Bring in the cows

"There they are, the keystone herbivores!" shouts conservation biologist Stuart Weiss. Framed by the front window of his Toyota truck, a small herd of cattle can be seen grazing near the crest of Coyote Ridge, a craggy promontory of serpentine rock that towers over the city of San Jose, Calif., unofficial capital of Silicon Valley. Ten cows and as many calves turn to stare, then go back to cropping grass turned green by winter rains.

No, laughs Weiss, these are not the infamous hooved locusts, tramplers of wildflowers, creators of cowburnt wastelands. These are well-mannered cows that don't overgraze because they are regularly moved from pasture to pasture. As a result, Weiss says, the grass-nibbling they do is beneficial, not harmful. In fact, their presence in these rugged rangelands may well be the only thing standing between an iconic insect, the Bay checkerspot butterfly, and near-certain extinction.

An affable 6-footer who makes his living as an ecological consultant, Weiss has followed the declining fortunes of the Bay checkerspot ever since 1979, when he was a freshman at Stanford University. For the past decade, he has been sounding the alarm about what he now considers the most pernicious threat to the beautiful bug's near-term survival -- the unholy synergy between invasive grasses and urban smog.

Smog, Weiss explains, contains not just carbon dioxide, the gas that drives global warming, but also a cocktail of nitrogen-rich compounds. Swept by the winds onto nearby rangelands, these compounds act like spray-on fertilizer, encouraging the rampant growth of Italian rye, wild oats and soft chess. Left unchecked, these aggressive annuals quickly overrun low-lying native plants, including dwarf plantain, the chief food source for Bay checkerspot caterpillars.

And this, of course, is where the cows come in.

To illustrate the point, Weiss parks his truck and hikes into a meadow the bovines have recently mowed. All around him, wildflowers are in various stages of bloom: California poppies, goldfields, red maids, tidy-tips, desert parsley, wild onion. Best of all is the silvery carpet of dwarf plantain that unfurls underfoot, "the most important plant in the world," Weiss calls it. Sure enough, in the midst of the plantain lies a fuzzy caterpillar, enjoying a postprandial nap.

"Isn't that the California lifestyle for you!" Weiss beams. "First you enjoy a salad of tender young greens, then you bask in the sun!"

Suddenly, the 49-year-old scientist starts to run, following a fluttering in the air, and just as suddenly he skids to a stop. In front of him, on a lichen-speckled rock, an adult Bay checkerspot spreads its wings to reveal a mosaic of orange and white set off by vivid black. Soon, thousands more like it will emerge from their tent-like chrysalises, and, over the course of a lifespan measured in days, they will mate, lay their eggs and die.

Were it not for the cows, Weiss says, these exquisite, ephemeral creatures could easily be the last of their kind.

Following a staircase of hoof prints, Weiss climbs to the top of a rocky knoll that affords a sweeping view of the valley and coastal mountains. Overhead a black-shouldered kite hovers; beneath us, a Black Angus cow and her calf duck into an oak-shaded ravine. Settling down on an inviting patch of ground, Weiss unstraps his backpack and pulls out a sandwich, a bar of chocolate and a thermos of green tea.

For a while we sit in silence, staring down at the cars and trucks streaming along U.S. Highway 101 a thousand feet below. Not visible from here is the full network of roads that connect San Jose and other Silicon Valley cities to the San Francisco Peninsula. Despite tighter emission controls, Weiss says, the vehicles that ply this congested corridor -- over 100,000 per day on U.S. 101 alone -- generate a significant amount of pollution, which the winds then lob onto Coyote Ridge.

• Wildlife