Nature does not work without connection.

– Mary Ellen Hannibal, The Spine of the Continent, 2012

Wildlife populations need to be able to connect to one another to provide access to additional water, food, mates and space. Our current intense human urbanization presents barriers to the free movement of wildlife populations. The resulting competition for space, food and water has led to exclusion of some wildlife from areas of intense urbanization.

There are a series of wildlife crossing (or overpasses) throughout Banff National Park. This crossings allow wildlife to cross the highway safely and maintain natural migration pattern. Photo: Michael Kwan, CC

To increase habitat connectivity, culverts and overpasses have been built to facilitate the movement of wildlife. According to Nancy Siepel of CalTrans, a few of these locations include: Banff, Alberta, Canada; Highways 260 and 93 in AZ; Hwy 395 near Susanville, CA; and Hwy 76 in San Diego. Several other locations are in the planning stages or they have been completed.  

The most ambitious plan seeks to provide a 5,000-mile wildlife corridor from now isolated Yellowstone National Park north to the Yukon. The noted naturalist E. O. Wilson has said that this type of project is “the most important conservation initiative in the world today.”

Animal populations in the Santa Cruz Mountains have significant dispersal barriers along Rte. 280, Hwy 92 and Hwy 17. Of all these barriers, Hwy 17 presents a challenge and an opportunity to mitigate dangerous wildlife crossings.

POST, along with other regional conservation partners, are researching the effectiveness of building wildlife crossings on Hwy 17. Providing safe passage for wildlife will improve their access to food and water, establishing their own territory, and locating greater numbers of viable mates to ensure genetically healthy populations, according to Tanya Diamond, Co-Principal, of Pathways for Wildlife, and Neal Sharma, Stewardship Project Manager, from the Peninsula Open Space Trust. Extensive animal dispersal fosters the health of wildlife populations and it usually leads to territories of sufficient size for each animal species.  


With freer access to a large territory, wildlife will have an enhanced chance of individual survival. So too, greater connectivity of habitats will lead to greater ease of animal movements, improve gene flow, reduce inbreeding and result in healthier wildlife populations.


A few animals are using the culvert at Trout Creek to cross underneath Hwy 17. However, many species, like these black-tailed deer, won’t enter such a tight space. Photo: Pathways for Wildlife

A study conducted by Pathways for Wildlife for Midpeninsula Regional Open Space District (MidPen) and POST, using motion-sensitive cameras, has recorded the number and species of animals that traverse existing culverts. A significant number of animals stop at the entrance and do not proceed, perhaps lacking a clear line of sight. At the Lexington Reservoir culvert, grey fox, red fox and bobcats, all medium-sized mammals, are the only animals that travel through the culvert.

Larger animals such as mountain lions and deer inspect the culvert, which is 10 feet tall by 10 feet wide, but they do not use it as an under-crossing possibly seeing it as too risky.  Instead, they turn around or take their chances by crossing Hwy 17.

Several agencies have collaborated in the effort to finance and build a more animal-friendly culvert to allow the passage of wildlife. These are POST, MidPen, UC Santa Cruz, CalTrans, and the California Department of Fish and Wildlife.

After careful review of available data, the structure is undergoing a feasibility design study to consider all aspects of installing a wildlife structure  for the proposed Hwy 17 crossing at Trout Creek. Precise cost estimates will be determined when the design of the new culvert is finalized. Nevertheless, POST and MidPen have both pledged to finance a portion of the project. The timetable depends on a process involving feasibility studies, permitting, and finally the construction phase.

Once completed, wildlife will be able to easily travel beneath the highway in both directions. Those coming from the Santa Cruz Mountains will be able to disperse into the Sierra Azul Preserve, the Gabilan Mountains, the Coyote Hills and across Hwy 101 to the Diablo Range. Another stream of wildlife will be able to cross the highway and disperse northward.

“This project rates very high on the list of POST priorities,” said Sharma, since it increases connectivity between the Santa Cruz Mountains and the largely undeveloped mountain ranges to the south and east. When finished, this important crossing will be more permeable to wildlife and it will connect to an extension of the Bay Area Ridge Trail, according to Julie Andersen at Mid Peninsula Open Space District.

With freer access to a large territory, wildlife will have an enhanced chance of individual survival. So too, greater connectivity of habitats will lead to greater ease of animal movements, improve gene flow, reduce inbreeding and result in healthier wildlife populations—an outcome that will especially benefit wildlife forced to live on the edge of our urban areas and subject to the damaging effects of habitat fragmentation.


Read part one of this series here.

Coauthored by Tanya Diamond, Pathways for Wildlife

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Peninsula Open Space Trust (POST) protects open space on the Peninsula and in the South Bay for the benefit of all. Since its founding in 1977, POST has been responsible for saving more than 87,000 acres as permanently protected land in San Mateo, Santa Clara and Santa Cruz counties. Learn more

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