Evaluation of Connectivity for Wildlife in Human-altered Landscapes: Mule Deer Landscape Genetics and Highway Crossing Rates for Deer and Carnivores at Key Southern California Linkages Active

Connectivity, the degree of movement of organisms or processes among habitat patches (Taylor et al. 1993), is essential to allow for the natural ranging behavior of animals among foraging or breeding sites and for the dispersal of wildlife from their natal ranges. Such movements facilitate exchange of genetic material among otherwise isolated populations and may be essential for natural range shifts in response to long-term environmental changes. Across large areas and over time, maintaining natural levels of connectivity may be essential to allow natural species’ range shifts in response to global climate change and other long-term transitions. Finally, connectivity is necessary to maintain the continuity of large-scale ecological processes and the flow of material, energy, or nutrients. Due to the threat that habitat fragmentation poses to natural environments, connectivity conservation is increasingly becoming incorporated into land-management plans worldwide.  

Urbanization is a leading agent of fragmentation and the associated loss of connectivity, and is a primary cause of species endangerment (Czech et al. 2000; Soulé 1991). Such is the case in coastal southern California, which is a hot-spot of biodiversity (Myers 1990; Wilson 1992) but where rapid urbanization and habitat fragmentation threaten much of this biodiversity (Dobson et al. 1997; Myers 1990; Wilson 1992). Fragmentation solely from roadways has had a significant impact across the Southcoast Ecoregions (Hunter et al. 2003), as highways connecting urban centers splice through natural areas and run perpendicular to animal movements. Major roadways are known to impede gene flow in a variety of species (Riley et al. 2006, Holderegger and Di Giulio 2010), effectively isolating populations and lowering genetic diversity. Highways may thus be potential sources of population decline. Given the continued pace of development and needs for transportation infrastructure, information on landscape connectivity in southern California is important for land and resource managers charged with maintaining natural habitats and species that depend on them. 

Mammalian carnivores can be effective focal species to evaluate the degree of landscape-level connectivity, or fragmentation, in a region. Large carnivores are particularly vulnerable to extinction in fragmented habitat because of wide ranges and resource requirements, low densities, slow population growth rates, and direct persecution by humans (Crooks 2000; Crooks 2002; Noss et al. 1996; Woodroffe & Ginsberg 1998). Consequently, top predators may not be able to persist in landscapes that are not connected by functional movement corridors. Using mammalian carnivores in conservation planning adds a critical layer of conservation strategy that may provide a robust method for protecting other species with less demanding needs (Carroll et al. 1999; Lambeck 1997; Miller et al. 1998). In southern California, medium carnivores such as bobcats (Lynx rufus) are an excellent focal carnivore species for the evaluation of connectivity (Crooks 2000; Crooks 2002), particularly where fragmentation levels are too high to support larger carnivores. Bobcats can persist in smaller habitat fragments, but only those that have adequate connections to larger natural areas. 

While connectivity for carnivores can support many other species, the assumption has not been tested for mule deer, a species that has received relatively little attention from research in the western United States and in southern California in particular. Mule deer are known to be sensitive to roadways but may have even fewer opportunities to utilize freeway underpasses than carnivores due to inadequate height and lack of suitable habitat near freeways (Ng et al. 2004). In a statewide analysis, Pease et al. (2009) found that southern California deer lineages exhibit lower genetic diversity than other California deer populations. However, this study’s scale was too coarse to apply to specific linkages within southern California and its sampling excluded large portions of coastal southern California. In this research we seek to evaluate connectivity for bobcats, deer, and other mammals, to provide local planning information and make predictions for wildlife responses and needs under continued landuse and environmental changes. Specifically we seek to understand site-specific and species-specific determinants of highway crossing rates at key linkage sites. We also address the questions, do freeways and urbanization act as barriers to gene flow for local mule deer populations? 

One of the key landscape linkages identified as important for movement and maintenance of large mammals and carnivores in southern California is the connection between the Santa Ana Mountains, Chino Hills, and Prado Basin (Spencer et al. 2010).  Large highways have been built between these three natural areas that are at the intersection of Orange, Riverside, and San Bernardino Counties, in a region of rapid, intensive urbanization and highway expansion. A four-lane, north-south highway, CA-71, lies between the Chino Hills and Prado Basin, and a 14-lane highway, CA-91, runs east-west and separates these areas from the Santa Ana Mountains. The Santa Ana River, the largest river in southern California, also runs through here parallel to CA-91 from the Prado Basin.

Several land and resource managers have responsibilities for the natural areas and transportation infrastructure in between them, including California State Parks, Irvine Ranch Conservancy, Caltrans, Orange County Transportation Agency, and U.S. Army Corps of Engineers (Corps). These groups work independently and together to address and meet connectivity requirements for wildlife as addressed in the 2001 U.S. Fish and Wildlife Service Biological Opinion (Ref: FWS-SB-909.6) for the area. The Corps has special responsibility for providing flood risk reduction to communities in the three counties, via its Santa Ana River Main Stem Project (SARMP).