Summer peregrinations: Wildlife need to beat the heat too

Summer is a season of transition for many animals in the Sky Islands. As temperatures drastically rise, many animals hike into the mountains to escape the heat and find new resources, or they travel from one mountain range to another. As water becomes even more scarce, wildlife congregates around the few remaining perennial sources. Cactus and trees are blooming, and predators will follow their prey to these resources. Along this wash near Borderlands Wildlife Preserve, we see a wide range of animals using it to travel, despite the nearby road noise and barbed wire fence. In a landscape increasingly dominated by humans, swimming through the seas of desert and grassland becomes ever more risky as wildlife face difficult challenges.

Javalinas captured by our wildlife cameras.

Summer challenges for migrating wildlife

Scorching roads and highways are already dangerous areas for animals, especially for those who do not perceive vehicles as a threat, or can not physically evade them in time. Often roads follow waterways because that is where people have naturally established towns and cities, and it is often easy terrain to traverse. For example, Highway 82 travels parallel to Sonoita Creek, the low point between a few of the Sky Island Mountains: the Santa Ritas and Patagonias, and the Canelo foothills that ultimately connect with the Huachuca mountains. Areas like this are critical for animals to move through in search of water and other resources, making road crossing unavoidable, and risky for both wildlife and drivers. 

Fencing creates a barrier for animals moving across the landscape. Often these fences are to mark property boundaries, keep livestock in a specific area, and to protect crops  — not necessarily to keep wildlife from crossing. And while many larger animals like ungulates (hoofed mammals) can jump over, adults can become separated from juveniles who are not able to jump high enough. The pronghorn prefers to crawl under fencing, as their grassland habitat rarely presents the need for them to jump over large objects, and they can become snagged on the lowest wire. Barbed wire fencing is also hard to see, especially for fast-moving ungulates and birds. 

Bobcat on the move.

Riparian areas are incredibly valuable in the Southwest because they are naturally few and far between, but provide critical resources and habitat for all kinds of wildlife in the region. Many animals use waterways to travel along because they can stay close to the resource and also take advantage of the shade and shelter that the vegetation along washes and streams provide. But riparian areas are dwindling as cattle drink, overgraze, and compact the area. Groundwater levels have dropped across the Southwest due to prolonged droughts, lack of infiltration, vegetation changes, and mining activities. Trees like Freemont’s cottonwood and Arizona sycamore that need access to the shallow water table near cienegas (wetlands) are in danger of not being able to obtain enough water to survive. As the canopy collapses, the extensive root systems of the trees can no longer help hold the soil together, leading to destabilized banks that are more susceptible to soil erosion. Streams become incised, and create a negative feedback loop as flow velocity continues to increase as channels straighten and deepen, with no vegetation to hold soil or water. This threatens an already fragile and fragmented resource for people and wildlife alike.

What can be done 

Wildlife crossings are structures designed to help wildlife avoid crossing roads and highways by providing an alternate way across. They can range from small culverts designed to help slow-moving reptiles and amphibians travel underneath roads, to large overpasses designed to connect the natural habitat on either side of a highway to encourage all wildlife, especially large ungulates and predators, to travel over the highway and completely avoid vehicles. 

A skunk passing by one of our wildlife cameras.

Wildlife crossings are a relatively new development in the effort to reduce wildlife-vehicle collisions. They are incredibly expensive and difficult to incorporate into existing infrastructure. But they are incredibly effective. In 2016, the Oracle Road wildlife bridge was completed, the first of its kind in the Sonoran Desert! Oracle Road consists of six lanes of traffic, and cuts between the Santa Catalina mountains and the Tortolita mountains. In conjunction, a large underpass was built just to the south, and wildlife fencing was installed to guide large mammals towards the crossing structures. Between completion in 2016 and 2020, over 12,000 wildlife crossings have been documented. Completion of the Wallis Annenberg wildlife overpass over the 101 in Agoura Hills, California is set for 2026, which will especially benefit mountain lions who require large home ranges, and are threatened by a lack of habitat connectivity and general loss of resources. 

Incorporating “wildlife-friendly” fencing whenever possible is a great way to avoid wildlife injury and death due to fencing collisions. This fencing can be designed in multiple ways to try and accommodate both wildlife and human needs, but in general, the top and bottom wire are not barbed so that animals crawling or jumping over do not snag as easily. The bottom wire is also usually placed higher to allow small and medium sized animals room to crawl under. The total height of the fence is ideally placed at 40 inches, to make jumping over easier for ungulates like elk and deer. 

A deer crosses a fence.

Protecting and restoring our watersheds and riparian areas is critical for maintaining healthy ecosystems in Southern Arizona. These areas support a high level of biodiversity, especially for Southern Arizona’s native fish, amphibian, and reptile populations, and provide important stops for migrating birds. They are also incredibly important recreational and spiritual places for people to be able to escape the heat while still spending time outdoors, admiring the intrinsic beauty that riparian corridors offer. 

Maintaining open space that is accessible and ecologically healthy is critical for the future of wildlife populations as human development continues to expand. Keeping known wildlife corridors like Borderlands Wildlife Preserve open is especially important for connecting habitat that enhances successful seasonal migration.