A group of biologists in the northern Rockies published a paper in 2018, “A fence runs through it: A call for greater attention to the influence of fences on wildlife and ecosystems.” In 2020, a meta-analysis in BioScience looked at all the studies of the effects of fences and found that their profound impacts are often ignored or greatly underestimated. The impacts extend far beyond blocking animal migration routes and include furthering disease transmission by concentrating animals, altering the hunting practices of predators and impeding access to key areas of water and forage. Fences are going up rapidly as border barriers and livestock farming increase.

In the case of the U.S./Mexico border wall intended to prevent illegal immigration, “The main threat ... is the landscape-level impacts of curtailing or completely precluding wildlife movement and eliminating landscape connectivity at large scales,” says Aaron Flesch, a wildlife biologist at the University of Arizona, who has studied the wall’s impacts. Bighorn sheep and jaguars are cut off from their kind on the other side of the border. That means that the genetic interaction needed to keep small populations of jaguars or ocelots healthy may be affected. It also means bighorn sheep in Mexico may not be able to migrate north to escape a hotter and drier climate.