue, Fort Lauderdale, FL 33314.
3US Geological Survey, Florida Integrated Science Centers, Center for Water and
Restoration Studies, 3205 College Avenue, Fort Lauderdale, FL 33314. *Corresponding
author - email@example.com.
278 Southeastern Naturalist Vol. 5, No. 2
this study is to examine possible trends in Gopher Tortoise burrow density,
activity status, and width at Cape Sable during a 22-year period from 1979 to
2001, using data from the survey by Kushlan and Mazzotti (1984) and two
surveys conducted in 1990 and 2001.
Cape Sable is the southernmost point of mainland Florida, consisting
of three main sandy capes (Northwest Cape, Middle Cape, and East Cape;
Fig. 1) described in detail by Kushlan and Mazzotti (1984). Gopher Tortoises
currently occur on both Middle Cape and East Cape. Although East
Cape was surveyed in 2001, we only consider the burrows at Middle Cape
in this analysis because it is the only area with three comparable samples
and contains the largest population of tortoise burrows (Kushlan and
Mazzotti 1984). The burrow surveys were concentrated in the open palm
savanna located between the beach and the dense mangrove forest that
fringes Lake Ingraham (Fig. 1). Using the vegetation classification of
Everglades National Park available in a geographic information system
(GIS; Madden et al. 1999), we calculated the area of open savanna grassland
with scattered Sabal palmetto Lodd. ex Schultes (palms) at Middle
Cape Sable to be 72.3 ha.
Figure 1. Map of southern Florida showing location of Cape Sable within Everglades
National Park with Northwest Cape, Middle Cape, and East Cape indicated.
2006 J.H. Waddle, F.J. Mazzotti, and K.G. Rice 279
The first survey of burrows at Middle Cape used in this study was
conducted in August 1979. This survey was described in Kushlan and
Mazzotti (1984), but only the density of all burrow types combined was
reported. They searched 37 seven-m-wide strip transects completely for
burrows. Transects were spaced 20 m apart, and ran from the beach across
the open savanna to the line of dense mangrove forest. Each transect was
oriented at a compass bearing of 48° from the beach. Because of the triangular
shape of Middle Cape, transects varied in length from 10–1013 m. In
February 1990, F.J. Mazzotti and others again surveyed Middle Cape for
burrows. In this survey, 30 seven-m-wide strip transects were searched for
burrows using the same technique as the 1979 survey. Although the 1990
transects were not in the exact locations of the 1979 transects, they were
arranged in a similar manner along the same orientation. The length of
transects in 1990 varied from 8–1208 m. In May 2001, we used a modification
of the distance-estimation technique developed by Lohoefener (1990)
and similar to that used by Hermann et al. (2002). Observers walked 31 line
transects arranged in the manner of the strip transects in the 1979 and 1990
surveys. Transect lengths varied from 11–998 m. The perpendicular distance
to all burrows observed from the line was measured to the nearest 0.1 m
Each burrow in all three surveys was assigned to an activity status
category following descriptions by Auffenberg and Franz (1982). Active
burrows were distinguished by the presence of fresh tracks, scat, or other
signs of recent use. Inactive burrows maintained the characteristic shape of a
tortoise burrow, but lacked signs of recent use, and abandoned burrows were
at least partially collapsed or filled with debris (Breininger et al. 1991, 1994;
Mushinsky and McCoy 1994; Stewart et al. 1993).
The density estimates of Gopher Tortoise burrows at Middle Cape Sable
in 1979 and 1990 were calculated by dividing the number of burrows
observed by the total area of the strip transects that was searched in either
survey. This density estimate was multiplied by the total area of the tortoise
habitat at Middle Cape Sable to produce an estimate of the number of
burrows. The variance for this estimate and a 95% confidence interval was
calculated using the equation for the ratio-estimator-of-abundance method
described by Cochran (1977) for use when complete detectability of individuals
is assumed on sample units of unequal area. Burrow densities for the
2001 sample were calculated using the program DISTANCE (version 4;
Thomas et al. 1998), with encounter rate and detection function estimates
pooled across transects. Density estimates of each type of burrow were
compared among survey years using the compare surveys feature in the
program AERIAL SURVEY (version 2000; Leban and Garton 1999), which
performs a chi-square test on the null hypothesis that the density of burrows
was the same in each year.
