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2011 SOUTHEASTERN NATURALIST 10(1):75–84
Distribution and Habitat Use of the Big Cypress Fox
Squirrel (Sciurus niger avicennia)
Danielle A. Eisenberg1,2, Reed F. Noss1,*, Jane M. Waterman1,3,
and Martin B. Main4
Abstract - Human population growth and development reduce the area and quality of
natural communities and lead to a decline of associated wildlife populations. Sciurus
niger avicennia (Big Cypress Fox Squirrel), a state-listed threatened subspecies endemic
to south Florida, appears sensitive to habitat fragmentation and fire regime. This research
assessed the distribution and habitat use of the Big Cypress Fox Squirrel from interviews
with biologists, private land owners, and golf course managers and by transect sampling
in natural areas on public and private lands in southwest Florida. Our findings indicate
that the distribution of fox squirrel populations is influenced by land use and understory
height. Conservation of this species in natural areas will require land management practices
that maintain open landscapes.
Habitat loss and fragmentation are widely recognized as leading causes for
declines in biodiversity (Groom and Vynne 2006, Noss et al. 2006). Florida
supports many endemic taxa that evolved in relative isolation for thousands
to millions of years (Adams et al. 2000, James 1961). With a growing human
population of 17 million and tourist population of 40 million per year, Florida is
losing native habitats due to demands for development (Kautz and Cox 2001). As
a result of this rapidly changing landscape, Florida faces one of the highest risks
of biodiversity loss of any US state, with hundreds of rare and imperiled native
species (Adams et al. 2000, Kautz and Cox 2001, Noss and Peters 1995).
Certain species can be useful indicators or “focal species” for determining the
quality of ecosystem remnants and for developing management strategies to restore
and conserve biodiversity (Carroll et al. 2001, Lambeck 1997). Habitat fragmentation
is a principal cause of endangerment for some tree squirrels because it creates
a mosaic of isolated patches and reduced habitat area (Koprowski 2005). Because
patches vary in suitability for survival and reproduction of individuals, habitat quality
is a key determinant of patch occupancy and the probability of local extinction.
Sciurus niger avicennia L. (Big Cypress Fox Squirrel) is a state-listed threatened
subspecies endemic to south Florida (Humphrey and Jodice 1992). It is found
in southwestern Florida, south of the Caloosahatchee River and west of the Everglades
(Ditgen et al. 2007). Jodice and Humphrey (1992) reported Big Cypress Fox
1Department of Biology, 4000 Central Florida Boulevard, University of Central Florida,
Orlando, fl32816. 2Current address - Department of Science/Wellness Education, 1000
Coconut Creek Boulevard, Broward College, Coconut Creek, fl33066. 3Current address
- Department of Biological Science, 186 Dysart Road, University of Manitoba, Winnipeg,
MB R3T2N2. 4Southwest Florida Research and Education Center, 2685 State Road
29 North, University of Florida, Immokalee, fl34142. *Corresponding author – rnoss@
76 Southeastern Naturalist Vol. 10, No. 1
Squirrel populations as rare and highly scattered in Collier, Lee, Hendry, and Monroe
counties. Fox squirrels are sensitive to fragmentation, roads, fire regime, and
other aspects of the structure and function of ecosystems (Koprowski 2005). The decline
of the Big Cypress Fox Squirrel mirrors the decline in mature Pinus spp. (pine)
and Taxodium spp. (cypress) forests as south Florida is increasingly transformed by
human activities (Wooding 1997). Pinus elliottii Engelm. (Slash Pine) flatwoods
declined by 79% from 1936 to 1987 (Wooding 1997), and remaining habitat has
been degraded due to fire exclusion and suppression (Doren et al. 1993).
