A commenter on this website, ‘Jonathan’, suggested that a good reason to destroy the Mount Sutro Cloud Forest would be to bring back habitat for endangered butterflies. Besides the Mission Blue Butterfly (for which there’s an attempted reintroduction on Twin Peaks) he suggested three others: The San Bruno Elfin, a tiny brown butterfly (pictured); the Callippe Silverspot; and the Bay Checkerspot. These are all butterfly subspecies found on nearby San Bruno mountain, which has some 2700 acres of preserved habitat. (A subspecies is what it sounds like: a division below the “species” level. Subspecies of animals or insects can often interbreed, but don’t usually because of geographic isolation.)
So we thought we’d look into it, (though we’d consider the destruction of this unique and beautiful forest a very high price to pay). The answer, as one might expect, is complicated.
According to a 2001 article by Sherri Graves and UC Davis’s Professor Art Shapiro, some 82 of California’s 236 butterfly species are already known to use introduced plants for feeding and/ or breeding. This can allow a species to expand its range or its breeding period, making it more successful. One example is the Anise Swallowtail, which now breeds on non-native fennel. Another is the Monarch butterfly, which uses the eucalyptus that has replaced all the other tall trees that were felled for lumber or land. [ETA: In a 2002 paper, Prof. Shapiro notes that around Davis – perhaps the most intensively studied butterfly habitat in the US – 29 of the 32 species breed at least partly on introduced plants. And 13 don’t seem to have a native host plant at all.]
Because insects have short life-cycles, they often can adapt to changed vegetation, and more may be in the process. For instance, some butterflies may lay their eggs on plants that poison their larvae. If there’s enough genetic variation in the population, a few larvae might actually survive. If they pass on their immunity to their offspring, they have another plant to use.
So what about these endangered butterflies? It turns out that at least at present, they all breed on very few plant species. Their caterpillars are picky. (We can also speculate that perhaps there’s not be enough variation within their diminished populations for them to adapt to other plants, though of course we don’t know this.) The Mission Blue breeds on a few species of lupine. The San Bruno Elfin needs broadleaf stone-crop. And the Calippe Silverspot (Speyeria callippe callippe, an endangered subspecies that shades into Callippe Comstocki to the south, and Callipe Liliana up in Marin) uses the yellow pansy as baby food.
Then there are the ants as nurses. Some species (the Mission Blue and the Callippe Silverspot, don’t know about the others [ETA: Also true of the San Bruno Elfin]) need native ant species to nurture their caterpillars. The ants, attracted by “honeydew” – a sugary caterpillar pee, protect caterpillars from enemies. Even on San Bruno Mountain, the relatively warm eastern side has native ant populations; the windy foggy western side has mainly Argentine ants. Mount Sutro, which is windy and foggy on all sides (because of the way the fog belt works) is unlikely to be a warm safe place for ants. [ETA: Argentine ants do apparently tend caterpillars, though it’s not known how effectively they defend them.]
So restoring butterfly habitat isn’t as simple as “if you plant it, they will come.” According to a 2007 report on San Bruno mountain – where the battle is to preserve habitat from housing development – there have been no instances of successful butterfly habitat restoration in 25 years: “The process of destruction, mitigation, and restoration does not address the site specific ecology of butterfly species, and the complexity of these grassland systems. It is entirely without scientific merit. The survival of these butterflies is not as simple as the farming of host plants, or the mass rearing of larvae (Matoon et al 1971). To replicate the system would require its associated group of organisms to all be present. To the best of my knowledge, no new Mission Blue nor Callippe Silverspot areas have been established by humans in the past twenty five years…”
Finally: These are grassland species, growing in flammable spaces. Pansies apparently need occasional fires to burn off competing plants. The lupines grow best in disturbed ground, where it’s burned or there’s been a landslide. Fire may be part of the ecological cycle that keeps their host plants healthy.
San Bruno Mountain’s had a bunch of fires. A prescribed burn in 2003 got out of control and burned over 70 acres. In August 2006, a 50-acre fire was put out with helicopter assistance. May 2008, a 7-acre grass fire was contained after it went to three alarms. Then in June 2008, a five-alarm fire burned 300 acres and threatened homes.
It’s bad enough in San Bruno, but that has 3,000 acres of open space. Mount Sutro Forest is 80 acres, surrounded by homes and a hospital. Converting it to a fire-prone ecology is a bad idea.