The width of 93 burrows located within a 1.45-ha plot at Middle Cape
Sable was measured in 1982 (Kushlan and Mazzotti 1984). Burrows were
measured at 50 cm inside the entrance to adjust for the often variable width
280 Southeastern Naturalist Vol. 5, No. 2
of the mouths of burrows (Alford 1980). No burrow measurements were
made in 1990, but a set of comparable burrow-width measurements was
obtained from active and inactive burrows during the 2001 burrow survey.
The distributions of burrow widths from 1982 and 2001 were compared
using a two-sample Kolmogorov-Smirnov Z test, which tests whether two
samples come from the same distribution (SPSS 2001).
During the 1979 survey at Middle Cape, 11.16 ha was searched for
Gopher Tortoise burrows. A total of 270 burrows was located, of which 162
were active, 69 were inactive, and 39 were abandoned. In 1990, a total area
of 11.25 ha was searched for burrows and 321 were found; of these, 208
were active, 97 were inactive, and 16 were abandoned. The methods employed
in the 2001 survey do not provide an exact area searched, but 275
burrows were detected of which 71 were active, 114 were inactive, and 90
were abandoned. The overall effective strip half-width (the effective detection
area for line transects) was 6.71 m (95% C.I. = 6.00–7.50), and the
burrow detection probability was 0.25 (95% C.I. = 0.22–0.28; Buckland et
The estimated densities of active Gopher Tortoise burrows at Middle
Cape in 1979 and 1990 were 14.51 and 18.49 burrows/ha respectively
(Table 1). The estimated density of active burrows in 2001 was 4.38 burrows/
ha. When these estimated burrow densities are multiplied by the area
of suitable habitat at Middle Cape, the estimated total number of active
burrows is 1049, 1337, and 317 in 1979, 1990, and 2001, respectively
(Table 2). The survey comparison test (Leban and Garton 1999) concluded
that the density of active burrows in 2001 was significantly lower than
densities in 1979 and 1990 ( χ2 = 14.0955, df = , p < 0.001). The density of
Table 1. Estimated density of burrows in each activity status category at Middle Cape for the
1979, 1990, and 2001 surveys.
Survey Est. density of Est. density of Est. density of Est. density of
year active burrows inactive burrows abandoned burrows all burrows
1979 14.51/ha 6.18/ha 3.49/ha 24.19/ha
1990 18.49/ha 8.62/ha 1.42/ha 28.54/ha
2001 4.38/ha 7.84/ha 6.93/ha 19.08/ha
Table 2. The estimated number of burrows of each activity status category occurring in the 72.3-
ha palm savanna habitat at Middle Cape in 1979, 1990, and 2001. The 95% confidence interval
of the estimate is shown in parentheses.
Survey Est. number of Est. number of Est. number of Est. number of
year active burrows inactive burrows abandoned burrows all burrows
1979 1049 (566–1533) 447 (203–690) 253 (97–408) 1749 (923–2575)
1990 1337 (615–2059) 624 (356–891) 103 (4–201) 2063 (1121–3005)
2001 317 (186–538) 566 (392–819) 501 (370–678) 1379 (1089–1747)
2006 J.H. Waddle, F.J. Mazzotti, and K.G. Rice 281
abandoned burrows was significantly greater in 2001 than in 1979 and 1990
(χ2 = 19.1289, df = 2, p < 0.001). No significant differences in burrow
density existed among years for the inactive-burrow status category (χ2 =
0.9862, df = 2, p = 0.6107) or for the estimate of all burrows (χ2 = 2.2638, df
= 2, p = 0.3224).
We measured the width of 180 active and inactive Gopher Tortoise
burrows in 2001. The frequency distribution of burrow widths in 2001 was
not significantly different in shape (Kolmogorov-Smirnov Z = 1.019, p =
0.25) from the 1982 distribution measured by Kushlan and Mazzotti (1984).
The 2001 distribution is bimodal, with peaks at 10 cm and 30 cm (Fig. 2).
The density of all burrows was not significantly lower in 2001 than in
1979 or 1990, but we found significantly fewer active Gopher Tortoise
burrows at Middle Cape in 2001. We have no direct estimates of Gopher
Figure 2. Frequency distribution of 180 Gopher Tortoise burrow widths measured at
Middle Cape Sable in 2001.
