Buenos Aires: The Grassland Boundary

The Sonoran Desert remains a surprisingly fragile ecosystem and has repeatedly shifted its boundaries in response to climate changes and other influences. In wet periods, grasslands expand into desert regions. During droughts and climate shifts, the grasslands retreat and the desert expands. Even insects and animals can affect the boundary between desert and grassland. Creatures whose presence can determine the boundary between two distinct habitats are called keystone species. For instance, kangaroo rats are so efficient at gathering up seeds and so well adapted to desert conditions that their industrious seed-gathering determines the boundary between grassland and desert. Harvester ants, which gather up huge quantities of seed, have a similar impact.

Human beings remain the most dramatic of the keystone species, since their activities have a huge impact on ecosystems and now even the climatic patterns of the entire planet. One of the best places to glimpse the impact of human beings on deserts and grasslands and the boundary between them lies in southern Arizona near the border with Mexico where a nature preserve protects a frail fragment of grasslands that once covered much of southeastern Arizona, but which now has largely become the Sonoran Desert.

The grass-graced Buenos Aires Wildlife Refuge, just 60 miles from Tucson, was established to salvage the masked bobwhite quail, pronghorn antelope, and a host of other grassland-dependent species. This refuge is nearly all that remains of a once sprawling grassland ecosystem that was consumed by a century of intense grazing, invading grasses, and smothered wildfires. The refuge lies in the long shadow of Baboquivari, which the Tohono O'odham believe forms the broken umbilical cord between Heaven and Earth where they believe the Creator, I'itoi, still lives. The refuge now harbors pronghorns, four sorts of quail, and a species count that includes 320 birds, 58 mammals, 42 reptiles, 11 amphibians, and more than 600 plants.

The plight of the once-common masked bobwhite quail demonstrates the human impact on the environment. The bobwhite, a droll, brick-red flurry of feathers in the waving, poppy-spattered grass, still holds out in portions of the Sonoran Desert in Mexico and on the Buenos Aires refuge. Keeping them company are the pronghorns, which the Apaches called “One Who Is Becoming” because its ghost face and unearthly speed made it seem part spirit, part animal.

The refuge runs up the Altar Valley, where giant ground sloths, camels, huge bison, horses, monster beavers, mammoths, and saber-toothed tigers wandered during the last Ice Age. Prehistoric hunters and then the Hohokam left their traces in the valley. The Apache arrived in the 1500s and 1600s to hunt the antelope, driving the farming-based Tohono O'odham back toward the fastness of Baboquivari.

Once the army subdued the Apache, the number of cattle in the southeastern grasslands rose from around 5,000 in 1870 to 1.5 million in 1990. Periodic droughts prompted the starving cattle to eat everything in sight, dramatically altering the grasslands. Droughts in the 1890s, 1920s, and 1950s left piles of cattle bones and converted most areas of the rolling grasslands into mesquite-dominated desert. Instead of a broad, grass-nourishing flow across the valley, storms scoured out a 20-foot-deep (6.9 m), 1,400-foot-wide (426.7 m), 40-mile-long (64.4 km) arroyo down the middle of the valley.

In the past 18 years in the refuge, the exclusion of cattle and the restoration of grassfires have dramatically increased native grasses, slowed erosion, and boosted key grassland species. However, the efforts to bring back both the pronghorn and the bobwhite quail illustrate the difficulty of putting Humpty Dumpty back together.

The refuge remains the only place in North America where four species of quail live, including Gambel's, scaled, Montezuma, and masked bobwhite. The masked bobwhite remains the most heavily dependent on thick, continuous grasslands for its survival, perhaps because it eats smaller seeds, feeds its young exclusively on summer grasshoppers, and relies on freezing into invisibility in the deep grass when threatened.

Despite the return of the grassland, the reintroduced bobwhites still struggle for lack of survival skills, since they do not have wild-smart parents to teach them how to survive sudden storms, sporadic freezes, intermittent droughts, hungry coyotes, patrolling hawks, and slithering snakes. Although the refuge's captive breeding program has released 22,000 captive-bred birds, the refuge's bobwhite population hovers at only 200 to 300 and has been dwindling in the face of the ongoing drought. Quail captured in Mexico and released on the refuge do better than the captive-reared birds, but overgrazing and drought in Mexico have nearly wiped out the wild populations there too.

Efforts to reintroduce the pronghorn have also struggled. Ice Age survivors, the pronghorns rely on binocular vision and spectacular speed. With enormous eyes that give them vision equal to binoculars with a wide field of vision, pronghorn can spot an approaching threat from four miles off. They can run for long distances at 40 miles an hour and sprint at 60 miles per hour by taking 27-foot (8-m) strides and landing on exquisitely evolved pads of cartilage. They burn oxygen three times as fast as most animals, thanks to a huge heart and a windpipe more than twice as big as a human's. Although they resemble African antelope, they are actually their own separate species, with feet like a goat and unique pronged horns that shed their sheath each year.

They thrived for 40 million years before Europeans arrived. But longrange rifles enabled hunters to slaughter them by the wagonload. At one point in the late 1800s, hunters sold miners four pronghorns for a quarter. A population estimated in the tens of millions plunged to 30,000 in 1920, including 650 in Arizona. Fortunately, nationwide conservation efforts have boosted the population back to 1 million pronghorn nationally and 60 at Buenos Aires.