North America’s wild horses are the feral descendants of animals brought by Europeans in the past few hundred years. Biologically speaking, this is the blink of an eye, far too short a time for horses to be considered native. More importantly, that’s much too short for native North American plants and animals to adapt to the pressures of coexistence.
Grasslands are protected by “biotic crusts” that consist of loose soil held together by tiny cyanobacteria, lichens, mosses, and green algae. They serve as a fragile glue that keeps desert soils from being washed or blown away. But these crusts are pulverized by horses, leading to poor water absorption, reduced fertility, and long-lasting environmental damage. Grasslands are disappearing as wild horse hooves crush biotic crusts, encouraging erosion that leaves wide swaths permanently degraded, replaced with barren rock.