PRIORITIZING MITIGATION FOR INTERSTATES USING WILDLIFE MOVEMENT INFORMATION

Many California interstates provide commuter traffic and goods movement among regions and cities through wild, protected areas. Collisions between wildlife and vehicles occur frequently, which has prompted Caltrans to seek assistance in assessing the nature, extent, and solutions to potential conflict between traffic and animals. The objectives of the study were to understand how wildlife were currently using available under-crossing structures, how wildlife in general and mule deer (Odocoileus hemionus) in particular interact with the highway and adjacent habitat, and to develop mitigation for risk reduction. Three types of wildlife observation data along 2 interstates were used to characterize wildlife movement: wildlife-vehicle collisions (WVC), images from wildlife camera traps at highway under-crossings, and deer movement patterns using GPS-collars. WVC occurrences were from Caltrans’ monitoring of carcass retrieval and disposal by Maintenance crews and opportunistic observations of carcasses by participants in the California Roadkill Observation System (http://wildlife crossing.net/california). Statistically-significant WVC hotspots were modeled using the Getis-Ord Gi* statistic, with Moran’s I being used to estimate distance bands. Beginning in July, 2011, camera traps captured still and video images at 4 street underpasses, 1 bicycle over-crossing, 5 railroad under-crossings, 6 culverts, and 10 wildlife trails adjacent to crossing structures. Species diversity and the relationship between wildlife passage and human use of structures were calculated. Twenty four female deer were tracked using GPS collars between December, 2011 and January, 2013. There was a strong negative relationship between the presence of humans hiking, driving, walking dogs, or riding bikes and the use of existing crossing structures by wildlife. In addition, there was very low species diversity observed using crossings at either interstate. Only 9 native mammal species were observed to use crossing structures at either interstate, which was not a function of camera sensitivity as they could detect movement of small lizards. Collared deer often approached and moved back and forth near the interstate. Only 5 of the 24 collared deer passed back and forth under the highway, all using the same crossing structure, a large vegetated underpass with a minor road. Mitigating WVC consists of where to act and what actions to take to reduce risk to drivers and animals. Managing conflict between vehicles and wildlife along interstates is likely to require identifying priority areas, fencing to keep deer and other animals from accessing the road surface, construction of new underpasses or enhancement of existing structures, and re-management of existing underpasses to reduce human use. Future research should focus on responses of wildlife to reduced human passage at underpasses, the different management required in developed vs. undeveloped areas, and methods to increase species diversity at crossing structures.