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WETLANDS GRAZING POOLS?
It can be hard to find a kind word for grazing livestock among environmentalists. But a study by Nature Conservancy ecologist Jaymee Marty, published two years ago in Conservation Biology, suggests that, at least in the case of Northern Sacramento Valley vernal pools, grazing may actually promote biodiversity.
Marty did her research on the Howard Ranch in Sacramento County, privately owned but with conservation easements protecting its vernal pools-seasonal wetlands that are home to unique annual plants and aquatic animals. The new easement holder, assuming that grazing is always bad for vernal pools, was about to fence out the cattle when Marty suggested an experiment.
She set up four different grazing regimes- continuous, wet-season, and dry-season grazing, plus ungrazed-in the ranch's vernal pool areas. For three years she measured native versus exotic plant cover, native plant species richness, and aquatic invertebrate biodiversity. The results: native plant cover was consistently higher in the continuous-grazing treatment, by 20 to 47 percent.
Exotic annual grass cover increased by up to 88 percent in the ungrazed pools, and a quarter of the native plant species in their edge and upland zones were lost. By the experiment's third year, ungrazed pools had the lowest invertebrate taxa richness. "I was surprised that the results were as clear as they were," Marty says. "Typically you don't see effects like I did in such a short period of time."
Marty's findings were no surprise to some vernal pool advocates, including vernal pool expert Carol Witham. "I've seen what happens when cattle are excluded, and it isn't pretty," she says. Case in point: the Vina Plains preserve near Corning, where grazing stopped when The Nature Conservancy acquired the land. "Three years later it was nothing but weeds," she recalls, "while the private hunting club across the road was solid wildflowers." At Jepson Prairie, grazing exclosures have "not a single native species growing in them."
Why would grazing make such a dramatic difference? Marty found that ungrazed pools dried down faster, good news for exotic grasses but bad news for fairy shrimp, tiger salamanders, and spadefoot toads. "Vernal pools are a miniwatershed," Marty explains. "There's a lot of water entering from the soil in the uplands. If exotic grasses are abundant, they'll be sucking up more water from the soil." The hooves of grazers may also keep the soil more compacted so that it holds more water. Vernal pool ecosystems evolved with the Central Valley's great herds of pronghorn and tule elk, and the Pleistocene megafauna before them. Today, cattle may be the best available proxies.
According to Marty, reaction to the Conservation Biology paper was mostly positive, with some land managers telling her she had just quantified the obvious. She acknowledges that what holds for Sacramento Valley vernal pools may not for other ecosystems: "Other systems such as riparian are very different. I wouldn't say that grazing is always good; it needs to be well managed."
Marty hopes others will try to replicate the Howard Ranch experiments, and Witham mentions research in progress at Dales Lake and elsewhere. For now, Marty's results have helped vernal pool advocates and cattlemen find common ground in the Rangeland Conservation Coalition, whose Rangeland Resolution is supported by groups from the California Native Plant Society and Defenders of Wildlife to the California Farm Bureau Federation. "In order to save the vernal pool species, you have to make ranching economically viable," Witham argues. If the ranchers can't make it, the developers are waiting.
CONTACT: Jaymee Marty, jmarty@TNC.ORG; Carol Witham, cwitham@ncal.net. JE
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