It comes as no surprise that natural areas are rapidly disappearing. Urban and suburban sprawl continue to fragment open space into “islands,” or little disconnected pieces of intact habitat surrounded by human development. Undoubtedly, this fragmentation has negative effects on wildlife. Several organizations and universities worldwide are studying the phenomenon of wildlife living in urban, suburban, and otherwise insular areas, with hopes of obtaining information to make wise and positive decisions for the future.
The Santa Monica Mountains National Recreation Area has been conducting a carnivore study since 1996, with a goal of understanding how carnivores are living in an urbanized and fragmented landscape, and how they are affected by these forces. Species of focus include mountain lions, bobcats, and coyotes. Large carnivores inherently require vast home ranges, or tracts of habitats in which the animal lives and travels, in order to acquire resources and avoid interference with competing individuals. This is more pronounced with solitary animals, including mountain lions and bobcats, who only live in the company of others during cubhood and mating. In an area of limited connected natural habitat, the question arises, how are these animals with large home ranges making use of the land? Do their home ranges encompass urban areas or require them to cross highways? Are their populations affected by this limited amount of intact habitat?
Highways are significant obstacles to wildlife movement. Despite the high risk of mortality, animals are crossing these daunting barriers. The Santa Monica Mountains carnivore study has tracked radio-collared animals and confirmed their movement across highways. In the San Francisco Bay and Monterey Bay areas, Connectivity for Wildlife is studying wildlife movement across anthropogenic barriers and determining key wildlife corridors. By employing the use of motion-activated infrared cameras, they have determined that many animals have been using culverts under highways and roads.
Although the animals are crossing these major obstacles, their movement between habitats on either sides of the roads is significantly impeded. The Santa Monica Mountains carnivore study has found that bobcat and coyote home ranges are mostly on one side of the 101 freeway or the other, and that there was twice as much genetic variation in bobcats between sites across the 101 than between sites on the same side of the freeway. Additionally, they have found significantly reduced genetic diversity in mountain lions south of the 101, which is one of the most insularized areas of mountain lion habitat. A study conducted by Dickson et al. in 2005 found highways to be major player in reducing gene flow between local populations in the Santa Ana Mountains.
In areas of isolated or insular habitat, many species may experience the founder effect, where a small population is segregated from the larger population and inbreeding results. A lack of genetic diversity can have negative effects on the fitness of individuals and the viability of the local population as whole. In order to preserve genetic exchange between different local populations, it is critical to determine and preserve key corridor areas, or areas linking intact tracts of habitats.
Multiple highways worldwide have recently been equipped with wildlife crossing structures. In many instances, animals have been using crossings not intended for wildlife, such as culverts, or underpasses. In other areas, park systems and governments have decided to install underpasses or overpasses specifically for wildlife. In Banff National Park, Canada, twenty-two underpasses and two overpasses were designed to increase habitat connectivity across the Trans Canada Highway, with a high degree of success. In Florida, twenty-four wildlife underpasses and twelve modified bridges have been implemented along a 40-mile stretch of Interstate 75, designed specifically to protect the endangered Florida Panther. The Netherlands has installed over 600 wildlife crossings to protect the endangered European Badger as well as red deer, roe deer, and wild boar.
The Santa Monica Mountains carnivore study advocates the construction of a culvert or tunnel at Liberty Canyon to conjoin the open space behind Chesebro Road with that on the South side of the 101 abutting the Santa Monica Mountains. This would provide connectivity between carnivore populations in the Santa Monica Mountains with those in the Simi Hills and Ahmanson Ranch areas. The fate of the open space behind Chesebro was up in the air until October 29th, when the Santa Monica Mountains Conservancy and Advisory Committee voted in favor of applying for grants to purchase the land from the City of Agoura Hills, and preserve it as open space.
For more information on carnviores in the Santa Monica Mountains and Greater Los Angeles area, visit Urban Carnivores