Meet the Bay Area Butterflies Fighting For Survival

When I lived in the San Francisco Bay Area in college, I was oblivious to the region’s status as a global epicenter for rare butterflies. The Mission Blue (Icaricia icarioides missionensis), the San Bruno Elfin (Callophrys mossii bayensis), and the Callippe Silverspot (Speyeria callippe callippe) are all found in and adjacent to the two-thousand-acre San Bruno Mountain State Park, located on the southern boundary of San Francisco. Other rare butterflies in the region include the Lotis Blue (Lycaeides idas lotis), the Behren’s Silverspot (Speyeria zerene behrensii), and the Myrtle’s Silverspot (Speyeria zerene myrtleae) to the north; the Bay Checkerspot (Euphydryas editha bayensis) to the south; and the Lange’s Metalmark (Apodemia mormo langei) to the east. In all, eight butterflies living within a sixty-mile radius of San Francisco are listed on the US Fish and Wildlife Service’s threatened and endangered species list. Of the twenty-six threatened or endangered butterflies in the United States, one of every three is found in this small region.

The Bay Checkerspot, a butterfly with a two-inch wingspan and wings covered with rows of orange and white or yellow spots against a black background, is one of the Bay Area’s rare butterflies. Of all the world’s rare butterflies, the Bay Checkerspot is one of the two most studied species (the other being the Large Blue in England). It was with this butterfly that Stanford University professor Paul Ehrlich conducted his classic ecological studies in population biology. His pioneering work resulted in some of the precursors to modern spatial ecology and conservation biology, disciplines that are key in the development of principles regarding how to manage populations in landscapes fragmented by people. Paul’s sustained research on the Bay Checkerspot led to a series of discoveries by his lab and collaborators that are now propelling the butterfly’s conservation.

As an undergraduate, I worked in Paul’s lab but not on butterflies. After failed starts in engineering and then economics during my first two years of college, I’d finally set my career path in conservation biology. I had no vision of what I would do in this field. At minimum, it would take me outdoors to conduct science. On my inaugural research trip, I remember walking across an open, grassy hill at Stanford’s Jasper Ridge Biological Preserve. I did not know it at the time, but this trip brought me within just feet of the Bay Checkerspot.

In 1990, I headed into the field to study birds as an assistant to one of Paul’s graduate students, Tom Sisk (now a professor at Northern Arizona University). I studied patterns of birds across the varied habitats of the California foothills, including oak woodlands, shrub-dominated chaparral, and grasslands. As I traversed the grassy ridge, I passed another student doing field research. It was not until months later that I learned that the student was studying the Bay Checkerspot. Even more surprising, my path wound through three sites that had supported a historic population of the butterfly. Unfortunately, the population at one of those sites had already been lost, and the other two were set on a downward trend. By 1998, the population at Jasper Ridge was extirpated (a term I use to differentiate the loss of a population from extinction, or the loss of an entire species or subspecies).

TWO CENTURIES OF DECLINE

The extirpation of the Bay Checkerspot population at Jasper Ridge was not the first. Over the previous decades, other populations nearby had also been lost. The forces of environmental change that caused the Bay Checkerspot (and other butterfly species living nearby) to become perilously rare were set in motion long before San Francisco’s rapid growth. As far as we know, the Bay Checkerspot lived in large areas that people have never converted to cities or farm fields. No one intended to destroy the Bay Checkerspot’s habitat. Yet, the population growth in the Bay Area led to large, unintentional changes that caused the Bay Checkerspot to decline. The sequence of change unfolded over two centuries.

The change began when Spanish settlers arrived in the Bay Area. In 1776 they established permanent occupancy, and an immigrant population soon gathered size and strength. In 1824, the Spanish missions began dividing their land holdings into separate parcels, ushering in a dramatic change in land use. Change in land ownership accompanied an increase in one particular use of this land: cattle ranching. By the 1830s, the Bay Area harbored hundreds of thousands of grazing cattle, horses, and sheep. Livestock farming transformed the landscape in two ways. First, the animals consumed and trampled native plants. Second, the farming practices spread populations of invasive grasses, which came to dominate the region.

