2009 SOUTHEASTERN NATURALIST 8(3):427–436
Use of Forest Edges by Free-ranging Cats and Dogs in an
Urban Forest Fragment
Britni K. Marks1 and R. Scot Duncan1,*
Abstract - Free-ranging Felis catus (Domestic Cat ) and Canis familiaris (Domestic
Dog) can greatly impact native prey populations, but little is known about their occurrence
in urban forest fragments. In this study, we used camera traps to photograph
(capture) cats, dogs, and native wildlife in a 409-ha urban forest in Birmingham, AL
from Jan–Apr 2007. Habitat treatments included forest interior and forest edges by
industrial lands, neighborhoods with higher house values, and neighborhoods with
lower house values. We employed both conservative (n = 31) and liberal (n = 64)
methods of tallying the number of individual dogs, cats, and native mammals captured.
Dogs and cats combined comprised 19% (conservative) and 26% (liberal) of
all photographic captures. Procyon lotor (Raccoon) were the most abundant of the
7 native species at 32% (conservative) and 53% (liberal) of all captures. Dogs were
more abundant in neighborhood edges, and cats were more abundant in the forest
interior. Cats and dogs combined were 75% (conservative) and 86% (liberal) of
captures from the forest interior. Captures of native species were far more frequent
in neighborhood edges (conservative = 86.9%, and liberal = 92.3%) than in other
treatments. These findings demonstrate that exotic predators can be an important
ecological presence in certain portions of urban forest fragments, and more extensive
studies of their impact are needed.
Introduction
The destruction and fragmentation of natural ecosystems is considered the
greatest threat to biodiversity (Groom et al. 2006, Harrison and Bruna 1999,
Murcia 1995, Oehler and Litvaitis 1996, Wilcove et al. 1998). One of the many
ecological changes that may occur with the urbanization of natural landscapes
is the introduction of non-native predators, particularly, Felis catus (Domestic
Cat; herein after referred to as cats) and Canis familiaris (Domestic Dog; herein
after referred to as dogs). Worldwide, free-ranging cats and dogs (including
pets—animals actively cared for and supervised by owners; strays—pets that
have escaped from owners; and true ferals—those surviving and reproducing
without dependence on human care) have been shown to prey on native
species and to compete with native predators (Butler et al. 2002, Coleman et
al. 1997, Manor and Saltz 2004). Such free-ranging exotic predators are also
known to spread zoonotic diseases, such as rabies, that may be fatal to native
fauna (Butler et al. 2002, Roelke et al. 1993). Since densities and distributions
of free-ranging cats and dogs often refl ect those of human populations, fragmented
habitats of urban landscapes may be particularly vulnerable to impacts
of these exotic predators.
1Biology Department, Birmingham-Southern College, 900 Arkadelphia Road, Birmingham,
AL 35254. *Corresponding author - sduncan@bsc.edu.
428 Southeastern Naturalist Vol. 8, No. 3
Free-ranging cats, due to their highly efficient predation of native wildlife,
are of particular concern. In the US, it is estimated that there are 25–40
million stray or feral cats, and many of the estimated 60 million house cats
in the US spend a substantial amount of time outdoors (Patronek and Rowan
1995). Cats can be found in densities 20–100 times those of native predators
(Kays and DeWan 2004, Woods et al. 2003). It has been estimated that
over a billion small mammals and hundreds of millions of birds are killed
by free-ranging cats each year in the US (Coleman et al. 1997, Crooks and
Soule 1999). In Great Britain, it was estimated that 57 million small mammals,
27 million birds, and 5 million reptiles and amphibians were killed
over a 5 month period by the ≈9 million resident cats (Woods et al. 2003).
Clearly, the native fauna of ecosystem fragments in developed areas can be
vulnerable to severe impact from free-ranging cat populations (Coleman and
Temple 1993, Hawkins et al. 2004).
The ecological impacts of dogs have been studied less than those of cats.
