Gaylord Mink has put together a short documentary on the history of horses in the the Horse Heaven Hills (When Wild Horses Roamed).
A very simplified order of events and impacts could be thought through that was covered by the film:
1) Pre horse the Horse Heaven Hills would have been grazed by a combination of pronghorn, bison, elk, deer and big horn sheep.
2) After the arrival of the horse, hunting by First Nations peoples likely ended the bison and pronghorn presence in the Horse Heaven Hills as well as other areas of eastern Washington and greatly diminished the elk and deer populations.
3) The horse rapidly became the dominant grazer in the Horse Heavens.
4) With few predators and rapidly increasing numbers combined with new invasive weeds, the ecosystem of the Horse Heaven scrub steppe would have changed rapidly.
5) With the arrival of farming with cattle, sheep and dry land wheat and economic shifts as well as petroleum powered machines huge round ups and slaughters greatly reduced and in most areas eliminated the wild horse population. Horses were pushed out into the more remote western Horse Heaven Hills.
6) Population of wild horses slowly rebounds and grows.
Thinking about the above process might inform how the remaining scrub steppe lands of the Horse Heavens will respond over time.
The wild horse population in the western Horse Heavens has been expanding rapidly and the range land they live on is being altered and likely is reaching its limits to support the horse population that is there. With farm fields in the valleys and to the east the range can not be expanded. Wild horses are now appearing with greater frequency along Highway 97 in the Horse Heavens on an increasing basis. Hence a very real challenge for the Yakima nation on how to manage the resources of the western Horse Heaven Hills.
But beyond the current wild horse range the change of grazing will and likely already has had an impact elsewhere in the Horse Heavens. Some limited areas still have open range cattle grazing on a seasonal basis (the range land both private and pubic is much better managed than it used to be). Other areas have no grazing of farm animals, but were grazed not that long ago. Isolated pockets have not been grazed and without a dominant grazer may be evolving in a markedly different manner. Past overgrazing combined with invasive grasses and weeds, particularly cheat grass, has greatly altered the intensity and frequency of wild fires in some areas. The presence of farm fields and roads has reduced the fire frequency in other areas thus allowing plants that are fire sensitive to become predominant at the expense of fire tolerant species. The Yakima Nation recently reintroduced pronghorns to the western Horse Heaven Hills.
All in all and interesting mosaic of changing land use and habitats across the Horse Heaven Hills despite the relatively uniform climate. The Horse Heaven Hills are a wide open, big sky kind of place, but on close look there is a lot of detail and change taking place.
View of the lower Yakima River valley from the Horse Heaven Hills
The above picture was taken a few days ago (nice to be out of foggy western Washington). This particular hill slope is on BLM managed land and is covered by bunch grass. It was formerly very heavily overgrazed, but has recovered with native grasses being the predominant species, but has a remarkable lack of scrub plants at least currently.