The Sunset District’s extinction distinction: a little blue butterfly
In the early 20th century the Sunset District neighborhood emerged on top of a massive field of sand dunes that covered most of western San Francisco, creating affordable homes for thousands of families but dooming a tiny, brilliantly beautiful creature to extinction.
The Xerces blue butterfly, no bigger than the end of a man’s thumb and boasting wings the color of a clear summer sky, lived nowhere else but the Sunset District and a few other spots in western San Francisco. What once was a favorite of Victorian-era collectors is believed to be the first American species of butterfly to become extinct through the urban development of its habitat.
Butterfly fades with the Sunset
Some sources place the species’ end in 1943, but according to entomologist Bob Langston, who studied similar species for many years, it actually came a few years earlier.
“I believe it was in 1936. Harry Lang from Davis found the last few,” Langston said. “It was before building the actual neighborhoods, but the neighborhoods came along soon afterwards.”
Langston said public and government attitudes at the time favored development over preservation, and famous developer Henry Doelger, whose sales office recently became San Francisco Landmark 265, turned the former Great Sand Waste into today’s Sunset District.
“Have you heard of Doelger homes? Well, they just bulldozed away,” said Langston. “They didn’t care. At that time, the more development the better.”
That development removed the Xerces blue’s main food for its caterpillars, the silver lupine, from the Sunset District. Although the plant is still widespread in other areas of coastal California and Oregon, without an abundant local source the butterfly couldn’t hang on.
“It couldn’t evolve into something that fed on something different,” Langston said.
The extinction of the Xerces blue may not have been widely considered important when it happened, as entomologist Langston said, but since then it has prompted the creation of the Xerces Society, an international nonprofit organization dedicated to the preservation of invertebrate species and their habitats.
Preserving an extinct species on the end of a pin
The Xerces blue isn’t completely gone from San Francisco, however. In the basement of the Academy of Sciences in Golden Gate Park, in a climate-controlled storage vault, the entomology department has three drawers of them, arranged neatly in rows on the ends of pins.
On a recent afternoon, academy entomologist Norm Penny wheeled a glass-covered tray of the little butterflies out of the vault.
The Xerces blue, known to science as Glaucopsyche xerces, was sexually dimorphic, Penny said. So only the males were really blue while the females were brown, a difference readily apparent in the academy’s specimens.
Some, facing down, show the bright blue wings of the males and the dull gray-brown hues of the females, while others are turned upside down to show the distinctive spot patterns by which scientists distinguish the Xerces blue from closely related species.
A close-up look at the tag that accompanies each individual butterfly shows where and when it was collected, such as a perfectly intact male that has outlasted his species for decades, collected June 23, 1935.
Similar species hanging on
The Xerces blue had a close relationship not only with the plants that fed its caterpillars, but also with other insects that depended on those caterpillars for food.
Those insects didn’t feed on the caterpillars, however, but rather from them. As the caterpillars fed they exuded honeydew, a sort of sugary liquid, and some native species of ant collected and ate the honeydew, also protecting their caterpillar food source from other insects.
As odd as that arrangement may sound it’s not uncommon and a number of other butterfly species have a similar relationship with ants, including some butterflies closely related to the Xerces blue.
One such species is the Mission blue butterfly. A federally endangered species, the Mission blue feeds on lupines just like the Xerces blue did.
It hadn’t been seen in San Francisco for three years, though other populations exist to the south on San Bruno Mountain and to the north in Marin, but three years ago it was reintroduced to the Twin Peaks area.
Sometimes it can be hard to pinpoint the effect the disappearance of one species has on an ecosystem or on people, particularly a species as localized and specialized as the Xerces blue.
But Penny recalled what one colleague said to him about the significance of losing a species here and a species there.
“‘It’s like an airplane flying along losing an occasional rivet,’” the colleague said. A loss or two might not be noticed in the short term.
“(But) at some point you’re going to lose one and the whole plane’s going to come down,” said Penny.
This article has been modified to correctly state the number of years the Mission blue butterfly is believed to have been absent from San Francisco.