In some rural areas of the Southeastern US, there exist dog populations
showing long-term pariah morphotypes that may be representative of, if not
partially descendant from, the ancestral dogs introduced by the first humans
to migrate to North America from Asia (Brisbin and Risch 1997). However,
little is directly known about their ecological role. In Alabama, feral
dogs were found to occasionally chase Odocoileus virginianus Zimmerman
(White-tailed Deer), but no evidence was found suggesting these chases
resulted in kills. Instead, feral dogs were shown to prey upon small animals
including rodents, rabbits, and endangered Gopherus polyphemus Daudin
(Gopher Tortoises) (Causey and Cude 1980, Scott and Causey 1973). In Africa,
there has been some concern and study of the effect of dogs on juvenile
ungulates (Butler and du Toit 2002, Manor and Saltz 2004). Because dogs
are effective scavengers, their densities can be especially high where there is
easy access to human waste (e.g., at landfills; Beck 1973, Causey and Cude
1980, Manor and Saltz 2004), and they may compete with native scavengers
(Butler et al. 2004). Beck (1973) reported that free-ranging dogs were common
in an urban area, with their densities being positively correlated with
garbage availability and abandoned buildings, which the dogs used for shelter.
Most of these urban dogs scavenged garbage and/or received handouts,
although a few also apparently preyed on rats and ground-nesting birds in an
urban park. Lowry (1978) reported that dogs from suburban communities in
Idaho chased deer, with chases often leading to the death of the deer.
Edge effects are one of the most obvious of the many negative consequences
for habitats that become fragmented (Groom et al. 2006, Harrison
and Bruna 1999, Murcia 1995, Oehler and Litvaitis 1996). The imposition
of artificial edges on an undisturbed ecosystem alters both abiotic and biotic
components of the remnant ecosystem. Unnatural forest edges, for example,
cause adjacent forest soils to become drier and warmer, and favor the establishment
and spread of invasive exotic plants and animals (Groom et al. 2006,
Murcia 1995). A combination of edge effects and reduced habitat availability
can cause the extirpation of native species from ecosystem fragments (Groom
2009 B.K. Marks and R.S. Duncan 429
et al. 2006, Murcia 1995, Wilcove et al. 1998). In particular, free-ranging cat
use of forest fragments and their edges is an effect that has been implicated as
one of the causes of bird population declines in forest fragments (Blair 1996,
Crooks and Soule 1999, Kays and DeWan 2004, Wilcove 1985).
Free-ranging cats and dogs are known to visit natural habitat fragments in
suburban and rural areas, but very little has been published regarding their use
of urban habitat fragments (Barratt 1997, Kays and DeWan 2004, Manor and
Saltz 2004, Scott and Causey 1973). In order to study free-ranging cat and dog
use of an urban forest fragment, we surveyed the fauna of a large urban nature
preserve in Birmingham, AL using camera traps at baited stations. Because we
speculated that free-ranging cat and dog use of the forest would be affected
by adjacent land-use patterns, we studied forest edges adjacent to areas of
industrial development, neighborhoods of lower house values (LHV), and
neighborhoods of higher house values (HHV). The forest interior was sampled
as a fourth treatment. Forest classified as industrial edge was located adjacent
to properties used for business and industry (e.g., auto parts salvage yards,
railroad freight yard, and small plants and factories). The density of free-ranging
dogs has been shown to be higher in lower-income neighborhoods (Beck
1973); our LHV neighborhoods had houses that were smaller, had smaller lots,
and were less expensive than those in the HHV neighborhoods. Our specific
goals were to a) determine to what extent free-ranging cats and dogs used this
urban nature preserve; b) compare cat and dog use of forest edge habitats varying
in adjacent land-use patterns; c) compare cat and dog use of forest edges to
use of the forest interior; and d) compare cat and dog use of these four habitat
treatments to use by native fauna also sampled by the camera traps. This case
study was also designed to draw attention to the management needs of urban
nature preserves, and provide useful information for managers concerned with
exotic predator use of such areas.