282 Southeastern Naturalist Vol. 5, No. 2
Tortoise abundance from any of the three samples, but the 76% decline in the
number of active burrows during the 11-year period from 1990 to 2001
suggests that there was a decline in the number of Gopher Tortoises at
Middle Cape or a decrease in tortoise activity. Air temperatures were warm
during the 2001 sample (maximum 32 ºC) and May is well into the tortoise
activity season (Eubanks et al. 2003), making it unlikely the occupied
burrows were scored as inactive or abandoned. The insular nature of Middle
Cape Sable makes it unlikely that emigration of Gopher Tortoises from the
area occurred, and the distribution of burrow widths suggests that recruitment
into the population from reproduction is occurring at the site. Thus,
emigration and lack of reproduction are unlikely explanations for the population
decline. We conclude that mortality at the site is the best explanation
for the apparent decline in Gopher Tortoises from 1990 to 2001.
Although the sampling methods employed in our third survey of Gopher
Tortoise burrows at Cape Sable are slightly different than the first two
surveys, the results are directly comparable. Both methods produce estimates
of the density of burrows. Additionally, we calculated confidence
intervals for the estimated numbers of burrows at Cape Sable from the first
two surveys for statistical comparison to the 2001 data. The variance estimates
from the 1979 and 1990 strip transects resulted in wider confidence
intervals compared to the confidence intervals estimated with distance sampling.
The compare surveys feature in the program AERIAL SURVEY is
considered to be a conservative test, especially when confidence intervals
are wide (Leban and Garton 1999). This underscores the importance of the
significant decrease in the numbers of active burrows and increase of abandoned
burrows between 1990 and 2001.
McCoy and Mushinsky (1992) examined changes in Gopher Tortoise
abundance at Cape Sable by comparing data on burrow abundance and
activity status collected in 1978–79 to data collected in 1987–88. They
found an increase in the total number of burrows and a slight increase in the
percentage of active burrows during the 10-year period, a pattern similar to
the one observed in this study between the 1979 and 1990 samples. Both this
study and that of McCoy and Mushinsky (1992) indicate that the Gopher
Tortoise population at Cape Sable was stable or slowly increasing in the
decade prior to 1990.
Aresco and Guyer (1999) found that Gopher Tortoises in southern
Alabama abandoned their burrows at a rate of 22% per year during a fiveyear
study, and they found a significant positive correlation between age
of an active burrow and tree density and basal area. With the exception of
a few small hammocks, the Gopher Tortoise habitat at Middle Cape Sable
is essentially treeless, so canopy closure is an unlikely cause of the increased
number of abandoned burrows at Cape Sable in 2001 relative to
earlier surveys. Guyer and Hermann (1997) found no difference in the
longevity of Gopher Tortoise burrows based on soil type or root structure
at pine forest sites in Georgia and Alabama. There is evidence that
2006 J.H. Waddle, F.J. Mazzotti, and K.G. Rice 283
tortoises in disturbed sites will readily abandon burrows to migrate to
areas of better forage as conditions change (Aresco and Guyer 1999,
Guyer and Hermann 1997). It is possible that tortoises at Cape Sable
abandoned burrows at a higher rate in 2001 than previously described
because of deteriorating habitat conditions.
Kushlan and Mazzotti (1984) noted that large tropical storms might be an
important cause of mortality in this coastal population of Gopher Tortoises.
From 1970 to 1990, no major tropical storms passed within 25 km of Cape
Sable. However, in 1992 the eye of Hurricane Andrew passed within 45 km
of Cape Sable as a category 3 hurricane with sustained winds of 202 km/h
(Pimm et al. 1994), and in 1999 the eye of Hurricane Irene passed within 10
km of Cape Sable as a category 1 hurricane with sustained winds of 120
km/h (Avila 1999). It is possible that a storm surge associated with either or
both of these hurricanes caused tortoise mortality at Cape Sable, but because
no survey of the Gopher Tortoises at Cape Sable was conducted directly
before or after these hurricane events, the impact of hurricanes on the
Gopher Tortoises at Cape Sable remains speculative.
It appears that the suggestion by Kushlan and Mazzotti (1984) that
instability in coastal areas may cause fluctuations in tortoise populations
was correct. There has been a significant decrease in active Gopher Tortoise
burrows despite evidence of reproduction at Cape Sable between
1990 and 2001. Reduction of habitat quality or the occurrences of tropical
storms are possible causes of the apparent decline in Gopher Tortoises at
Cape Sable. Continued monitoring of this population would provide more
information on the long-term dynamics of survival and reproduction at this
site. More frequent burrow surveys and surveys after major storm events
would provide insight about the vulnerability of Gopher Tortoises at Cape
Sable to tropical storms.
