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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