Methods
Study area and design
This study was conducted at Ruffner Mountain Nature Center (RMNC) in
Birmingham, AL. At 409 ha (1011 acres), RMNC is among the largest urban
parks in the US. Between 1880–1960, RMNC was mined heavily (surface and
subterranean) for iron ore, limestone, and chert (Raney 2007). Public use of
RMNC is limited to trail hiking. While the preserve requires that dogs be on
leash at all times, this rule is often disregarded by preserve users. The original
forest was largely destroyed during this period of industrial use, and this
preserve is now almost entirely composed of secondary forest including xeric
and mesic pine-hardwood forest, xeric and mesic calciphytic forest, and other
forest types (Fig. 1; Raney 2007). The dominant topographic feature of the
preserve is a portion of Red Mountain, a long, low-elevation ( approximately
335–366 m at RMNC) mountain stretching for many kilometers in a NE–SW
orientation along the NW edge of the Valley and Ridge Physiographic Province
(Fig. 1). The preserve is roughly rectangular with its long axis running parallel
430 Southeastern Naturalist Vol. 8, No. 3
with the mountain ridge. Camera traps were located along RMNC’s two longest
edges, the NW-facing and SE-facing borders. Industrial edge forests were
located along the SE-facing border of the park. The NW-facing border of the
preserve is adjacent to residential neighborhoods. Along this border, the HHV
neighborhoods were located in the northern section, and the LHV neighborhoods
were located in the southern section (Fig. 1). Within each of the four
habitat treatments, three sites were chosen for placement of camera traps. Site
selection was infl uenced by factors affecting the function of the camera trap
(e.g., slope, tree density). Border locations were approximately 50 m from
the forest edge and were separated by a minimum of 250 m to reduce the frequency
of multiple photo captures of the same animals during a sample period.
Adjacent to each treatment zone, a forest interior camera site was established
near one treatment site set back 100 m from the forest edge. This distance was
chosen because previous studies have shown that cat and dog use of forest fragments
declines strongly at or before 100 m from the edge (Crooks 2002, Kays
and DeWan 2004, Oehler and Litvaitis 1996); a secondary reason is that at
distances beyond 100 m from the edge in the HHV and LHV habitat treatments,
the slope increases dramatically and the forest composition begins to change.
Each of the four treatments was sampled on three different occasions
(deployments) from January to April 2007. Having only three cameras, it
was not possible to sample all treatments simultaneously. Instead, habitat
Figure 1. Map of
Ruffner Mount
a i n N a t u r e
Center, Birmingham,
AL, and
approximate positions
of camera
traps for each of
the four sampling
treatments (see
text for details).
Preserve boundary
outlined in
black. Shaded
areas in the preserve
indicate
different forest
types and topographic
features
of the preserve.
Base map courtesy
of Patrick
Raney, Ruffner
Mountain Nature
Center.
2009 B.K. Marks and R.S. Duncan 431
treatments were sampled sequentially on a rotating basis. Deployments
within a treatment were separated by 4–5 weeks. During a deployment, all
three sites for a treatment were sampled simultaneously. Cameras were left
to photograph animals for approximately 48 hrs at each site, including two
nights. The totals of camera trapping hours for the four treatments were as
follows: industrial edge, 436 hrs; LHV edge, 421 hrs; HHV edge, 432 hrs;
and forest interior, 425 hrs.
Camera traps and baiting statistics
Camera traps are a useful and relatively inexpensive method for monitoring
populations of secretive and rare animals (Azlan and Sharma 2003,
Hegglin et al. 2004, Heilbrun et al. 2006, Locke et al. 2005, Rowcliffe and
Carbone 2008). The cameras are triggered by movement and/or heat sensors.
