In the hills of San Mateo County, a special band of horses became part of a pilot project in grassland management for the Peninsula Open Space Trust. With 20 acres of overgrown property near Pescadero to manage, POST needed the four-legged “experts” to do what they do best: eat like horses.

“These animals eat 18 hours a day,” said Sam Harpur, founder of Rediscovering Horses, a nonprofit organization near Santa Cruz that gives unwanted horses another chance. “Instead of using tractors or burns to manage the land, we thought: Why not bring the horses to the land, for the benefit of each?”

The herd was a mixed lot — former racehorses, breeding horses, and show horses — all acquired after each somehow “failed” to serve their previous owner, although Harpur would say it was the other way around. For 12 weeks, the trial partnership with POST gave the animals plenty of room for “forage, friendship, and freedom,” three basic needs of healthy horses, according to Harpur. In turn, the happy grazers pared down the plant growth and the threat of wildfire.

“It’s been a pretty good relationship,” said POST’s stewardship assistant, Taylor Jang, of the fledgling partnership. “Historically, horses were kept in Burns Valley, so the project was a nice continuation of that, and we got vegetation management.”

With a hospitable Mediterranean climate zone, the San Francisco Bay Area is one of the nation’s top hotspots for grassland biodiversity. At the same time, California’s native grasslands rank among the most endangered ecosystems in the country. With only about 20 percent of those grasslands on public lands, good stewardship is key to their survival.

Not so long ago, livestock were considered the bane of healthy grasslands — too many mouths might denude the hillsides while too many hooves could trample habitats. But, more recently, there’s been a push to reintroduce cattle, horses, sheep, and goats to open spaces, said Jang. Although teaming up with Rediscovering Horses was a new project, POST uses “conservation grazing” on several other properties, including Driscoll Ranch in La Honda. They aren’t the only ones to use grazing as a management tool; among other sites, the Midpeninsula Regional Open Space District recently returned cattle to Mindego Ranch, a 1,000-plus acre property located on the coastal side of Skyline Ridge.

More than half of the 117,000 acres of land owned by the East Bay Regional Park District is managed with grazing animals. An additional 700 acres use goats as part of the district fire department’s fuel break program. “We could not manage this large an area of grassland with our existing staff,” said Denise Defreese, the wildland vegetation program manager for EBRPD.

Forty herd owners lease EBRPD land for feeding their animals; almost all graze cattle, although two still use sheep and horses. “It’s like a prescription,” said Defreese. “For each property, you look at what you want to accomplish and then try to determine the best stock to use.”

Management goals might include promoting native purple needlegrass or reducing yellow starthistle, an aggressively spreading weed that is toxic to horses, but readily eaten by cows, sheep, and goats.

When land managers choose which animals to use for grazing, they take into account the attributes and preferences of each species. Cows, for instance, only have teeth on their lower jaws so they can’t eat the tough woody stuff that serves as fodder for horses and goats. They also prefer to keep their feet on level ground, noted Defreese. Sheep are ‘intermediate grazers’ that eat grasses and also browse broad-leafed plants such as white clover. Goats, with nimble feet and a full set of teeth, are good on steep terrain and browse a variety of forage. But — myth buster — goats won’t eat everything.

“They are much pickier than people expect,” said Genevieve Church, the general manager for City Grazing, a San Francisco-based company that rents out goats to control vegetation in areas of two acres or less. Goats turn up their noses at oleander or boxwood, which are toxic to them, she said. However, the small ruminants have hearty appetites for blackberry brambles, dense “coyote brush,” and even poison oak (although only during part of the year); such tastes make the goats suitable for clearing everything from public parks to hidden city yards.

Sometimes, two species work better than one. In Pleasanton Ridge Regional Park, Defreese doubled down on reducing an invasive weed called “medusa head” by letting a herd of sheep eat their fill for nine days, and then putting cattle on the same plot a month later. She has to wait until spring, when the medusa head comes up again, before she’ll know if those efforts paid off. “But that’s the cool stuff you can do with livestock,” she said. “Otherwise the treatments would be burning, mechanical, or herbicides.”

With targeted approaches, grazing not only helps weed out non-native grasses and promotes native plant growth, it can even save habitats for endangered species. In one classic example, cattle on Coyote Ridge, near San Jose, helped rebound the declining Bay checkerspot butterfly population. By grazing down invasive Italian ryegrasses, the cattle made room to grow for a native plant that provides food for the butterfly.

Just keeping grasses short helps maintain habitats for other species. Well-grazed lands tend to have higher populations of ground squirrels, a common critter whose underground complexes may be used by Western burrowing owls, native California tiger salamanders, and other animals. As a side benefit, the man-made “stock ponds” that supply cattle with water also make good breeding spots for the California red-legged frog, which is listed as threatened under the U.S. Endangered Species Act.

“Right after the total conversion of land to some other use, one of the biggest threats to grassland endangered species isn’t grazing — it’s the removal of grazing,” said Lawrence Ford, a conservation land management consultant and research associate in environmental studies at UC Santa Cruz.

Left alone, native grasses and habitats are lost as the grassland rapidly undergoes succession to shrubland, then woodland. From Ford’s perspective, the traditional year-round, extensive grazing practiced by “grandfather and grandmother” was the most effective method of management. “It’s what gave us the hotspot of biodiversity that we enjoy today,” he said.

But these days, grasslands are managed more intensively to bolster an entire ecosystem, not just livestock production. And grazing isn’t a panacea; hoof stock can also gobble up native plants, spread the seeds of invasives, and negatively impact soil erosion and water quality.

“Now we need to take ‘grandpa’s’ grazing strategies and shave off the rough spots,” Ford said. “Let’s remove the really egregious practices like damage to riparian areas, damage to water quality, over-concentrated grazing areas that destroy open space aesthetics, and find better ways to deal with conflicts between predators, wildlife, and grazers.”

Devising better methods through scientific studies takes time and money — both of which are often in short supply. However, in one new rangeland resilience pilot project, EBRPD is working with the Alameda County Resource Conservation District and the San Francisco Public Utilities Commission to study ways to improve water sources on a cattle grazing site in Sunol. With better distribution of cattle across the landscape, the reduced impact on creeks and riparian areas can improve water quality, according to Defreese.

Elizabeth Devitt covers open space for the Monitor.