Ecological impact of inside/outside house cats around a suburban nature preserve

While subsidised populations of feral cats are known to impact their prey populations, little is known about the ecological impact of inside/outside hunting cats (IOHC). We studied IOHC around a suburban nature preserve. Mail surveys indicated an average of 0.275 IOHC/house, leading to a regional density estimate of 0.32 IOHC/ha. A geographical model of cat density was created based on local house density and distance from forest/neighbourhood edge. IOHC hunted mostly small mammals, averaging 1.67 prey brought home/cat/month and a kill rate of 13%. Predation rates based on kills brought home was lower than the estimate from observing hunting cats (5.54 kills/cat/month). IOHC spent most outside time in their or their immediate neighbours’ garden/yard, or in the nearby forest edge; 80% of observed hunts occurred in a garden/yard or in the first 10 m of forest. Radio-tracked IOHC averaged 0.24 ha in home range size (95% minimum convex polygon (MCP)) and rarely entered forest. Confirming this, scent stations detected cats more often near the edge and more cats were detected in smaller forest fragments. There was no relationship between the number of cats detected in an area and the local small mammal abundance or rodent seed predation rates. Cold weather and healthy cat predator populations are speculated to minimise the ecological impact of IOHC on this area. Domestic cats (Felis silvestris catus) are of special concern for conservation biologists because of their record as a subsidised exotic predator of native species. Because they often receive supplementary food from human caretakers, they can reach densities of 100 times or more higher than native carnivores (Coleman & Temple, 1993). The consequences of their hunting have been especially severe on oceanic islands with no native mammalian predators, where native prey species have little innate ability to elude cat predation (e.g. Bloomer & Bester, 1992; Stiling, 1996). Evidence is accumulating that continental cats may also be a conservation concern because of the effects their subsidised populations can have on native prey (Crooks & Soule, 1999; Baker et al., 2003). However, F. silvestris lives in a number of distinct domestic and wild situations and it is important to consider this variety when evaluating their conservation risk in any particular situation. Studies on their ecological role have typically focused on worst case scenarios such as feral cats, farm cats and ‘cat colonies’ (mostly neutered strays fed and given shelter at one centralised structure: Clarke & Pacin, 2002). Truly feral cats receive little or no food from humans and hunt as much as four times more than domesticated animals (Liberg, 1984; Paltridge, Gibson & Edwards, 1997). Most cats living on farms ∗All correspondence to: R. W. Kays. Fax: 518-486-2034; E-mail: [email protected] or managed colonies receive supplemental nourishment from humans, but also hunt for meals. These cats live at such high densities that their impact on the local fauna can be substantial (Hawkins, 1998). Managed colonies, in particular, pose a significant threat if located near protected areas (Weber & Dailly, 1998; Clarke & Pacin, 2002). However, most cats are not as free-ranging; rather, they live as well-fed house pets. Estimates put the owned cat population of the USA at around 60 million and the stray/feral cat population at between 25–40 million (Patronek & Rowan, 1995). It is estimated that over 50% of these owned cats spend time outside their house (APPMA, 1997). These inside/outside house cats (IOHC) typically receive all their food from their owners; their hunting appears to be more recreational or opportunistic, rather than directed towards feeding themselves or their litters (Leyhausen, 1979). Surprisingly, few studies have documented the ecological effects of IOHC; even ranging patterns and hunting rates of this type of cat are poorly known (Fitzgerald, 1990; Fitzgerald & Turner, 2000). What is the ecological effect of this recreational hunting? A number of studies have sounded the alarm against letting house cats roam outside based on the number of small prey they bring home (Meek, 1998; Robertson, 1998; Woods, McDonald & Harris, 2003). However, only one study has actually documented a negative impact of these IOHC on a wild population of 2 R. W. KAYS AND A. A. DEWAN prey in natural areas (scrub breeding birds: Crooks & Soule, 1999). In fact, some authors have warned against wide ranging civic regulation without more data about the hunting habits and ecological impacts of domestic cats (Fitzgerald, 1990; Jarvis, 1990). Although small mammals are the most common prey of housecats (Liberg, 1984; Meek, 1998), the impact (or lack there of) of IOHC on natural small mammal populations is rarely mentioned and most discussion has centred on their impact on birds. This question has important ramifications past the mere conservation of the small mammals themselves, since they often play important ecosystem roles as seed dispersers/predators and vectors of diseases (Ostfeld, Manson & Canham, 1997; Ostfeld & Keesing, 2000). Because IOHC are so prevalent in society, it is important to understand the conservation implications of their predation on native prey in natural habitats. With this as our overall objective, we studied a population of IOHC around the suburban Albany Pine Bush Preserve (APBP), New York, documenting their density, use of the landscape, hunting habits, effect on small mammal populations and effect on rodent seed predation rates using a suite of complementary techniques. If IOHC are a major conservation threat to the preserve, we would expect them to range widely through the preserve, hunt native prey within the preserve at a high rate and reduce the abundance of prey in areas where they hunt the most, thereby also affecting the ecological processes associated with these prey populations. METHODS AND MATERIALS