Baiting stations are often used to lure animals into the camera’s range (Andelt
and Woolley 1996, Kays and Dewan 2004). We used WildView Xtreme
3.0 Digital Scouting Cameras (Wildview, Bedford, TX) to document animal
visitations to bait stations. This camera is triggered by a passive infrared
heat-and-motion sensor, has a fl ash feature effective for up to 9 m, and prints
the time and date on every image. We lured animals to the camera traps using
an 8-oz can of water-packed sardines secured directly in front of each
camera trap (Andelt and Woolley 1996). This bait was placed 3 m from the
camera trap (Hegglin et al. 2004), and the can was punctured several times
to allow the odor and liquids to escape. Where soil conditions permitted,
cans were secured to rebar driven into the ground; on rocky soils, cans were
secured to a tree. A photo was automatically taken when the camera was triggered
by the sensor. A 5-minute delay between photos was used to reduce
the occurence of multiple photos of the same animal during the same visitation
to the bait. Cameras were secured to trees with chains and locks (height
= 0.5 m) to avoid theft and oriented to avoid being triggered by the rising
and setting of the sun. Cameras were placed on trees at least 4.5 m from any
human walking trails.
Statistics
A capture was defined as any time the camera trap took a picture of an
animal. When sample size was sufficient (n = 5 captures per treatment),
chi-square goodness-of-fit analyses were used to compare the number of
observed versus expected captures among treatments (alpha = 0.05); the
number of expected captures was calculated as the total number of observed
captures divided equally among the treatment categories. One limitation
of camera trapping is that it is difficult to know when separate pictures of
the same species are of the same or different individuals. We found that we
could readily distinguish between different individuals of the same species
for both cats and dogs, but not for the native species. This bias could lead
to an infl ated count of native species relative to exotic species. To address
this issue, we employed two analyses of our data. In the “conservative”
analysis, we did not count the same species more than once within a 12-hr
432 Southeastern Naturalist Vol. 8, No. 3
period at each trap during a deployment. One exception was made for two
different dogs captured within a 12-hr period at a HHV station; however, we
found that inclusion of the second dog did not alter the findings of this study
relative to analyses excluding this second dog. In the “liberal” analysis, we
based our analyses on the total number of captures, rather than an estimation
of the number of individual animals which may have been involved.
Results
Sixty-one photos were taken of mammals that were identifiable to species
during a total of 108 days and 72 nights of camera trapping; 30 of
these were excluded for the conservative analysis. In most cases, however,
the trends within conservative and liberal analyses were very similar. The
number of captures generally declined as average temperature and day
length increased during the spring.
Procyon lotor (Raccoon) were the most frequently captured species,
with 32.3% (conservative) and 53.1% (liberal) of all captures, respectively.
Raccoons were the species whose proportion of total captures differed the
most between conservative and liberal analyses. In the conservative analysis,
Domestic Dogs were the second most common species (16.1% of captures),
while Domestic Cats were captured at the same frequency (9.7%) as
Vulpes vulpes (Red Fox), Sciurus carolinensis (Eastern Gray Squirrel), and
Didelphis virginiana (Virginia Opossum ) (Table 1). In the liberal analysis,
cats, dogs, and opossums were tied (9.4% of captures) for the second-most
common species. Together, cats and dogs totaled 25.8% and 18.8% of captures
in the conservative and liberal analyses, respectively.
With all species pooled, most captures were in the HHV (48%, conservative;
51% liberal) and LHV (32%, conservative; 33% liberal) edge
Table 1. Total photographic captures of Domestic Cats, Domestic Dogs, and native mammals
from four habitat treatments at Ruffner Mountain Nature Center, an urban forest fragment in
Birmingham, AL. Treatments included forest interior and edge habitat adjacent to neighborhoods
of lower house values (LHV), higher house values (HHV), and industrial (I) lands.
Results of conservative and liberal counts of photographic captures are presented, with the
latter in parentheses.
