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Use of Forest Edges by Free-ranging Cats and Dogs in an Urban Forest Fragment
Britni K. Marks and R. Scot Duncan

Southeastern Naturalist, Volume 8, Number 3 (2009): 427–436

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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 - 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. Literature Cited Andelt, W.F., and T.P. Woolley. 1996. Responses of urban mammals to odor attractants and a bait-dispensing device. Wildlife Society Bulletin 24:111–118. 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