Although Big Cypress Fox Squirrels are generally found in pine flatwoods,
their specific habitat requirements are poorly understood (Humphrey and Jodice
1992, Williams and Humphrey 1979). Approximately 384,000 ha of potential Big
Cypress Fox Squirrel habitat may remain, with 223,000 ha (58%) within conservation
lands (WilsonMiller, Inc. 2002). Potential fox squirrel habitat was estimated
by WilsonMiller, Inc. (2002) using January 1995 USGS digital ortho quarter-quad
(DOQS) imagery to map pine, Quercus spp. (oak), and cypress forests, which are
consistent with potential fox squirrel habitat as described by Humphrey and Jodice
(1992). Areas with less than 10% canopy cover were excluded from the potential
habitat estimate. Understory characteristics that are important to fox squirrels
(Ditgen et al. 2007) apparently were not factored into the WilsonMiller, Inc. (2002)
analysis; moreover, this GIS-based study identified potential habitat only and did
not evaluate presence/absence of squirrels. Even though a large amount of potential
fox squirrel habitat may remain, squirrels avoid many areas because of dense
understory growth or other factors (Wooding 1997).
Big Cypress Fox Squirrel populations are also reported to occur in pasture and
ranch lands that were historically pine flatwoods and upland prairie (Wooding
1997). Previous research found higher fox squirrel abundance in suburban areas
than in natural habitats (Ditgen 1999, Jodice and Humphrey 1992). Golf courses
and rural residential lands have little to no understory, may serve as habitat
corridors, and, somewhat ironically, may provide more suitable habitat for fox
squirrels than many remaining natural areas, at least in the short term while large
trees remain available in these developed landscapes (Ditgen et al. 2007, Jodice
and Humphrey 1992, Wooding 1997).
This research evaluated the distribution and habitat use of the Big Cypress
Fox Squirrel in urban greenspace, private ranch lands, and conservation lands
throughout southwest Florida. Assuming fox squirrel occurrence depends on the
availability of open understory, and open understory is more prevalent in intensively
managed landscapes such as golf courses and grazed ranchlands, we predict
(H1) that Big Cypress Fox Squirrels are present more often in intensively managed
landscapes than in natural areas. If fox squirrel occurrence depends on understory
characteristics, we predict (H2) that Big Cypress Fox Squirrels will be found more
often in areas with lower understory growth than in areas with dense understory.
This study attempted to conduct a range-wide status survey of the Big Cypress
Fox Squirrel; therefore, field sites were located in multiple locations in southwest
2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 77
Florida (Fig. 1). The climate of the region is humid subtropical with warm, rainy
summers and cool, dry winters (Chen and Gerber 1990). Field surveys were conducted
between January 2005 and October 2007 along 1-km transects to census
fox squirrels and measure habitat variables. We identified transect locations in a
stratified random manner from areas of potential squirrel habitat identified through
a combination of geographic location, land use, and accessibility (Bourgeron et al.
2001). From a pool of 40 potential sites in natural areas, private ranch lands, and
urban greenspace, we randomly selected 20 transect locations (12 in natural areas,
5 in golf courses, and 3 in private ranches). Transect orientation (N–S, E–W) was
randomly assigned to each transect by the flip of a coin. Universal Transverse Mercator
(UTM) coordinate locations for each transect were recorded using a Garmin
12 Global Positioning System (GPS, Garmin 12, Olathe, KS).
Big Cypress Fox Squirrel surveys were conducted along 1-km x 20-m belt
transects (Avery and Burkhart 2001), resulting in a surveyed area of 0.02 km2 along
each transect. Three surveys were conducted along each transect, with each survey
comprising a 2-hr walk to search for fox squirrels. If no fox squirrels were observed,
the site was characterized as fox squirrel “absence”. If at least one fox squirrel was
observed, the site was characterized as fox squirrel “presence”. Presence/absence
methods are useful when the species of interest exists in low numbers or is difficult
to detect (Joseph et al. 2006). Perkins and Conner (2004) found that season did not
affect habitat use by southeastern fox squirrels; hence, surveys were conducted in
both dry (November through April) and wet (May through October) seasons (Chen
and Gerber 1990). Fox squirrels were surveyed along each transect at least once in
both dry and wet seasons and at least once in the morning (0700–1100) and once
in the afternoon (1300–1700). Occasionally, surveys were conducted mid-day
(1100–1300) due to constraints in scheduling and access to private lands. Lacking
quantitative studies of variation in fox squirrel detectability, we were not able to account
for potential differences in fox squirrel behavior and detectability in response
to temperature, weather conditions, time of day, or other factors.
