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Features

Restoring the Sonoran pronghorn

Wildlife

At the US-Mexico border in southwestern Arizona, the old Peligroso/Danger signs dangling from the barbed wire facing Mexico do little to stop a furtive flood of foot traffic through the desert, despite its unforgiving conditions. Here, 14 undocumented Mexican immigrants perished in May 2001.

While humans are ill-equipped for the harsh conditions of the Sonoran desert, the endangered Sonoran pronghorn may be even less equipped for the widespread consequences of human activity in a region where moisture is already a rare commodity. In conjunction with range fragmentation and habitat degradation, recent extended periods of low rainfall during hot summer months are presenting serious problems for the Sonoran pronghorn, which was listed by the Federal government as an endangered species in 1967.

A goatlike animal often mistakenly called an antelope, the Sonoran pronghorn is one of five subspecies of Antilocapra americana, the only species in the Antilocapridae family. As much an icon of the Sonoran Desert as the buffalo was of the prairie, thousands of Sonoran pronghorn likely once graced the landscape in bands of 25 or so, roaming like caravans across vast expanses of the North American desert.

Because of hunting in the early part of the 20th century, along with livestock overgrazing, new diseases introduced through cattle, and ever-increasing habitat fragmentation, the subspecies now numbers less than 500. There are three isolated populations: two in Mexico and one confined to federal lands in the United States, including Cabeza Prieta National Wildlife Refuge in Arizona, where US Fish and Wildlife Service biologist John Morgart tracks and monitors the herd.

"It's life versus death out here," Morgart said. A glimpse through his high-powered binoculars confirms this. The only perceivable movement in a wide desert valley is that of two rival buzzards poking for morsels at the underside of a sun-bleached skeleton. Cicadas buzz in erratic drumbeats.

Those who claim this vast Sonoran Desert arena as home - the turkey vulture, the desert bighorn sheep, the coyote, the desert tortoise, the saguaro cactus, and the Sonoran pronghorn - have evolved over time to survive under notoriously austere conditions. Traveling long distances in response to rainfall across a landscape teeming with hungry predators, the pronghorn has two distinctive survival strategies: great speed and a pair of enormous eyes positioned for a wide-ranging view.

All three populations of Sonoran pronghorn contend with roads, fencing, and railroad tracks. Border fencing and Mexico's Highway 2, which runs alongside it, have divided the US population of pronghorn from Mexico's northernmost population on the El Pinacate Biosphere Reserve. Further south, the largest population of some 311 individuals - more than 60 percent of the entire population - is isolated by the Gulf of California on one side and Mexico's Highway 8 on the other.

Border-dwelling pronghorn are challenged by migrant and drug traffic. Foot traffic means disruptive, if temporary, human presence on pronghorn stomping grounds.

Still more menacing are the makeshift roads that litter both sides of the border. John Hervert, a wildlife program manager for the Arizona Game and Fish Department, has observed some of the more subtle, but long-lasting deterioration caused by the network of illegal roads. "On more heavily used roads, the hydrology is being altered to the detriment of plants," he said. "On first glance, you can see how a road crushes plants or cuts through the natural flow of vegetation. But even worse is what you cannot see right away. The movement of water in slightly sloping desert valleys is very slow, and heavily used roads will effectively divert moisture away from lower level vegetation." In short, pronghorn forage dies where roads make incisions across the land.

Overgrazing has also taken a toll on native vegetation, particularly in the El Pinacate Biosphere Reserve, where hungry domestic livestock deplete the greenery and make the inland soils especially vulnerable to erosion. Much of the native vegetation that pronghorn graze, such as dune bursage, mistletoe, and mesquite leaves, is fading away at an accelerated pace, giving way to parched earth and scrub.

"We suspect that livestock grazing can significantly alter the equilibrium of the plant community, evidence of which exists on both sides of the border," said Hervert. "The dominance of creosote in certain areas is a good example of how the relative balance in the native plant ecology has been upset."

To Hervert, an overabundance of creosote is a reliable indicator that a desert ecosystem is in disrepair. A sturdy native desert shrub that provides forage for neither cattle nor pronghorn, creosote can out-compete neighboring palatable vegetation. By degrees, patches of the shrub will fan out, grow taller, and dominate an area. Ultimately, a landscape of thick, inedible vegetation is unattractive to an animal like the pronghorn, who prefers open vistas where it can use its extraordinary vision to spot predators while it forages.

