This year, the numbers have been coming in on the low end, due in large part to the fact that the spring of 2008 (when these future butterflies hatched) was the fourth driest since 1895. But it's not just climate that regulates the population, Weiss says. Checkerspots also respond to internal rhythms. In some years, for example, caterpillars become so numerous on a particular hillside that they decimate the dwarf plantain, setting the stage for a population crash in the next generation.

What makes Coyote Ridge so valuable is its size. If a newly emerged butterfly finds itself on a slope with few flowers, it can fly off to find a better location without ending up smashed on a windshield or marooned in a grass-choked field. Coyote Ridge is also topographically rugged, another important asset. Flowers on north-facing slopes bloom up to four weeks later than those on south-facing slopes, which extends the seasonal availability of nectar. In similar fashion, dwarf plantain stays green longer in shaded locations.

Weiss is encouraged by the fact that government officials in San Jose and Santa Clara County have gotten behind a plan to preserve much of the South Bay's remaining serpentine grassland, the largest portion of which lies on Coyote Ridge. Significantly, the plan -- which envisions protecting some 4,000 acres through conservation easements or outright land purchases -- embraces grazing as a management tool.

As lengthening shadows announce the end of the day, Weiss heads back down Coyote Ridge, passing through the series of gates that divide one property from another. At the last gate, I find myself looking back at the ecological equivalent of the border between two countries. Outside the gate lies a monotone of grass; inside, wildflowers have created a canvas worthy of Claude Monet or Vincent van Gogh. "Eye candy," Weiss smiles, and indeed it is, for people and butterflies alike.

J. Madeleine Nash is a freelance journalist and science writer based in San Francisco, California.