We measured understory variables at 100-m intervals along each transect. A
2-m cover pole was used to estimate understory height in nested 1-m2 subplots
5 m from the center of each plot in all 4 cardinal directions (Griffith and Youtie
1988). A total of 40 measurements were collected from each transect and then
averaged to obtain a mean understory height value for each study site. Lee et
al. (2009) classified understory height into two categories: short (≤1 m) and tall
(>1 m). For better discrimination of short understory important to fox squirrels,
we defined “minimal” understory as less than 50 cm tall, and each site was
grouped into two mean understory height classes: less than 50 cm and ≥50 cm.
Interviews and analysis
One thousand interview questionnaires (60% to private land owners, 30% to
golf course managers, and 10% to biologists and field personnel of public landmanaging
agencies) were distributed to collect information about the historic
and current trends in Big Cypress Fox Squirrel distribution and abundance. To
enhance the reliability of participant observations, we included a photo of the
78 Southeastern Naturalist Vol. 10, No. 1
Figure 1. Locations of 1-km transect study sites. Map legend: 1 = Big Cypress National
Preserve, 2 = Caloosahatchee Regional Park, 3 = Club at Olde Cypress, 4 = Club at
Pelican Bay, 5 = Collier-Seminole State Park, 6 = Corkscrew Swamp Sanctuary, 7 =
Fakahatchee Strand State Park, 8 = Flint Pen Strand, 9 = Florida Panther National Wildlife
Refuge,10 = Hickey’s Creek Mitigation Park, 11 = Imperial Marsh Preserve, 12 =
Picayune Strand State Forest, 13 = Pine Lake Preserve, 14 = Private Ranch 1, 15 = Private
Ranch 2, 16 = Private Ranch 3, 17 = Royal Palm Golf Club, 18 = Royal Poinciana Golf
Club, 19 = Royal Wood Golf Club, 20 = Six Mile Cypress Slough.
2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 79
species of interest and a comparison photo depicting the common Sciurus carolinensis
Gmelin (Gray Squirrel). Big Cypress Fox Squirrels are distinguishable
from Gray Squirrels because southeastern fox squirrels have characteristic black
and white face markings and are polymorphic in pelage color ranging from buff
to black (Humphrey and Jodice 1992), while Gray Squirrels are standard agouti.
Participants were asked to indicate their personal sightings on the following
scale: none (no sightings in at least ten years), rare (once or twice a year), and
common (weekly or daily sightings). Land use was divided into three categories:
urban greenspace, farm/ranch, and park/preserve.
Data collected from questionnaires and field surveys were analyzed with
contingency analyses to examine relationships between the presence/absence of
fox squirrels and land use. Specifically, a G-test of independence, which is an
alternative to the chi-square test in which only the observed frequencies are used
to calculate the expected frequencies (Zar 1998), was used to test the hypotheses.
G-tests of independence were used to examine if the Big Cypress Fox Squirrel
is more widely distributed in urban greenspace and other managed landscapes
where there is minimal understory growth than in natural areas. JMP (JMP 7,
Statistical Analysis Software) was used to perform contingency analyses. Alpha
(α) was set at 0.05.