Habitat treatments
Edges Forest Total
Species HHV LHV I interior captures
Procyon lotor (L.) (Raccoon) 5 (19) 5 (15) 0 0 10 (34)
Canis familiaris L. (Domestic Dog ) 3 (3) 1 (2) 0 1 (1) 5 (6)
Didelphis virginiana Kerr (Virginia Opossum) 2 (5) 1 (1) 0 0 3 (6)
Felis catus Schreber (Domestic Cat) 1 (1) 0 0 2 (5) 3 (6)
Sciurus carolinensis Gmelin (Eastern Gray Squirrel) 2 (2) 0 0 1 (1) 3 (3)
Vulpes vulpes L. (Red Fox) 2 (3) 1 (1) 0 0 3 (4)
Urocyon cinereoargenteus Schreber (Gray Fox) 0 0 2 (3) 0 2 (3)
Canis latrans Say (Coyote) 0 1 (1) 0 0 1 (1)
Sylvilagus fl oridanus J.A. Allen (Eastern Cottontail) 0 1 (1) 0 0 1 (1)
Total captures 15 (33) 10 (21) 2 (3) 4 (7) 31 (64)
2009 B.K. Marks and R.S. Duncan 433
treatments, with differences found among the four treatments (conservative:
χ2 = 13.5, P < 0.0036, d.f. = 3; liberal: χ2 = 35.3, P < 0.0001, d.f. = 3). There
was a trend for cats to be captured most frequently in the forest interior, with
no cats being captured in the industrial and LHV edge treatments (conservative:
insufficient sample size; liberal χ2 = 11.3, P = 0.0101, d.f. = 3). No significant pattern emerged for dog use of the habitat treatments (conservative:
χ2 = 3.8, P = 0.2839, d.f. = 3; liberal: χ2 = 3.3, P = 0.3435, d.f. = 3), though
nearly all dog captures were in the HHV and LHV edges. Of the dogs photographed,
one wore a collar, three did not (four in liberal analysis), and one
was undeterminable. Together, cats and dogs represented the majority of captures
from the forest interior in both conservative and liberal analyses (75%
and 86% of captures, respectively).
Captures of native species (pooled) differed among habitat treatments
(conservative: χ2 = 13.0, P = 0.0046, d.f. = 3; liberal: χ2 = 41.2, P < 0.0001,
d.f. = 3), and were most frequent in the HHV and LHV edges. This trend was
driven primarily by Raccoons, as they were more likely to be captured in the
HHV and LHV edges than the other habitat treatments (conservative: χ2 =
10.0; P = 0.0186, d.f. =3; liberal: χ2 = 34.9; P < 0.0001, d.f. =3). The only
captures from the industrial edge treatment were of Urocyon cinereoargenteus
(Gray Fox), all probably of the same individual (same deployment and
site). Native canids (pooled) were captured in similar frequencies among the
edge treatments, with none being captured in the forest interior. Finally, excluding
Sciurus carolinensis (Eastern Gray Squirrel), all native species were
captured at night while most cat and dog captures were in the day (75% for
both liberal and conservative analyses).
Discussion
While this is just a case study of one urban forest fragment, our results
show very clearly that an urban forest can be frequently used by free-ranging
cats and dogs. Cats and dogs together were between 20% to 25% of all captures,
and were the great majority of captures in the forest interior. Dogs were
captured at frequencies very similar to that of native canids, while no native
felid (e.g., Lynx rufus Schreber [Bobcat]) was captured. These findings show
that these exotic predators are present deep within the forest (100 m from the
edge) where they might have been expected to be less frequent.
