Bay checkerspot butterflies are medium-sized insects and have brilliant markings in a mosaic of white, black and reddish-orange. This butterfly was only found in Santa Clara County until recently when it was reintroduced to San Bruno Mountain and Edgewood County Park in San Mateo County, California. This species was listed as threatened in September 1987. The U.S. Fish and Wildlife Service recommended uplisting to endangered in 2022.

Historically, the subspecies occurred in the vicinity of the San Francisco Bay area from San Bruno Mountain, west of the bay, Mount Diablo, east of the bay, to Coyote Reservoir, south of the bay. The current range of the subspecies is limited to Santa Clara and San Mateo counties, California and all occurrences are on serpentine or serpentine-like grasslands except for San Bruno Mountain where it occurs on non-serpentine nonnative annual grasslands. Since 2009, Bay checkerspot butterfly reintroductions or translocations continued in Santa Clara County at Tulare Hill and in San Mateo County at Edgewood Natural Preserve and San Bruno Mountain. The Edgewood and Tulare Hill reintroductions have had limited success, while the San Bruno Mountain reintroduction has the potential for success. 

At the time of listing, habitat damage resulting from urban development, highway construction, drought and overgrazing was noted as having caused the disappearance of four populations of Bay checkerspot butterfly. The threat from urban development has reduced over time as land protection has increased, and currently most of the butterfly’s habitat is protected or expected to be protected under the Santa Clara Valley Habitat Conservation Plan/Natural Communities Conservation Plan. While overgrazing may still be a threat to Bay checkerspot butterflies, there is increasing consensus that grazing is the most cost effective and flexible tool for managing California grasslands. In fact, the local extirpation of the Bay checkerspot butterfly has occurred at sites after cattle grazing was removed, for example, at Tulare Hill, which results in the displacement of larval host plants and nectar plants by non-native invasive annual grasses. The listing rule also noted climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change , habitat degradation and pesticide use as threats to the species. Today, wildfire and small population size coupled with pesticides, non-native invasive plant species, extreme weather and climate change threaten the species. The spread of non-native invasive plants is exacerbated by atmospheric nitrogen deposition from vehicle exhaust that fertilizes the naturally nutrient-limited serpentine soils allowing nonnative plants to invade and displace the butterfly’s larval host plants and nectar plants. 

The butterfly continues to be threatened by: 

• Habitat degradation
• Non-native invasive plant species exacerbated by atmospheric nitrogen deposition from vehicle exhaust
• Climate change
• Wildfire
• Fire retardant
• Pesticides
• Small populations