Boom, Boom on the Range

Where endangered Sonoran pronghorn antelope play under the roar of U.S. Air Force jets on live-fire bombing exercises

There are, however, three areas on the range where live ordnance--including 1,000-pound bombs and Maverick air-to-ground missiles--is used. It is in one of these areas, at a place called High Explosive Hill, where John Hervert and Laura Thompson-Olais, a Fish and Wildlife Service ecologist, noticed pronghorn were congregating.

"During our telemetry flights of the spring and early summer of '95," Hervert says, "we saw some pronghorn near H.E. Hill, and we pondered why the pronghorn were there, because they seemed to be there in frequency greater than random groupings of animals.

"During one of those flights, we spotted the water hole."
The water hole actually is a massive bomb crater, 30 feet across, blasted into an area where rainwater collects. An underlying layer of clay keeps the water from seeping away. The crater is deep enough--Hervert says the water has been as deep as seven or eight feet--to retain water for months at a time.

Although it hasn't rained on the range in weeks, the crater contained water last week when Luke officials took New Times for an inspection of H.E. Hill and the crater.

Water isn't the only thing that lures pronghorns to High Explosive Hill. Soil in the pronghorns' habitat is so hard-baked it's called desert pavement. But explosions at H.E. Hill apparently loosen the soil, allowing seeds that otherwise would dry up and blow away to sprout, producing forage.

The irony that weapons of mass destruction might indirectly nurture an endangered species is not lost on those charged with its recovery.

But neither is the threat to that species. It doesn't take a Ph.D. in biology to recognize that endangered species and high explosives don't mix.

Except for two months each year when H.E. Hill is closed for cleanup, the site absorbs 150 to 250 bombs per month, including 1,000-pound bombs, rendering it a pocked moonscape of pulverized, volcanic cinder.

Major James "Jake" Alwell, a flight instructor and No. 2 officer in Luke's Range Management Office, says H.E. Hill teaches pilots the dangers of friendly fire. He says shrapnel from a 1,000-pounder can fly 2,500 feet from the point of impact, so pilots are trained to keep their distance and time their own bomb drops so as to avoid the explosive "bubble" left by a preceding warplane.

Standing more than a quarter-mile from H.E. Hill last week, Alwell picked several footlong fragments of bomb casings that blanketed the ground, held them up and explained that such shrapnel could easily bring down an F-16.

Or, presumably, an antelope.

Biologists charged with preserving an endangered animal such as the Sonoran pronghorn face a conundrum. The fact that there are so few of the species makes it risky, at best, to capture and collar them, or even observe them closely.

Yet without scientific study, it is difficult to know whether--and why--their numbers are increasing or decreasing.

A dearth of data produces disagreement over the most basic pronghorn facts. One biologist might say the species is on the verge of extinction, for example, while another may postulate that since nobody has ever known the true population, it cannot be said with scientific certainty that their numbers have declined.

Consequently, one of the most pressing tasks facing the biologists charged with recovery of the Sonoran pronghorn (to a "sustainable population" of 300 in the U.S.) is getting a good idea how many of them exist now. This is done painstakingly, if not precisely, by aerial survey, with spotters in single-engine planes flying a grid pattern over their massive range.

John Hervert and U.S. Fish and Wildlife Service officials also take weekend flights over the range--when no warplanes are about--to track movements of radio-collared Sonoran pronghorn.

The species has been listed as endangered since 1967, yet has shown no apparent recovery in nearly 30 years. Much remains to be learned, but here is some of what is known about the enigmatic ungulate:

Its Latin name is Antilocapra americana sonoriensis, one of five subspecies of Antilocapra, all of which live in North America. Other subspecies of Antilocapra exist in Arizona (americana americana and americana mexicana), but they are not endangered.

Travelers on Camino del Diablo, the treacherous frontier trail that linked Caborca, Mexico, and Yuma, told of seeing hundreds, even thousands, of pronghorn en route. During a border survey conducted in the 1890s, Sonoran pronghorn were observed in every valley between Nogales and Yuma. Old-timers recall ranchers chasing Sonoran pronghorn into pens, then challenging their greenest wranglers to lasso them.

Today, the Sonoran pronghorn's territory is much smaller and more fragmented than it once was. Its current range is bounded roughly by Interstate 8 on the north, Caborca on the south, the Lechugilla Desert on the west and U.S. Route 85 on the east.

Two former free-flowing sources of water and accompanying forage--the Gila River to the north and Rio Sonoyta across the border in Sonora--now flow intermittently due to dams and diversions for agriculture.

But the drying of those rivers and the decimation of their riparian areas do not necessarily sound a death knell for the Sonoran pronghorn. As deserts go, the Sonoran is capable of supporting a veritable riot of flora and fauna, because it has two rainy seasons--the intense summer monsoons and, in normal years, more sedate winter showers. The two rainy periods mean that as some perennial plants begin to die off, others begin to sprout. Still, average annual rainfall on the gunnery range is a paltry five inches.

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