Invasive species caused immediate change and set the stage for future habitat degradation. The Bay Checkerspot lives in grasslands that cover the foothills that encircle the bay. At first glance, it looks as if there should be plenty of habitat for the butterfly; however, not all grasslands are equal. Native California grasses and herbs, including the host plants with which the butterfly evolved, no longer dominate vegetation in the foothills. Invasive Italian Ryegrass (Lolium multiflorum), Common Wild Oat (Avena fatua), Soft Brome (Bromus hordeaceus), and other species do. These species grow fast, produce many seeds, and establish easily. These and other characteristics allow them to displace native species. Now the butterfly’s host plants are relegated to a few small habitat patches scattered across the Bay Area.

The landscape’s degradation continued apace with the gold rush of the 1840s and 1850s. Houses and buildings covered some of the Bay Checkerspot’s habitats. More important, urban expansion near the species’ dwindling habitat caused unusual changes that proved the most enduring cause of butterfly decline.

DISCOVERY AND DISTRIBUTION

By the early twentieth century, most of the Bay Checkerspot’s habitat had been lost to invasive species, and the butterfly persisted in tiny fragments of remaining habitat. By the time Robert Sternitzky of the Pacific Coast Biological Service discovered the Bay Checkerspot in 1933 (it had been unknown to science before his discovery), the butterfly occupied only a few small areas that were unique in their ability to withstand invasion by exotic plants. The butterflies found a foothold in grasslands that grow on a soil type that is uncommon in the Bay Area. This soil forms on outcroppings of serpentine rock, which has a greenish hue and slippery feel and comes to or near the surface of the thin soil layer. Serpentine soils have an unusual composition of nutrients. They are low in some nutrients that are essential to living things, such as nitrogen and calcium, and high in other elements that can be toxic, such as magnesium and nickel. Besides its unique blend of nutrients, serpentine soil dries quickly. These features of the environment support a unique community of plant species.

Bay Checkerspots lay most of their eggs on Dwarf Plantain (Plantago erecta). This host plant for the species’ caterpillars is a small herb that grows to about one foot in height, often in serpentine soils. Its narrow leaves cluster around the plant base. Other aspects of the butterfly’s biology besides its selective diet enable it to persist in serpentine habitats. Unlike most butterflies, which lay their eggs individually or in small clusters, Bay Checkerspot females dump large masses of up to 250 eggs on single, small plants.

After the Bay Checkerspot’s eggs hatch, tiny young caterpillars form webs and live communally. Dining in concert, they are able to ravish their diminutive host plant. Unlike many caterpillars, which can feed on the plant on which they hatch until they become a chrysalis, Bay Checkerspot caterpillars exhaust their first host and must crawl to other plants nearby. When they set out, their destination is not only another Dwarf Plantain. The Bay Checkerspot has two other host plants, Purple Owl’s Clover (Castilleja exserta) and Denseflower Indian Paintbrush (Castilleja densiflora). The three host plants grow in different seasons. The Bay Area’s winter rains are followed by eight arid months. In summer or fall when I scan the grass-covered foothills, the only color I see is brown. As the landscape dries out, Dwarf Plantain desiccates. It can do so before the caterpillars have finished eating. If it does, the caterpillars must search for a new host plant. Purple Owl’s Clover and Denseflower Indian Paintbrush emerge later in the wet season and remain green for a short time into the dry periods. The caterpillars’ goal is to make it through about half their life cycle before all of their host plants dry out. When they have made it this long, they retreat under dead leaves or rocks, where they enter diapause for seven months. After the rain returns, their host plants form new leaves, and the Bay Checkerspots can complete their life cycle. In a good year, about half the caterpillars that live through diapause will complete their life cycle. In a bad year, for example when plants are under stress from extreme drought, only a quarter may survive this gauntlet.

Serpentine soils provide safe havens for Bay Checkerspot populations; however, the dozen populations that remained on these soils in the middle of the twentieth century were not safe. They ranged over fifty miles, from just south of San Francisco to just south of San José. Some areas where they thrived in and near San Francisco, such as Twin Peaks, became part of the city, covered over with homes and other buildings. A large freeway sliced into Edgewood Park, just south of San Francisco, where another Bay Checkerspot population occupied about 115 acres until 2002. The now-extirpated population at Jasper Ridge once inhabited a small grassland covering just twenty-five acres. The best conservation outcome would see Bay Checkerspot reestablished in the minuscule number of areas distinguished by serpentine soils that remain undeveloped.

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Adapted from The Last Butterflies by Nick Haddad. Used with permission of Princeton University Press, © 2019 Princeton University Press