Urban Nature

Swallowtail butterflies thrive along SF’s gritty streets

July 13, 2012

San Francisco residents are well-acquainted with the seagulls and pigeons in their midst. Take a closer at the green spaces amid the cityscape and you’ll notice one of most beautifully understated wild residents: the swallowtail butterfly.

As the butterfly aficionados gear up for the annual Big Butterfly Count, which takes place July 14 through August 5, taking a few extra moments to seek out our pretty, fluttering friends is a great way to check in on our physical surroundings.
 Amazingly, San Francisco boasts the highest population of swallowtail butterflies in the country, according to a national count, thanks to the diverse plant and wildlife species unique to the region.
“It’s an unlikely habitat, but it works,” said Amber Hasselbring of Nature in the City on a recent tour of swallowtail habitats. “The types of plants and trees here, along with relatively mild weather, have made it perfect for the Swallowtail.”
There are 33 types of butterflies living in San Francisco; the swallowtail is the most populous.
Photo by Lydiat on Flickr.

These nectar-eating insects thrive not only on the native plants along the ocean dunes, but also within the rich array of human-altered landscapes. Even in the gritty Tenderloin district of the city, an impressive patchwork of parks, planters and community gardens is helping to keep the butterflies fed and happy.

In particular, the London plane tree, a sycamore hybrid, lines many city streets and provides a safe haven in which butterflies can lay their eggs. To us, a row of London Plane trees are a welcome splash of green amid gray. To a riparian-dwelling butterfly, it’s the next best thing to a river corridor. The trees form a dense canopy like those found near streams, offering a protected home close to food sources for the early stages of life.
 Of course, the swallowtails need different types of food throughout the life cycle, which is where being a city butterfly can become tough. Adults need nectar to thrive. Just as the mid-Market corridor is devoid of many healthy food options for humans, it’s a relative food desert for butterflies as well. This is where people could step in and make it more hospitable.
 “We could do a lot better with our urban planting,” said Hasselbring.
She pointed out that the ubiquitous bamboo and shrubs found around Civic Center offer virtually no food sources for butterflies, birds or bees. Dandelion, thistles, buckwheat or berry-producing plants would provide much-needed nectar sources for pollinators. In the ravenous caterpillar stage, leafy foods like nettle and parsley are perfect. Meanwhile, adults need flowering plants like zinnias, asters, tithonia and berry-producing plants with multiple florets to facilitate resting, feeding and pollinating.
 “Planning green spaces is more than just leaving buildings out,” said Elizabeth Stampe of WalkSF. “We could do a lot to help both people and wildlife thrive by choosing which things to plant.”
Of course, it’s impossible to please all sides when discussing urban planning. Hasselbring pointed out how many of the plants that are great for pollinators are actually invasive species, which run counter to many environmental advocates’ plans for a greener San Francisco.
As it stands, San Francisco does a good job at providing a home for the butterflies, while also serving its two-legged residents. Several community farms such as the Tenderloin People’s Garden and the Please Touch Community Garden in the heart of the city yield a two-fold benefit. People in need get healthy, locally-grown food and pollinators have an excellent food source, too.
Fennel and members of the carrot family are welcome treats for butterflies and are extremely common in community gardens. Modest planters featuring fragrant sage, rosemary and lavender are abundant throughout the city and provide an excellent nectar source. Even street weeds, like the pellitory cascading beneath a sewer grate, serve as nourishing larval plant food, proving that a species can thrive just about anywhere.
 Overall, while there is definitely room to grow, San Francisco has fostered a welcoming environment for the little swallowtail.
“We have a lot more nature than we realize,” said Hasslebring.
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