Stuart Weiss, a Stanford biologist who is spearheading the repopulation effort, tried to reintroduce the checkerspot butterfly to San Mateo County in the spring of 2007.
That attempt failed due to unusually dry weather conditions, which killed off the food supply before the 1,000 caterpillars that had been hand-carried into the Edgewood Park were able to mature and reproduce, Mr. Weiss said.
"When we tried this back in 2007, we just happened to pick the fourth-driest year since 1895," he said.
Along with the 21 females and 20 males released March 30, volunteers in February brought in more than 4,000 caterpillars and scattered them throughout the hills and low-growing native grasses of the 467-acre park.
The checkerspot butterfly, which is a federally listed endangered species, numbered around 4,500 adults in the Edgewood area in 1997, Mr. Weiss said.
Nitrogen contained in exhaust from vehicles traveling on nearby Interstate 280 created an artificial fertilizer, which allowed invasive grass species to grow and crowd out the native species upon which the checkerspot butterflies depend, he said.
The checkerspots were extinct in the area by 2003.
Using funds from the U.S. Fish and Wildlife Service, the California Department of Food and Agriculture and Pacific Gas & Electric, park managers have been able to eradicate the invasive weeds in sections of the park, allowing native grasses and wildflowers to gradually repopulate.
The return of the checkerspots' natural food supply and a renewed effort to reintroduce the butterflies in greater numbers gives Mr. Weiss and his volunteers hope that this year's efforts will succeed.
Mr. Weiss carefully removed each butterfly from containers in the ice cooler and placed them individually on budding wildflowers such as desert parsley, dwarf plantain and tidy tips.
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