Critters Along the Creekside

Oaks ‘n” Folks – Volume 14, Issue 2 – August, 1999

California’s expanding human development is causing extensive habitat fragmentation and an expanded urban-wildland interface that threaten wildlife conservation. Undisturbed areas become islands of habitat surrounded by development, and habitat islands become smaller and more isolated as developments expand. Animals requiring large patches of woodland to survive must be able to move among remnant habitat patches to find adequate resources and avoid population inbreeding.

In an effort to conserve wildlife populations in fragmenting landscapes, managers and conservation biologists have proposed the use of habitat corridors, strips of intact habitat between two or more otherwise disjunct habitats. Despite theoretical assumptions, few studies provide empirical evidence that wildlife use corridors. Of the existing corridor studies, few examine spatial scales large enough to be relevant to landscape management. Few studies focus on mammals, such as carnivores that exist at low population densities. Most large mammals need corridors to survive. Most wildlife corridors, both natural (sometimes called de facto or “by default”) and designed, are riparian corridors that are essential to protect river systems. Without knowing if wildlife preferentially use riparian corridors in highly modified landscapes, developing appropriate management practices for maintaining riparian corridors as a means of ensuring habitat connectivity for wildlife movement is difficult.

A pilot study was initiated in Sonoma County in July 1998 to begin testing wildlife use of corridors on a landscape scale. Several recent land-use issues in the region made a riparian wildlife corridor study appropriate and timely. Decline of anadromous fisheries, invasion of exotic plant species into riparian zones, and increased hillside vineyard expansion has prompted concern about protecting and restoring riparian corridors.

We sampled wildlife in 3 types of areas in or adjacent to oak woodland habitat of the Mayacumas Mountains in Sonoma Valley: a vineyard, an oak woodland strip, and undeveloped oak woodland. Each area contained a riparian and a non-riparian site. The vineyard area included a site within a vineyard and a riparian site (narrow corridor) along a creek where the creek passed between vineyards. Natural vegetation extended approximately 10 meters from the creek bed. Vineyards and housing are adjacent to the 1-mile wide oak woodland strip that connects the Mayacumas and Sonoma Mountains. The strip contained an oak woodland site and a riparian site along a creek (wide corridor). In the undeveloped oak woodland area, a woodland site and a riparian site were sampled. Live-traps were used to detect the species composition and relative abundance of rodents. Remotely triggered cameras were used to detect carnivores over a 60-day period. Wildlife use of a small and a large highway underpass was assessed with a camera at each site.

A total of 87 individual rodents of six different species were captured. Capture rates in non-riparian sites were low. In riparian sites, the least number of species and fewest individuals were captured in the vineyard riparian site. Dusky-footed wood rats were not observed in this site, but were present in both the oak woodland strip site and undeveloped riparian site, which suggests that dusky-footed wood rats may be sensitive to vineyard development. Black rats and house mice, non-native rodents, were present in both the vineyard riparian site and the oak woodland riparian site, but in neither of the undeveloped riparian sites. Non-native species often are found near human-altered landscapes like vineyards, and their presence is a concern because they may displace native rodents in moderately disturbed areas.

Cameras documented seven carnivores in 159 photos: raccoon, opossum, gray fox, domestic cat, striped skunk, coyote, and bobcat. The domestic cat, another non-native species that may molest or compete for food with native wildlife, was most abundant in the vineyard riparian site, again suggesting that vineyard sites may be preferred by non-native wildlife. Neither bobcats nor coyotes were observed in either of the vineyard sites. Bobcats were observed in both oak woodland strip sites, and bobcats and coyotes were detected in the undeveloped oak woodland area, which suggests that, like the dusky-footed woodrat, these species also may be sensitive to vineyard development. During 40 days of monitoring, only one animal (a raccoon) was documented using the small underpass. The camera at the large underpass took 43 animal photographs, indicating that larger underpasses may be necessary to facilitate wildlife movement.

The results of this pilot study are tentative. However, narrow riparian corridors in vineyards appear to lack some native species and harbor exotic species. Wide oak woodland strips may support native species sensitive to more disturbed areas, such as vineyards, but also may contain exotic species. This suggests that active planning of wildlife corridors may be important to maintaining native mammal populations in vineyard landscapes. Where roads intersect wildlife corridors, underpass design may affect the utility of the corridor. Based on the outcome of the pilot study, extended research focusing on differences in species composition among sites using non-baited cameras at independent oak woodland sites across multiple seasons is underway.

prepared and edited by Richard B. Standiford, Justin Vreeland, and Bill Tietje

Jodi Hilty, Graduate Student, Dept. of Environmental Sciences, Policy, and Management, UC Berkeley