Opening the corridor

San Francisco is a dangerous town for butterflies

San Francisco is a dangerous town for butterflies. Xerces blue, a species that once thrived in the city's dunes, suffered a catastrophic demise in 1941, the first butterfly extinction in the United States caused by urban development.

In the years since, local butterflies haven't fared much better. According to lepidopterist Liam O'Brien, 24 of 58 local species have been wiped out in regional extinctions caused mainly by habitat destruction. Another three or four, he said, will likely be gone within the next five years.

The green hairstreak is one of these on-the-brink butterflies. Boasting brilliantly verdant wings, the nickel-sized hairstreak lives only in the Inner Sunset's Golden Gate Heights neighborhood and at Battery Crosby in the Presidio. Survival of the species depends on linking two populations on Rocky Outcrop (14th Ave. and Noriega) and Hawk Hill (14th Ave. and Rivera).

Separated by just five blocks — less than a mile but enough concrete to be the edge of the earth for smaller butterflies — the two hilltop populations are islands whose fluttery inhabitants have become genetically threatened by full sibling inbreeding.

Female hairstreaks rely on one of two native plants, coast buckwheat and deer weed — both of which once grew abundantly on natural dunes — as sites for their eggs. As O'Brien told the Guardian, "The females disperse, and they just disperse into oblivion if they don't have the host plant to keep it going."

The Green Hairstreak Project is O'Brien's plan to build a botanical bridge. "We could keep this butterfly alive in the city if we just totally bombard that area with these two plants," he said, adding that starters are being grown in preparation for October planting.

The project is a program of Nature in the City, an organization devoted to the ecological stewardship of San Francisco. Founding Director Peter Brastow said the city is full of "reservoirs of indigenous biodiversity," and believes that the whole urban landscape is a potential habitat. "The other piece of the puzzle," he said, "is connecting up wildlands via corridors."

O'Brien is considering various corridor-constructing strategies, from knocking on doors and giving buckwheat and deer weed plants to residents (he's mapped potentially usable front yards) to professional dune restoration. During this past hairstreak season, between mid-March and the end of May, he led walks to introduce future stewards to the resident butterfly.

"Literally, can we please just put this plant in your front yard? It's not complicated," O'Brien assured would-be-hosts, adding that he would like San Francisco to be celebrated for what it saved, not just for a species it destroyed. "Here's a butterfly that flew at the same time Xerces did. Are we going to step up and do something?"

O'Brien's hairstreak haven is not the only corridor being mapped out. A few neighborhoods east, artist Amber Hasselbring is building a series of native plant plots that zigzag along Mission District sidewalks. "Think about looking down from Dolores Park," she said, "and seeing this whole thing just unfolding in front of you so the park does not have a border anymore, [but] just flows into the next one."

At Mission Playground on 19th Street and Linda, Hasselbring explained her Mission Greenbelt Project, also a Nature in the City program. From her initial, mammoth vision to "daylight" the buried Mission Creek, she wondered instead about connecting the spaces, and people, that are already part of the community.

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