Xerces Blue Butterfly (Lepidoptera: Lycaenidae)

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Scientific Name: Glaucopsyche xerces
Status: Extinct

The Xerces Blue butterfly is the first butterfly in North America known to have become extinct due to human disturbance. This butterfly formerly inhabited the sand dune systems of San Francisco until this habitat was almost entirely destroyed by urban development. The species was first described in 1852, at which time small populations were probably widely dispersed around the peninsula. Populations of this butterfly's food plant, legumes in the genera Lotus and Lupinus still persist at some of the butterfly's former localities. Thus the exact reasons for the butterfly's disappearance are uncertain. Numerous possible explanations can be proposed from population genetic theory (see our essay Population Genetics and Endangered Species Conservation). Obviously the areas which any one population could occupy was diminished. Possibly remaining areas were not large enough to maintain the diversity necessary to cope with the many other radical environmental changes occurring. It is likely also that the peninsula populations together functioned as a metapopulation. When the subpopulations' connections to each other were severed, the whole was no longer sustainable. Another possible explanation comes from consideration of the butterfly's life history. Like many lycaenid butterflies, the Xerces Blue was probably associated with ants in its larval stages. In such associations the ants may provide significant care to the developing larvae. At about the time of the earliest declines in the Xerces Blue's populations a new species of ant had been introduced from South America, the Argentine ant (Iridomyrmex humilis) which has displaced numerous native ant species, particularly in urban and suburban areas. Because the Argentine ant is not adapted to any kind of relationship with California butterflies, any dependent on native ants' care would have suffered losses at this time.

Whatever the specific causes, all of the above factors are associated with urbanization. Though only the Xerces Blue has received widespread attention, the growth of San Francisco has undoubtedly brought about the decline or extinction of many undocumented insect populations, subspecies, and species. Certainly San Francisco is not unique in this respect either. Cities and native biodiversity are to some extent mutually exclusive. However, there are ways of lessening our impact as we grow which would make our cities much more environmentally friendly. We, and all of the other organisms we share our space with, would stand to benefit from more consideration of Urban Ecology.

For further reading:
Emmel, T.C. and J.F. Emmel, 1993. The Xerces Blue, Glaucopsyche xerces (Boisduval). in: T.R. New (ed.), Conservation Biology of the Lycaenidae (Butterflies). Occasional Papers of the IUCN Species Survival Commission No. 8.