Authors: Seth P. D. Riley, Raymond M. Sauvajot, Todd K. Fuller, Eric C. York, Denise A. Kamradt, Cassity Bromley, and Robert K. Wayne.
Urbanization and habitat fragmentation are major threats to wildlife populations, especially mammalian carnivores. Conversion of natural habitat to human uses reduces the amount of intact natural habitat and fragments what remains. Wide-ranging and low-density species such as mammalian carnivores are particularly vulnerable to the processes of habitat loss and fragmentation. Bobcats (Lynx rufus) and coyotes (Canis latrans) are common predators throughout North America, yet little is known about how these two relatively adaptable but also quite different carnivores respond to urban development and habitat fragmentation.
To better understand the conservation of carnivores in the face of urbanization, we studied the use of a fragmented landscape by bobcats and coyotes. We estimated the level of exposure to development for these two carnivores by measuring the percentage of developed and altered area within their home range, or their “urban association.” Our goals were to determine whether exposure to urban areas was related to species, age/sex class, home range size, degree of nocturnal or diurnal activity, survival rates, and causes of mortality.
The study area was in the central Santa Monica Mountains and Simi Hills of southern California, west of the city of Los Angeles (Fig. 1). Land cover was a patchwork of natural vegetation and human land uses.
We captured, radiocollared, and monitored 50 bobcats and 86 coyotes (Fig. 2). All location data were entered into a database for GIS analysis in ArcView 3.2 (ESRI 1998). We determined 95% minimum convex polygon (MCP) home ranges (Hayne 1949) for 35 bobcats and 40 coyotes using the Animal Movement Extension (Hooge, et al. 1999). We calculated survival and cause-specific mortality rates, and, when an animal was located, activity level and the time of day was recorded.
We generalized a digital land-use map (SCAG 1993) into natural areas, developed areas, and altered open areas (Fig. 1). “Natural areas” consisted of large, contiguous, areas of natural vegetation. “Developed areas” included commercial and residential areas with at least one house per 0.40 ha. “Altered open” areas included large open landscaped areas, a landfill, graded areas, and small patches or strips of natural vegetation within high-density residential development (e.g., stream corridors).
We measured urban association as the percentage of each animal’s home range consisting of these non-natural land uses. We overlaid the 95% MCP home ranges for each animal onto the land use classification map and calculated the percentage of its home range consisting of development, altered open areas, or both (referred to as “non-natural area”). We also overlaid individual locations on the land use map to determine urban association for all animals to examine the relationship of urban association with cause-specific mortality rates.
Results & Discussion
Even though these carnivores utilized developed areas, all groups were predominantly associated with natural areas (Table 1). For bobcats, age and sex affected their sensitivity to urbanization. Male bobcats were significantly more urban associated than female bobcats (Table 1, Fig. 3). This difference was most striking for adult males and females. The low level of urban association of adult female bobcats may be related to the species’ polygynous social system, in which females care for the young. Areas frequented or modified by humans may be perceived by adult female bobcats as unsafe for raising young.
For some predators, such as red foxes and raccoons, density increases and home range size decreases in urban areas, presumably because of high-density food supplies and sufficient habitat requirements. Fedriani et al. (2001) suggest that coyote density may be enhanced by human-related food items in this study area, and in comparison with other populations, the home ranges of bobcats and coyotes in our study were small. The productivity of environments in coastal California may allow bobcats and coyotes to meet metabolic requirements with small home ranges, but the constraints of urban habitat fragmentation may also restrict home range size.
However, although developed and altered open areas may offer increased food resources, we observed a positive relationship between home range size and urban association. For all bobcats, home range size was larger for animals with higher urban association. This positive relationship was significant for adult males and young females. For all coyotes home range size increased with percent range non-natural but not with percent range developed. For adult male coyotes the relationship was stronger. This suggests that non-natural areas are less suitable than natural areas in some important aspect. Secure resting and denning locations may be more dispersed in developed areas, and although coyotes, adult male bobcats, and young female bobcats may forage in the neighborhoods, they may be less willing to rest there.
A decreased sense of security around humans would also explain the shift toward nocturnal use of more developed areas in both species. Based on the analysis of all radiolocations, both coyotes and bobcats shifted their use of altered open areas, and particularly of developed areas, to the night. Additionally, at the individual animal level there was a relationship between urban association and night activity.
The survival rates for bobcats and coyotes in our study were similar to those reported in other unexploited populations and survival rates did not vary with urban association. So, contrary to our expectations, there were no differences in survival rate relative to urban association. We followed mostly full-grown animals that were at least 6-9 months old, so differences in survival rate could be more evident in young animals. Perhaps when bobcats and coyotes have learned to negotiate their first few months in the urban landscape, the lack of human exploitation or larger carnivores produces high survivorship. Additionally, in our study area almost every animal lives in a fragmented and urbanized landscape: 4 coyotes and 2 bobcats had home ranges consisting entirely of natural area. Consequently nearly every bobcat and coyote was potentially affected by human-associated mortality sources. And, while vehicles and rodenticides were among the principal causes of death for bobcats and coyotes, mortality rates from specifically human-related causes were not positively related to urban association. For bobcats the vehicle death rate was highest in animals with the least urban association. In this landscape roads are omnipresent and roads traversing open space can be particularly dangerous, especially if used by many vehicles traveling at high speed.
Woodroffe and Ginsburg (1998) suggest that minimizing carnivore mortality at the boundaries of nature reserves may be more critical than the size of the reserve. Our results further indicate that in fragmented urban landscapes, human-caused mortality may affect all animals. Carnivore conservation efforts in these landscapes must account for the pervasive impacts of humans and development, even within reserves.
Bobcats are more sensitive to urbanization than omnivorous canids such as coyotes and gray foxes (Riley 1999). The higher sensitivity of adult female bobcats in particular is important for bobcat population viability because lands that are inhospitable for females cannot produce new animals. The most marginal areas used by bobcats may be a population sink if dispersing bobcats reach these areas from nearby source populations but do not reproduce there. An important management question then becomes, what are the minimum requirements for adult female bobcats to survive and successfully reproduce? Adult female home ranges averaged 1.7 km2, and we know that fragments of 3.15 and 4.45 km2 supported at least three female bobcats. However, the habitat quality of a particular patch is important, and intensive human alteration will limit its value. We do not know the reproductive success of bobcats in habitat fragments or how it compares with that in nearby wildlands.
Even the highly adaptable coyote utilizes natural areas more than developed areas, expands its home range in increasingly urbanized areas, shifts its use of developed areas to periods of decreased human presence, and is vulnerable to vehicle collisions and poison. Ultimately, we must not only learn the requirements of carnivore species in developing landscapes, we must also educate people to value carnivores and promote their conservation by preserving open space, using rodenticides sparingly and correctly, providing usable crossing points under freeways and major roads, and driving slower where carnivores cross roads.
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