The Mission Blue butterfly ranges from the San Mateo Peninsula near Crystal Springs Reservoir to the southern part of Marin County. As an endangered species, it is most readily observed at San Bruno Mountain, Milagra and Sweeney Ridges, Twin Peaks, the Marin Headlands, and Oakwood Valley. It inhabits small specks of grassland and rocky outcrops along these coastal hills and ridges. Its immediate year-to-year survival is dependent upon specific lupines on which it lays its eggs. In addition, its long-term prosperity may hinge upon native ants which protect its larvae, conservation minded humans, and a diverse and intact ecological system.