New Study Reveals an 86% Reduction in Wildlife Crossings at the Border Wall. Our Experts Explain Why…
and what this means for biodiversity in the Borderlands.
A mountain lion at the border wall. Photo by Sky Islands Alliance/Wildlands Network
Wildlands Network and Sky Islands Alliance published a study in Frontiers in Ecology and Evolution that revealed the impact of the U.S.-Mexico border wall on wildlife movement and habitat connectivity. The U.S.-Mexico Borderlands are home to some of the most biodiverse ecosystems in North America, which harbor species like pronghorn, black bear, jaguar, Mexican gray wolf, ocelot, javelina, and many more.
However, the study’s results demonstrated that only 9% of wildlife interactions with the border wall led to successful crossings – an 86% reduction in crossings when compared to vehicle barriers, including a 100% reduction in crossing for large animals like black bears, mountain lions, deer, and wild turkeys. We monitored 13 small wildlife passages that proved to facilitate 16.7 times more crossings at the border wall, which were particularly helpful for small-to-medium-sized animals like the javelina and coyote. The study underscores that more frequent and larger openings are critical for the survival of many species and for maintaining connectivity among regional populations of wildlife.
We sat down with our experts who co-authored the study, Myles Traphagen, Borderlands Program Coordinator, Aaron Facka, Senior Wildlife Biologist, and Michael Dax, Western Program Director, to understand what these findings mean for the Borderlands and what next steps are needed to reconnect this divided land.
“We need to facilitate wildlife access to food and water and the ability to move under changing conditions. It is our responsibility as good stewards of the land and its inhabitants.” - Myles Traphagen
1. This study reveals an 86% decrease in wildlife crossings due to the U.S.-Mexico border wall. What does this reduction mean for the long-term health of wildlife populations in the region?
Myles: The Borderlands is an inherently arid landscape, with scarce water resources often separated by vast distances. In times of more frequent drought and increasing disturbance like wildfires, animals need more space, not less, to survive. I am concerned about regional die-offs among larger animals, like black bears, deer, and mountain lions if they are not able to access their historic water and food sources because they happen to be stuck on the wrong side of the wall.
Those are potentially the immediate and near-term impacts, but the long-term effects could be far worse regarding genetic diversity and evolutionary processes.
Never before in the history of North America has a barrier existed that blocks migration on a continental scale.
A female deer and her fawns unable to cross through the border wall. Video by Sky Island Alliance/Wildlands Network
Map of border barriers and different wall construction types, including the bollard-style steel border wall and vehicle barriers. Map by Myles Traphagen
2. How was the study conducted?
Aaron: We conducted the study by collecting short videos through remote (trail) cameras to monitor multiple locations along the border wall between August 2022 and July 2024. We chose locations where we could observe different types of wall construction, including the border wall, vehicle barriers, and small wildlife passageways cut into the wall. We then placed each camera close enough to the wall to observe animals and their interactions with the wall.
After routinely collecting the video data from the cameras, our partners at Sky Island Alliance categorized each animal’s interaction with the wall into: 1) no interaction, 2) some interaction that did not result in the animals moving through the wall (or other construction types), and 3) animals successfully moving through the wall. We then could quantify what proportion of all these observations resulted in successful crossing through the wall.
We then calculated an average proportion by species and construction type which is the backbone of our current results.
Graphic by Myles Traphagen
Key study species. Photos by Sky Island Alliance/Wildlands Network
Myles Traphagen installing a wildlife camera to monitor the border wall. Photo by John Kurc
3. What are wildlife openings in the border wall? How have small wildlife passages facilitated movement for different species, and what infrastructure changes are needed to better support connectivity?
Michael: Dedicated wildlife openings that currently exist are 8” x 11” passages at the base of the bollard wall. In the 100 miles of border wall and vehicle barrier that we studied, we only documented 13 of these small openings. Despite the small size and volume, we found that these openings have been utilized by a range of smaller species like bobcats, coyotes, and javelinas. In certain sections, the border wall also includes larger floodgates that are opened seasonally during periods of heavy rain, which allows for the passage of larger species like deer and black bear.
To support wildlife connectivity, we need more frequent and larger openings. We also need to ensure floodgates are open as much as possible to maximize wildlife’s ability to locate and use them.
Thanks to a small wildlife passage, a female mountain lion and her three cubs successfully crossing the border wall. Video by Sky Island Alliance/Wildlands Network
However, large animals like this black bear - unable to use the wildlife passage - attempt to cross the border wall without success. Photo by Sky Island Alliance/Wildlands Network
Our study area and metrics.
4. What additional scientific insights have we gained from monitoring wildlife movements in the study area?
Aaron: We also evaluated small differences in gap sizes between the wall’s panels to see at what point certain species could and could not cross. For example, when we measured the javelina crossing rate against the average size of each gap, javelinas couldn’t cross when the gap was 11 centimeters wide or smaller. However, once the gap got to 13 centimeters, the crossing rate starts to increase significantly. This demonstrates that even a two-centimeter difference in the size of an opening can increase connectivity.
The average crossing rate for javelinas relative to the size of gaps between bollard-style steel slates. Photo by Sky Island Alliance/Wildlands Network
A javelina unsuccessfully attempts to cross the border wall through a 13-centimeter gap between steel slates. Even subtle changes in gap distance could make the wall much more permeable for small-to-medium wildlife. Video by Sky Island Alliance/Wildlands Network
In addition to the information we report in the paper, we also documented other animals including small mammals (like rodents), birds, and even some lizards and snakes. These show us what types of animals exist in different places and their relative abundance. We can compare this data to other locations or to the same location in the future. We also got a sense of the times of day and year (or seasons) that different types of animals are most easily detected which may indicate seasonal patterns of movement, habitat preferences, or even predator-prey dynamics.
5. What are the policy implications of these results?
Michael: While the border wall has done little to deter human migration, these results suggest that it has impacted wildlife movement patterns significantly both for large mammals as well as almost every species in areas where small openings have not been installed. While we would prefer more nuanced border security strategies be prioritized, we expect a renewed push to build more wall with the incoming Trump Administration.
With that in mind, our results indicate that a range of species can effectively utilize the small openings. We would encourage the Department of Homeland Security to include as many of these as possible in any new sections of the border wall.
Myles Traphagen and our partner at Sky Islands Alliance, Eamon Harrity, measuring a small wildlife passage at the border wall. Photo by Emily Burns
6. What gaps remain to be studied?
Aaron: The gaps in our knowledge are also still quite large. We do not yet know if differences within the same species are important for their ability to move through the wall. For example, while females and juveniles can move more freely through small wildlife passages, are large male mountain lions able to cross?
We also have not yet documented important species, including pronghorn, bighorn sheep, wolves, or jaguars. These species may have different or more magnified responses to the border wall or small wildlife passages.
Additionally, how does other infrastructure or human activity affect how wildlife responds to the border wall? The presence and associated traffic of roads near the border structures or lights that illuminate the wall could impact wildlife movement as well.
In many ways, our work on the interactions between border structures and wildlife is just beginning. With this information, we’re working to identify and maintain key corridors and habitats to reconnect landscapes and allow for continued connectivity across the border wall.
Construction of the border wall. Photo by Myles Traphagen