Of the 1000 questionnaires we distributed to document recent and historic
sightings of Big Cypress Fox Squirrels, 145 were returned (52% from private
land owners, 32% from golf course managers, 16% from biologists and field
personnel of public land-managing agencies). Contingency analyses revealed a
significant interaction between fox squirrel presence/absence and land use. The
presence/absence of the Big Cypress Fox Squirrel in a site is not independent of
land use (G = 10.24, df = 2, P < 0.01, n = 145). According to the surveys returned
to us, Big Cypress Fox Squirrels were reported as present in 78% of sites in urban
greenspace, but at only 51% of farm/ranch sites and 62% of park/preserve lands
(Fig. 2). A significant interaction (G = 20.65, df = 2, P < 0.01, n = 96), also was
found between fox squirrel sightings and land use. Big Cypress Fox Squirrel
sightings were not independent of land use. Big Cypress Fox Squirrel sightings
were reported as common in 63% of urban greenspace sites, but common sightings
were only reported in 35% of farm/ranch sites (Fig. 3). No (0%) fox squirrel
sightings in park/preserve lands were reported as common.
Big Cypress Fox Squirrels were observed in only 7 (35%) of the 20 transect
study sites, and 5 of those sites were urban greenspace lands. Fox squirrels were not
observed along transects in 10 (83%) of the 12 sites located in natural areas. Gray
Squirrels were observed at all study sites, including along transects in natural areas
where fox squirrels were not observed. Contingency analyses revealed a significant
relationship between fox squirrel presence/absence and understory height (G = 5.94,
df = 1, P < 0.05, n = 20). Big Cypress Fox Squirrels are primarily found in areas with
minimal understory growth, less than 50 cm in height (Fig. 4). Fox squirrels were not
observed at sites where mean understory height was greater than 1 m.
80 Southeastern Naturalist Vol. 10, No. 1
Natural areas, private ranch lands, and urban greenspace all support Big
Cypress Fox Squirrels, but individuals are widely distributed throughout southwestern
Florida. In this study, Big Cypress Fox Squirrels were most commonly
Figure 3. Percentages of responses from biologists, golf course managers, and private
land owners for land use types characterized by Big Cypress Fox Squirrel sightings as
rare and common (n = 96).
Figure 2. Percentages of responses from biologists, golf course managers, and private
land owners for land-use types characterized by the absence and presence of Big Cypress
Fox Squirrels (n = 145).
2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 81
found in urban greenspace areas such as golf courses and other intensively managed
lands, but were rarely observed in parks, preserves and other natural areas.
These results are in accordance with findings from previous studies (Ditgen 1999,
Jodice 1990) where, despite intensive searches in the Big Cypress National Preserve
and other natural areas, very few fox squirrels were found. Thus, we accept
H1 that Big Cypress Fox Squirrels occur more often in intensively managed landscapes
than in natural areas. We also accept H2 that Big Cypress Fox Squirrels
are found in areas with minimal understory growth because fox squirrels in our
study primarily used sites characterized by less than 50 cm of growth and were
not found in areas where the understory height averaged greater than 1 m. These
results suggest that land uses that maintain open and low understory vegetation
may provide suitable habitat for the Big Cypress Fox Squirrel. A caveat is that
golf courses, pastures, and many residential areas may have large pines and oaks
and support fox squirrels today, but may not support fox squirrels in the future
due to low or nonexistent recruitment of appropriate trees (Wooding 1997).
Fox squirrels historically existed in a landscape where lightning fires maintained
a grassy, open forest floor (Kiltie 1989). Fire exclusion has altered the natural
fire regime such that many areas within parks and preserves now have a dense and
relatively tall understory. Although prescribed fire is commonly a component of
habitat management plans in southwest Florida, budgetary and regulatory restrictions
reduce the ability of land managers to burn as often or extensively as needed
to replicate historic vegetation structure. Periodic fires are vital for maintaining
habitat quality in many Florida ecosystems (Abrahamson and Harnett 1990). Pineoak
forests thrive with frequent fires that release nutrients required by trees and their
mycorrhizal fungi, and the open-canopied stands and patches of bare mineral soil
Figure 4. Percentages of transect sites characterized by the absence and presence of Big
Cypress Fox Squirrels in terms of understory height class (n = 20).
