Pronghorn (Image from Wikipedia)
I have a clear memory of our family encyclopedia with the a great chart showing animals running speed. Cheetahs were the fastest on the chart with the pronghorn of North America second. Cheetahs can reach 70 mph and pronghorns clock in at 60. But pronghorns can maintain high speeds for long distance and for that I always admired them. The same chart showed that a top human marathoner could out run all animals with one exception - the pronghorn. Being a long distance runner I have always been a great admirer of the pronghorn.
A few years ago while traversing an alluvial fan in Nevada on foot in a snow storm I surprised a pronghorn that was in the incised stream channel. Being within 25 feet of a pronghorn that bolts was a thrill. It put hundreds of feet between us in seconds. After a minute of running it stopped and turned to look back at me. I realized it was at least 3/4 of a mile away! I was no match. Nor would any known predator match the running of a pronghorn.
Significant portions of eastern Washington look a lot like Nevada and eastern Oregon. In fact pronghorn preserves are located in southeastern Oregon and northern Nevada. While we have wild horses like Nevada on the scrub steppe (HERE), there are no pronghorns in Washington State. This question has perplexed me as well as a number of biologists.
Pronghorns used to live in eastern Washington. Lyman (2007) provides an excellent summary of the information that has accumulated regarding pronghorn from archaeological sites. Based on the archaeological record, pronghorn lived in eastern Washington throughout most of the last 10,000 years and were a food source for First Nations people; however, they were never as abundant as elsewhere in their range and eastern Washington was likely marginal habitat with limited connectivity to other pronghorn habitat. Pronghorns appear to have disappeared in eastern Washington prior to the Lewis and Clark Expedition and other explorations into eastern Washington as there are no unambiguous documented pronghorn observations. Bison were similar to pronghorn in that they were formerly in eastern Washington in numbers less than elsewhere in their range.
While hunting likely impacted pronghorn and bison populations, there were plenty of pronghorn and bison east of the Rocky Mountains well through the mid 1800s. Hence, other factors likely played a role besides hunting by First Nations peoples given that northern plains peoples were highly dependent on hunting.
Lyman (2007) and Lyman and Wolverton (2002) suggests a combination of forage quality, migration obstacles and human predation limited pronghorn and bison populations in eastern Washington. Williams (1987) suggests water content of snow played a factor.
Pronghorns prefer forb plants over grass. Grassier areas of eastern Washington hence would limit pronghorn range. In addition, eastern Washington is very dry in the summer so nutrient values, particularly protein in forage are lower during critical summer months while mothers need milk for young. Access to water would also be a limiting factor for pronghorns in eastern Washington. Deep long lasting snow also poses a problem for pronghorns particularly in areas where forb plants are low growing. A deep snow winter is not uncommon in significant parts of eastern Washington and in areas with generally low snow, one bad year would devastate the population.
Migration corridors between eastern Washington and other pronghorn habitat areas is very restricted. The limited scrub steppe and grass lands of eastern Washington are relatively isolated from other pronghorn areas. Pronghorn habitat in eastern Oregon is continuous with vast tracts of similar habitat in Nevada and Idaho and points beyond. But the Oregon range is separated from eastern Washington by the Blue Mountains and other high forested areas between the Blues and the Cascade Ranges with only a relatively narrow area of suitable habitat connecting the two areas. The very large Columbia River and Snake River as well as a few other rivers also present a significant barrier to in migration. The limited migration would play a role if numbers declined due to any number of factors. One very bad winter with deep wet snow even if rare would be take a long time for population recovery due to the limited migration routes.
Regardless of the natural challenges facing pronghorns in eastern Washington, pronghorns were apparently present throughout the past 10,000 years up until just before the first American and fur trading explorers arrived in the early 1800s. The predator-prey balance with humans may well have been tilted against pronghorns when eastern Washington First Nations obtained horses in the 1700s. The fact that pronghorns were present in eastern Washington for 10,000 years until they disappeared when they did certainly implies some change had taken place. Perhaps a marginal population could not survive the added pressure of hunters on horse back.
I would propose another factor to be added to the forage, migration and snow factors - the local geology and topography. Eastern Washington would be full of potential hunting traps to drive pronghorns toward. Large rivers and cliffs in the scab lands would provide excellent drive locations for hunting relative to other areas where humans and pronghorns interacted. Add the use of horses and perhaps the human predation would have been just enough to finish off the already low pronghorn numbers.
Given our current impacts on the landscape, it is extremely unlikely that pronghorns will ever return to eastern Washington on their own. For one thing, despite their amazing running ability they go under or through fences as the are lousy jumpers. Pronghorns were introduced as a game animal to eastern Washington in the early 1900s, but the population did not survive. Some First Nations groups are considering pronghorn introduction. Perhaps portions of The Yakima Nation Reservation would support pronghorns or the Hanford Reach National Wilderness Area in combination with the adjoining Saddle Mountain National Wildlife Area, Hanford Nuclear Reserve and the Yakima Firing Range or the public lands of the Telford scabland area. I for one would love to see pronghorns eyeing the cars crossing through the Hanford area or on the high ridges between Yakima and Ellensburg.
Lyman and Wolverton, 2002, The Late Prehistoric Early Historic Game Sink in the Northwestern United States, Conservation Biology.
Lyman, 2007, The Holocene History of Pronghorn (Antilocapra americana) in Eastern Washington State, Northwest Science.
Williams, 2005, Spatial Precipitation Variability, Snowfall and Historical Bison Occurrence in the Northwestern United States, Anthropology Theses Georgia State University.