Outlook Dims For Giant Saguaro; Invasive Grass, Failing Monsoons Cut Survival, Repro Rates [View all]
Native to Africa, buffelgrass was imported to Arizona for cattle forage and erosion control in the 1940s. Like many such imports, which seemed like a good idea at the time, this one has gone out of control (the classic example is kudzu, which is in the process of swallowing the Southeast). Approximately 2,000 acres of the park are currently covered with buffelgrass, and can spreading at a rate up to 35 percent per year. It has the potential, Stonum said, to turn the expanses of Saguaro-dominated landscape into grassland.
“What it does,” Stonum said, “is out-compete the native grasses, the native plants, and the cactus, and it literally excludes all the new plants.” This is a problem for creatures like the desert tortoise that feeds on native plants. Young tortoises may be somewhat protected from predators by the buffelgrass, but by the time tortoises are big enough to reproduce, the decrease in native plants can make it harder for the tortoises to do so. This reduces the tortoise population overall.
Compounding the problem, he explained, is that buffelgrass thrives on fire. The Sonoran desert ecosystem where the park lies is dotted with vegetation, but with significant gaps where there’s nothing keep a fire going. Fires may start — Tucson is a lightning hot spot in the U.S. — but the gaps keep them from burning out of control. Where the buffelgrass fills in, fire can spread more easily and burn hotter. The roots of the grass survive, so it grows back easily. Saguaros, which are lucky to generate a single mature offspring from tens of millions of seeds, can’t. “They melt and die,” said Stonum.
Worrisome as the increased danger from fire is, the Saguaro and other park species are threatened by changing weather patterns as well. Currently, the region’s bone-dry conditions are interrupted by two intensely rainy seasons: one in winter and one in summer. “This is really advantageous to the cacti,” Fisher said. “They’ve adapted to have their flower season, their fruit season, and their seeds timed just perfectly to the monsoon rains.” Climate modeling studies by the Southwest Climate Change Network project that in the next century, southern Arizona’s climate could see a 10 percent decline in precipitation.