82 Southeastern Naturalist Vol. 10, No. 1
produced by fire result in better pine reproduction (Weigl et al. 1989). Fire suppression,
and also probably a greater use of winter as opposed to growing-season burns
(Tanner and Mullahey 1999), has led to increased cover of Saw Palmetto (Serenoa
repens) and other shrubs. These practices have created a dense understory unsuitable
for fox squirrels, which travel and forage extensively on the ground (Ditgen et
al. 2007, Jodice and Humphrey 1992). Loeb (1999) found that Gray Squirrels may
outcompete fox squirrels in areas with dense understory growth. Big Cypress Fox
Squirrels were found to coexist with Gray Squirrels in urban greenspace areas like
golf courses and private ranch lands. Big Cypress Fox Squirrels probably occur
more often in urban greenspace areas than natural areas because understory vegetation
in suburban areas is typically intensively managed.
The Big Cypress Fox Squirrel was recently denied federal listing, in part because
of its opportunistic use of golf courses and other suburban lands (Ditgen
et al. 2007). Although fox squirrels are attracted to suburban areas because of an
open understory and year-round food supply (Ditgen et al. 2007), populations
may not be prospering at such locations and may not persist in the long term due
to a deficiency of large trees (Wooding 1997). Ditgen (1999) documented Big Cypress
Fox Squirrel population levels in a variety of golf courses and developed a
landscape evaluation index that ranked golf courses in terms of habitat suitability
for fox squirrels, concluding that fewer than 5 of 48 golf courses were capable
of maintaining populations during the period of intensive development that is
expected through the year 2020.
Suburban areas may have attractive habitat cues for fox squirrels such as a
minimal understory. Nevertheless, although the habitat structure in these urban
greenspace areas may be currently suitable for fox squirrels, the presence of
roads, pets, and other risks may make these areas ecological traps or “attractive
sinks” (Delibes et al. 2001). A species has source-sink dynamics if births exceed
deaths in some habitats (sources), while deaths exceed births in other habitats
(sinks). Source-sink dynamics often are related to habitat fragmentation (Pulliam
1988). Although large, unfragmented tracts of forest are not a prerequisite
for successful management of southeastern fox squirrels, the effects of various
land-use practices on population dynamics can differ greatly among areas, even
within subspecies (Lee et al. 2009). Additional study is needed to determine
whether fox squirrel recruitment on golf courses and other suburban areas represents
local reproduction or dispersal from other areas. Such data would help
illuminate source-sink or other metapopulation dynamics and clarify whether
these populations are likely to remain viable in the long term. Long-term studies
of tree recruitment in habitats that appear suitable today also are needed.
Big Cypress Fox Squirrels were found to be more common in urban greenspace
and other suburban areas than in natural areas. Although the ability to detect fox
squirrels in natural areas (with greater than minimal cover) was likely lower than
in suburban areas (with clear open landscapes and habituation of squirrels to human
presence), results from our surveys of natural area biologists and managers
corroborated these findings. Understory height may be the key determinant in
fox squirrel avoidance versus preference of habitats, whereas other factors, such
as road density, traffic volume and fire frequency may separate source from sink
populations. Understory growth due to fire exclusion has resulted in less suitable
2011 D.A. Eisenberg, R.F. Noss, J.M. Waterman, and M.B. Main 83
habitat for fox squirrels, which require open-canopied, mature pine forests with
minimal understory (Wooding 1997). Therefore, habitat management practices
such as more extensive and frequent prescribed burning in parks, preserves, and
other conservation lands may be a critical tool to enhance habitat quality in natural
areas for Big Cypress Fox Squirrels. Spatially explicit population viability
analyses would aid future decision-making and the development of policies to
best conserve the Big Cypress Fox Squirrel.
The US Fish and Wildlife Service provided the funding for this project. We thank J.F.
Weishampel and J.D. Roth for their input and guidance throughout this research project.
We also thank C. Knickerbocker for generating the map showing study site locations and
J.E. Fauth and P.F. Quintana-Ascencio for statistical advice. Special thanks are due to
the members and staff of the golf courses and the private land owners for allowing use
of their properties in this study. Finally, we thank the various park and wildlife biologists
who provided vital information for this research.
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