In fact, wherever native habitat has been altered in its current, fragmented range, the Sonoran pronghorn suffers. And each injury is made worse by the recent spate of dry seasons. Morgart said that with the extreme hot and dry conditions of the past several years, "border fencing and other obstacles are severe deterrents for an animal in search of nutritious forage and water." Worse still, added Morgart, the harsh natural pattern of drought-like conditions in the Sonoran Desert is "possibly exacerbated by global warming."

Even desert critters need water for survival. Pronghorn typically do not drink water when the moisture content of their regular forage is at an adequate level. Although adults can retrieve moisture from a variety of plants, growth of their preferred nutrient-and-moisture-rich forage coincides with the rainfall the animals would instinctively follow were it not for the barrage of obstacles throughout their range.

The drought has significantly diminished the animals' success at nurturing young. The better forage a mother can access, the more nutrients she can divert to her fetus. And after birth, the mother is better equipped to provide nutritious milk during the critical nursing stage. If malnourished, a fawn is likely to die. Because the pronghorn's life span is generally short, between 10 and 12 years, the time it has to reproduce is precious.

Is hope for the pronghorn as diminished as the recent rainfall? "No," said Morgart. "As long as we don't let our guard down."

Morgart heads a collaborative recovery team that includes scientists from both sides of the border. In the United States, the Fish and Wildlife Service, the Arizona Game and Fish Department, Organ Pipe Cactus National Monument, the US Bureau of Land Management, the US Air Force, the US Marine Corps, and the University of Arizona are working for the pronghorn. Recovery team members from Mexico include the El Pinacate Biosphere Reserve and the Instituto del Medio Ambiente y el Desarrollo Sustentable de Estatio de Sonora. Together the team's ultimate goals are to increase Sonoran pronghorn numbers and to improve and expand their current range.

Experimental techniques are also part of the process. For example, John Hervert and his colleagues from the Arizona Game and Fish Department have been hauling water tubs to remote areas on the wildlife refuge where they have tracked pronghorn activity. The four-mile hikes with five-gallon jugs of water in 105-degree temperatures is proof of their dedication; the fact that the animals drink the water is proof of their desperation.

The biologists have affixed cameras to snap pictures of activity at water tubs to gather information that may help make the process more effective, and are charting the survival rate of fawns that have access to the water tubs.

The adjacent Barry M. Goldwater Range, a military training ground used by the Air Force, plans to fund a forage-enhancement project on its land. Already the military branch is negotiating with the Bureau of Reclamation to drill two test wells as a source of water for the forage-enhancement project. Depending on the well drilling results, site preparation for the project was scheduled to begin as early as June 2002. By clearing creosote and supplying moisture during below-average rainfall, biologists hope to increase forage. If these efforts achieve the desired results, other partners may initiate similar projects throughout pronghorn range.

The recovery team has proposed several additional action items in a recent Supplement and Amendment to the 1998 Final Revised Sonoran Pronghorn Recovery Plan. The team believes that its comprehensive documentation will provide the guidance necessary to increase pronghorn survival and improve habitat. "It may be a long, hard road to recovery ahead," said Morgart, "but the shorter road leads only to extinction."

To stave off the possibility of extinction, the team is discussing the possibility of establishing a captive breeding program. "We hope it doesn't come to that," said Morgart. "We hope that conditions will improve enough for pronghorn to re-establish their numbers on their own."

Only time and the weather can determine if conditions will improve. The winter months of 2000 and 2001 fortunately provided more precipitation than in preceding years, and recent surveys conducted by Arizona Game and Fish reveal that, as a consequence, a significant number of fawns were born into the US population. The estimated ratio of fawns to does reveals the highest productivity ever recorded for Sonoran pronghorn. Up from an estimated total US population of 99 individuals in 2000, US pronghorn now number approximately 140, according to biologists.

But Morgart remains cautiously circumspect. "We still can't afford to assume that things are good," he said. "This past winter has been extremely dry. As of March, does have been dropping [giving birth to] fawns. If there is not an adequate amount of rainfall to allow for some green-up, followed by an early and widespread monsoon, these fawns are probably all going to die."

The Sonoran desert is a primordial stage for an archetypal drama; it is, as Morgart said, life versus death here. But modern circumstances threaten to destroy the players completely. Over a relatively short period, human presence has disturbed patterns of evolution, patterns that developed to afford a fighting chance to the natural inhabitants of a harsh land. Pronghorn recovery efforts represent a small step towards balance to an equation that tilts towards death.

Ben Ikenson is a freelance writer and a wildlife biologist with the United States Fish and Wildlife Service.

   

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