Biology of the Bay Checkerspot Butterfly

Paul Ehrlich began long-term studies of the biology and demography of the Bay Checkerspot Butterfly (Euphydryas editha bayensis) shortly after he arrived at Stanford University in 1960 as a newly appointed assistant professor. In the four and a half decades since, he and his associates have monitored population sizes annually and done detailed analyses of larval food plant specialization, adult nectar feeding preferences, and studied many other aspects of the basic ecological relationships in this species and its relatives. There may be more information about the population size fluctuations of this butterfly than for any other invertebrate.²

The Bay Checkerspot Butterfly is currently listed as a threatened species by the U.S. Fish and Wildlife Service. It used to be found in two, largely separate, metapopulations that are about 50 miles apart--one highly fragmented one in San Mateo County, another more or less intact in Santa Clara County, California. The largest part of the metapopulation in San Mateo County, subpopulations on the Jasper Ridge Biological Preserve of Stanford University, went extinct in 1997. A small population remains on a separate serpentine outcrop about 4 miles to the north. The butterfly is restricted to patches of native grassland that support its native host plants (Plantago erecta and Castilleja purpurascens³) and adult nectar sources (including Lomatium, Lasthenia, Layia, and others). With only one exception, the patches supporting native grassland are outcrops of serpentine soil.⁴

Basic life-history⁵

• Univoltine--one generation per year.

• Adults fly from late February to early May.

• Females lay masses of up to 200 eggs at the bases of the larval host plants--Plantago is the primary host plant, Orthocarpus is used as a secondary host when Plantago senecesces before the larvae diapause.

• Diapause extends through the summer drought. It is broken by late summer rains when the larvae begin feeding.

Population dynamics

• Population size of reproductive adults largely regulated by larval mortality in preceding spring.

• Larval mortality largely controlled by timing of rainfall, adult emergence, and senescence of food plants
  • Spring rainfall delays senescence of food plants, decreases mortality

  • Warm temperatures increase rate of larval development

  • Microtopographic influences on soil moisture (host plant senescence) and temperature (larval development, adult emergence) have large effect on larval survival rates--larvae that hatch from eggs laid on cool slopes by females that emerge early from warm slopes are most likely to survive

  • Severe droughts, e.g., 1975-1977, cause severe population declines, even local extinctions.
    • Three subpopulations at Jasper Ridge Biological Preserve, smallest went extinct, but was re-established a few years later, presumably by colonists from one of the other Jasper Ridge populations. Now that all populations at Jasper Ridge are extinct, the USFWS continues to include the serpentine grasslands there in its critical habitat designation in the hope that migrants from remaining population units will re-establish it.

    • In Santa Clara County all populations except one went extinct during the 1975-1977 drought.

  • Local population dynamics can be extremely local--two subpopulations at Jasper Ridge separated by less than 500m showed opposite changes in population in 6 out of the 25 years from 1960-1985.

• Increased nitrogen deposition associated with air pollution is increasing the fertility of serpentine soils, which are naturally low in N, and allowing exotic grasses to invade. As the exotic grasses invade they exclude the food plants and nectar sources on which the bay checkerspot depends. In the large metapopulation south of San Jose removal of cattle from grasslands harboring the butterfly led to a dramatic decrease in butterfly population size. Grazing leads to a net export of N as cattle are removed for slaughter. Thus, moderate grazing enhances the viability of an endangered butterfly [4].