M. Deacon, A. Watts, C. Zweig, and two anonymous reviewers provided valuable
editorial comments on this manuscript. This project would not have been
possible without help in the field provided by M. Caudill, M. Cherkiss, G. Cook, M.
Crockett, M. Delong, A. Dove, P. George, J. Graham, L. Hord, S. Howarter, B.
Jeffery, A. Maskell, L. McKercher, C. Vischer, and J. Williams. Surveys were
conducted under NPS permit #2000074.
Alford, R.A. 1980. Population structure of Gopherus polyphemus in northern
Florida. Journal of Herpetology 14:177–182.
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
Auffenberg, W., and R. Franz. 1982. The status and distribution of the Gopher
Tortoise (Gopherus polyphemus). Wildlife Research Reports 12:95–126.
284 Southeastern Naturalist Vol. 5, No. 2
Avila, L.A. 1999. Preliminary report: Hurricane Irene 13–19 October 1999. National
Hurricane Center, Miami, FL.
Breininger, D.R., P.A. Schmalzer, and C.R. Hinkle. 1991. Estimating occupancy of
Gopher Tortoise (Gopherus polyphemus) burrows in coastal scrub and slash pine
flatwoods. Journal of Herpetology 25:317–321.
Breininger, D.R., P.A. Schmalzer, and C.R. Hinkle. 1994. Gopher Tortoise
(Gopherus polyphemus) densities in coastal scrub and slash pine flatwoods in
Florida. Journal of Herpetology 28:60–65.
Buckland, S.T., D.R. Anderson, K.P. Burnham, J.L. Laake, D.L. Borchers, and L.
Thomas. 2001. Introduction to Distance Sampling: Estimating Abundance of
Biological Populations. Oxford University Press, New York, NY.
Cochran, W.G. 1977. Sampling Techniques. Wiley, New York, NY.
Eubanks, J.O., W.K. Michener, and C. Guyer. 2003. Patterns of movement and
burrow use in a population of Gopher Tortoises (Gopherus polyphemus).
Guyer, C., and S.M. Hermann. 1997. Patterns of size and longevity of Gopher
Tortoise (Gopherus polyphemus) burrows: Implications for the longleaf pine
Ecosystem. Chelonian Conservation and Biology 2:507–513.
Hermann, S.M., C. Guyer, J.H. Waddle, and M.G. Nelms. 2002. Sampling on private
property to evaluate population status and effects of land use practices on the
Gopher Tortoise, Gopherus polyphemus. Biological Conservation 108:289–298.
Kushlan, J.A., and F.J. Mazzotti. 1984. Environmental effects of a coastal population
of Gopher Tortoises. Journal of Herpetology 18:231–239.
Leban, F., and O. Garton. 1999. Aerial Survey 2000. University of Idaho, Wildlife
Resources, Moscow, ID. Available at: http://www.cnr.uidaho.edu/fishwild/programs/
Lohoefener, R. 1990. Line transect estimation of Gopher Tortoise burrow density
using Fourier series. Pp 44–69, In C.K. Dodd, Jr., R.E. Ashton, R. Franz, and E.
Wester (Eds.). Proceedings: Eighth Annual Meeting of the Gopher Tortoise
Council. Florida Museum of Natural History, Gainesville, FL.
Madden, M., D. Jones, and L. Vilchek. 1999. Photointerpretation key for the Everglades
vegetation classification system. Photogrammetric Engineering and Remote
McCoy, E.D., and H.R. Mushinsky. 1992. Studying a species in decline: Changes in
populations of the Gopher Tortoise on federal lands in Florida. Florida Scientist
Mushinsky, H.R., and E.D. McCoy. 1994. Comparison of Gopher Tortoise populations
on islands and on the mainland in Florida. Fish and Wildlife Research
Pimm, S.L., G.E. Davis, L. Loope, C.T. Roman, T.J. Smith III, and J.T. Tilmant.
1994. Hurricane Andrew. BioScience 44:224–229.
SPSS Inc. 2001. SPSS for Windows, Release 11.0.0. Chicago, IL.
Stewart, M.C., D.F. Austin, and G.R. Bourne. 1993. Habitat structure and the
dispersion of Gopher Tortoises on a nature preserve. Florida Scientist 56:70–81.