Along the wall at the biodiverse US-Mexico border, study saw decrease in some animal populations

In one of the most ecologically diverse corridors in North America, where both black bears and jaguars coexist, an environmental nonprofit is studying wildlife movement in southern Arizona’s borderlands and has found a decrease in certain mammal species, prompting questions about what is going on.

Driving through the grasslands just north of the border, a sea of gold and white-hued grasses spread out for miles one morning in late April. Eamon Harrity, the wildlife project manager at Sky Island Alliance traveling to the organization's motion-censored cameras, pointed out pronghorn antelope in the distance and a virtual Border Patrol surveillance tower amidst the natural landscape.

In southern Arizona's borderlands exists a biodiverse area where mountains rise up out of the desert and where the desert sits next to oak woodlands and grasslands, among other habitats. The area is home to more than 7,000 species of plants and animals, which includes half of the birds in North America, according to the Sky Island Alliance.

The environmental advocacy group began its Border Wildlife Study in 2020.

Three years later, the data from that study has revealed a slight decrease in medium to large mammals in the Patagonia and Huachuca mountains. In the San Rafael Valley, which sits in between the two mountain ranges, the cameras have documented a slight increase.

"The trend is not super steep, but it is indicative of a pattern. More needs to be explored about what is driving these patterns," Harrity said.

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Data reveals coyote detection rates are declining in some areas

The study found a significant decline in the number of coyotes detected during May and June of 2020 compared with the same period in 2022.

Specifically, the cameras documented a 60% decline in the Patagonia Mountains, a 54% decline in the San Rafael Valley, and a 68% decline in the Huachuca Mountains. However, during the second half of 2022, the numbers of coyotes seemed to have started to rebound slightly in the San Rafael Valley.

Across all cameras this corresponded to 86 detections of coyotes in May and June 2020 and only 35 times across those same cameras in May and June 2022.

The study also documented a 41% decline in mule deer in the San Rafael Valley, with mule deer detected on the cameras 39 times in May and June 2020 and only 23 times during that same period in 2022.

These numbers are based on average detection rates for the species in each section of cameras and compared across years.

“We’ve been noticing these patterns and the next step is to dig into why,” Harrity said.

Harrity said potential causes could be environmental factors, including continuing effects from extreme drought that hit Arizona in 2020 which was heavily felt throughout 2021, as well as prey availability.

Coyotes are omnivores and eat plants like cactus fruit, mesquite beans, flowers and also insects, rodents, lizards, rabbits, birds, and snakes, among others.

Harrity said they are known for being an adaptable and resilient animal and have stable population numbers around the country.

Conversely, while the study found that lagomorphs, various species of jackrabbit and cottontail, were decreasing at the beginning of the study, they have been increasing more recently throughout the study area.

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Causes of the declines remain unknown

To study wildlife movement in the borderlands and the effects of the border infrastructure on wildlife, the Sky Island Alliance placed more than 100 cameras throughout the borderlands with 22 of them studying how animals interact and navigate through the different kinds of border infrastructure.

Infrastructure includes steel bollards and vehicle barriers often made of old railroad tracks, according to the Wildlands Network, and barbed wire fencing that has been in place in some areas for over 100 years, Harrity said.

While the causes of the decrease in coyotes are unknown, this data can be used as a reference point to find out how the permanent steel bollard border wall impacts wildlife.

When the wall was constructed between 2017 and 2020, all environmental review requirements were waived to streamline construction. These reviews are designed to look for environmental impacts of projects.

“Without such reviews, it is difficult to measure the true extent of what has been lost and damaged by the construction process and assess the long-term impacts of the barriers once they are constructed,” he said.

Harrity noted that while comparing the baseline data to data collected in areas with and without the steel bollard border wall is not as direct as a comparison of the “before and after” of the same areas, it is a helpful alternative.

“Our wildlife monitoring efforts ensure we can use data-driven evidence on the diversity of this region and the impacts of border infrastructure on wildlife to advocate for more sensible and wildlife-friendly border infrastructure,” he said.

The Arizona Game and Fish Department, which manages wildlife within public land in the state, said rabbit populations throughout the state have been impacted by RHDV2 disease, which led them to recently lower their bag and possession limits.

"Coyotes and rabbits have a highly predictable oscillating population dependency. We do not monitor these populations at that level, but when rabbit populations decline predictively, we would expect coyote populations to follow," said Josh Avey, terrestrial wildlife program branch chief with the Arizona Game and Fish Department.

He noted that any reductions of other species would likely be due to the drought conditions in previous years.

How wildlife is affected by the wall

Anecdotally, Sky Island Alliance has seen that animals cross frequently through areas with barbed wire and vehicle barriers, and much less in areas with the wall, which consists of 18-foot or 30-foot steel bollards.

Scientists have seen deer pacing back and forth along the wall, and carnivores trying and failing to get through.

In the short term, the inability for larger animals to pass through will be seen relatively quickly and could lead to individual animals failing to breed or unable to access food or water, Harrity said.

“The impacts to populations and entire species will play out across longer time periods and will be harder to address if border infrastructure continues to be so impermeable to wildlife,” he said.

Harrity said if a species cannot cross the wall, they are essentially split in half. This leads to a smaller gene pool, which is shown to have unintended consequences.

Data shows animal activity declined in areas that bordered the shipping container wall

In December, former Gov. Doug Ducey placed shipping containers along the U.S.-Mexico border, through Coronado National Forest land. Construction of the makeshift wall was stopped, eventually deemed illegal and dismantled soon after.

In such a short time, the Sky Island Alliance documented a 25% decrease in daily mammal detections when the containers were present compared with average rates before and after the containers.

The makeshift wall blocked seasonally flowing drainages and wildlife migration corridors.

"Our study spans across one of the last areas that doesn’t have the big steal bollards, we are hoping to keep it that way," Harrity said, in a presentation about the data. "So, our data are important for providing a context and argument against any construction of border wall."

This article originally appeared on Arizona Republic: Along US-Mexico border, study saw some animal populations decreased