It is unclear whether the dogs photographed were feral, free-ranging
pets straying from residences, or pets accompanying humans walking in
the preserve. None of the dogs photographed, however, showed the longterm
pariah morphotype that would be expected of dogs that were part of
a population breeding without artificial selection (Brisbin 1977). All dogs
photographed resembled large breeds or breed-hybrids, including pit bull
terrier and Labrador retriever. Only a minority (25%) of those photographed
wore a collar. While the preserve requires that dogs be on leash at all times,
this rule is often disregarded by preserve users. Free-ranging dogs can function
as both predators and scavengers (Butler and du Toit 2002, Causey and
434 Southeastern Naturalist Vol. 8, No. 3
Cude 1980, Manor and Saltz 2004, Scott and Causey 1973), and some dogs
are eager to chase native wildlife. All off-leash dogs can pose a threat to
both native wildlife and humans visiting the preserve. In his study of freeranging
dogs in Baltimore, MD, Beck (1973) found that free-ranging dog
densities were higher in poorer neighborhoods where garbage availability,
vacant buildings, unused lots, and unfenced yards were more frequent than
in wealthier neighborhoods. In contrast to Beck’s findings (1973), there was
a trend in our study for more dog captures in HHV edges than LHV edges,
though sample size was limited.
Cats are known to be very effective hunters and can have a signifi-
cant detrimental impact on populations of some small native prey species
(Coleman et al. 1997, Crooks and Soule 1999, Woods et al. 2003). Previous
studies have found that cats use forest fragments (Crooks and Soule 1999,
Oehler and Litvaitis 1996, Wilcove 1985), but most of the evidence thus far
suggests they prefer edges (Crooks 2002, Kays and DeWan 2004, Oehler and
Litvaitis 1996). In our study, a trend emerged for more cats to be captured in
the forest interior, but more study is needed to confirm this pattern. Cats and
dogs were more frequently captured in the day (7 of 9, using conservative
analysis; 9 of 12, using liberal analysis), while nearly all native species were
captured at night. It is possible that cats and dogs may reduce their nighttime
activity in the forest to avoid encounters with native species, or that their
owners restrain them or bring them indoors during the night.
Across species there was a clear trend for more captures in the HHV and
LHV residential edge treatments than in either the industrial edge or forest
interior treatments, which had very few captures in comparison. Previous
findings have also suggested that mammals in developed landscapes often
prefer edge over interior habitats (Groom et al. 2006, Kays and DeWan 2004,
Oehler and Litvaitis 1996). In our study, Raccoons, the most common species
captured, and Virginia Opossums were only detected in the residential
edges; both species are effective scavengers in residential areas where food
refuse and outdoor pet food sources are available. The native canid species
were also found exclusively in edge treatments. Gray Foxes were the
only species photographed along the industrial edge. The canids visiting
the edges may be attracted to edges if densities of their prey species (e.g.,
rodents) are higher than in the forest interior. Some native canids may also
be scavenging outdoor garbage and pet food resources available along the
edges. Only one Canis latrans (Coyote) was photographed during the study;
this may under-represent Coyote densities as Coyotes have been shown to be
wary of camera traps (Sequin et al. 2003).
As is true of RMNC, control and/or elimination of exotic species is
often a top priority for preserve managers (Raney 2007). The reduction of
free-ranging cats and dogs in preserves benefits many stakeholders. Freeranging
exotic predators may depredate, compete with, spread disease to, or
simply harass native wildlife (Butler et al. 2004). In addition, pets visiting a
preserve may be injured, killed, or contract a disease from encounters with
2009 B.K. Marks and R.S. Duncan 435
native wildlife (Olson et al. 2000), and dogs (including pets) may threaten
or harm preserve visitors. Our findings demonstrate that exotic predators can
be a significant presence in urban forest fragments and that camera-trapping
can be effective for surveying exotic predators (and native wildlife) to identify
where dog and cat abatement strategies might be needed. More study
is needed to determine whether these patterns are widespread across other
urban forest fragments.
Acknowledgments
We thank Ruffner Mountain Nature Center for permission to conduct research
in their preserve. We especially thank RMNC naturalist Marty Schulman for field
assistance and logistical support, and BSC student Mark Bentley for field assistance.
Finally, we thank Birmingham-Southern College for its support of this research.
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