Crossed Fingers for a Threatened Butterfly If you stand with your back to the sun and inspect the place on the ground where your shadow is cast, you might spot the tiny larvae of the threatened Bay checkerspot butterfly. "It takes some practice," said Stuart Weiss, a conservation biologist from the Creekside Center for Earth Observation. But when you see a little black object that stays black, that's usually a checkerspot. Dr. Weiss explained the technique as we hiked through the grasslands of Edgewood Park, a 500-acre nature preserve on the San Francisco peninsula where I have been volunteering. To reintroduce the checkerspot to the park, Dr. Weiss and his team placed 4,000 larvae, or caterpillars, here last winter. Over the next several weeks, he and several volunteers whom he refers to as "checker spotters" will be searching their shadows for signs of success: a new batch of larvae, descendants from last year's transplants, that will be transformed into thousands of bright orange and black spotted butterflies in late March. While the work may seem tedious, especially for an insect whose adult life as a winged butterfly spans a mere 10 days, the study of this tiny creature has far broader implications. The checkerspot, Dr. Weiss explains, "has become the poster child for the effect of nitrogen on the environment." The impact on California's biodiversity could be even more profound than that of global warming, many scientists say. Vehicle exhaust from nearby Interstate 280 deposits up to 15 pounds of nitrogen per acre each year in the park. The interstate runs along the park's western border, carrying more than 100,000 cars between San Francisco and San Jose on a busy day. Nitrogen from car exhaust fertilizes the soil just as common lawn fertilizer does and favors faster-growing nonnative plants. "It's a matter of competitive ability - the nitrogen-rich soil produces a very strong growth response in annual grasses, like Italian ryegrass," Dr. Weiss said. They thatch over and smother native plants. As native plants disappear, so do the native species like the Bay checkerspot that depend on them. "When you continue to dump nitrogen into the atmosphere, it has all sorts of negative impacts on the ecosystem, on human health, on the air you breathe and the water you drink," Dr. Weiss said. The Bay checkerspot once thrived at Edgewood but has not been seen here since 2002. Since then, an aggressive program of mowing and weeding has helped keep invasive plants in check and restore the native grasslands. Using grant money from the San Mateo Parks Foundation to finance the work, Dr. Weiss and his team collected 4,000 Bay checkerspot larvae last winter from Coyote Ridge park in San Jose, the only remaining habitat for the species. They carried the larvae to Edgewood and placed them over 20 acres on the west side of the park, where its favorite meal, the native California plantain, is plentiful. During this year's hunt, which runs throughout February and March, the team hopes to find 4,000 new larvae. A lot depends on the weather conditions, Dr. Weiss said. A wet winter and a cool spring are ideal, as sufficient moisture in the soil helps extend the lifespan of the plants that support the larvae and adults. Ultimately, the project will be counted a success if 2,000 adults are observed flitting among the wildflowers in late March. Once the adults appear, over the course of about 10 days, they will feed on nectar from the wildflowers, mate, lay their eggs on the native plantain and then die. The annual cycle begins again. Based on early counts of larvae, Dr. Weiss is optimistic about this year's outcome, although he expects it will take additional cycles to re-establish the butterfly at Edgewood. The checkerspot is not the only threatened species that occupies his time. In a remote corner of the park, he points to a few square meters of ground that hold the only known remaining patch of the San Mateo thornmint, an endangered annual wildflower. The plant has also fallen victim to invasive Italian ryegrass, its population dwindling to a few hundred plants by 2008. Thinning the nonnative grasses and carefully replanting thornmint seeds has brought the population back to nearly 3,500 plants. He is hopeful that the plant has reached critical mass and may now come back on its own without his continuing intervention. Although the tiny thornmint has not attained the celebrity status of the Bay checkerspot, Dr. Weiss says that nurturing the species is important work. "It's an utterly unique form of life and we shouldn't let it go extinct on our watch," he said.