Glaucopsyche xerces
The Xerces Blue butterfly is the first North American butterfly known to become extinct as a result of human interference in its habitat. The Xerces Blue, named for the Persian king Xerxes (but with the French spelling, thus the c instead of the x) by French entomologist Jean Alphonse Boisduval, inhabited the undeveloped coastal sand dune regions of the San Francisco peninsula and, specifically, where the Lone Mountain Cemetery, Presidio military installation, San Francisco's Sunset District, and Lake Merced can now be found. The Xerces Blue was first described in 1852, when small populations of the butterflies could be found dispersed throughout the peninsula, largely isolated from one another.
The Xerces Blue's dominant, eye-catching feature was the males' iridescent blue-violet upper wing colouration (the females were brown in this area). In addition, for a few years it was widely thought that the Xerces Blue was not one but a number of different butterfly species due to the wide variety of wing shape and markings that could be found amongst the butterflies. Eventually, observers noticed that despite the differing appearances of the butterflies, they were breeding with each other. The Xerces Blue would have made a wonderful subject to study the genetics and evolution of, had they survived beyond the 1940s.
The butterflies primarily fed on legume-type plants of the genera Lotus and Lupinus. Deerweed (Lotus scoparius) seems to have been preferred by the butterflies for food and laying eggs on. Other plants the Xerces Blue seemed to eat and/or lay its eggs on were the Beach Lupine (Lupinus aboreus) and a blue-flowered species of lupine believed to be Lupinus micranthus. Though much of the butterfly's habitat was removed by urbanization of the area, these plants can still be found in other areas and even in some of the previous habitats of the Xerces Blue. This has led to some speculation as to how, exactly, human development in the area caused the extinction of the Xerces Blue.
It's possible that the smaller areas where the Xerces Blue could thrive as urban development encroached on its habitat wre unable to support the diversity necessary within the butterfly's population to allow the species to deal with other dramatic changes to its environment. Another possibility is that the spread out, isolated groupings of Xerces Blue could have functioned together as a metapopulation and that when many of them were destroyed by human interference, the remaining subpopulations couldn't connect to each other normally. This would lead to problems with biodiversity within the surviving subpopulations.
Another aspect of urbanization which may have affected the Xerces Blue is the introduction of the Argentine ant (Iridomyrmex humilis) from South America to California. Some experts believe that the Xerces Blue's larvae may have been partially cared for somehow by indigenous ant populations. When the Argentine ant was introduced to California, it displaced much of the indigenous ant populace. The Argentine ant, having no experience aiding developing Xerces Blue larvae, would have been unable to assume the role that other ants had and may have resulted in the death of many of those larvae. This theory has to support it the fact that the first noticed decline in Xerces Blue numbers coincided with the introduction of the Argentine ant to the Xerces Blue's habitat.
These factors, combined with the geographic isolation of the Xerces Blue (otherwise the species could have survived elsewhere, where its primarly sources of food still existed plentifully), led to the butterfly's extinction in the 1940s. As early as 1875, the decreasing number of Xerces Blue was noticed. A San Francisco lepidopterist named Herman Behr wrote to a colleague that year that the Xerces Blue was "now extinct, as regards the neighborhood of San Francisco. The locality where it used to be found is converted into building lots, and between German chickens and Irish hogs no insect can exist besides louse and flea."*
By the 1930s the butterfly could only be found in vacant lots. Once the United States entered World War II, a massive reduction in the Xerces Blue's habitat would come when officers of the Presidio military base recommended the undeveloped sandy areas to the south of San Francisco (which the butterflies lived in) be urbanized. This recommendation came based on the concept that if the area remained largely empty of human development, it would prove to be a vunerable piece of land that the Japanese could use to land an invasion force.
On 23 March, 1941, W.H. Lange collected the last known specimens of Xerces Blue at the Presidio. The last known sighting of the Xerces Blue was in 1943: Dr. Edward Ross and Harry Davis of UC Davis witnessed some of the butterflies near a blue-flowered Lupine while in the vicinity of the marine hospital above Lobos Creek. Not long after, the land the butterflies were seen flying over was flattened and built upon by the army.
And that was the end of the Xerces Blue... or was it?
There exists a method of reintroducing extinct species into the environment known as resurrection ecology. The idea behind resurrection ecology is to take some of a species closely related to the now-extinct species, release it into the previous habitat of the now-extinct species (assuming the habitat is still or once again habitable for such species), and allow the species to gradually adapt to its new environment, hoping it takes on the same attributes of its extinct relative. What results from this might not be the exact same species as the Xerces Blue but could be considered a very closely related subspecies. This has apparently happened in Florida, where an extinct butterfly's population was replaced with that of a closely related type from Cuba that went on to become almost identical to the species that had died out.
The Palos Verde Blue butterfly (Glaucopsyche lygdamus palosverdesensis), itself thought to be extinct until a small surviving population was found later, and a subspecies of the Silvery Blue butterfly (Glaucopsyche lygdamus) that lives on Santa Rosa Island off the coast of California are both considered by some to be likely candidates for "resurrecting" the Xerces Blue. There exist several major differences between the species and the Xerces Blue. The Palos Verde Blue and Silvery Blue do not have the diverse wing design and markings that the Xerces Blue did. In addition, the Silvery Blue seems to lay its eggs and eat only deerweed and have significantly less magnificent colouration on its wings. Nevertheless, one day, an attempt to reintroduce the Xerces Blue or something similar may take place.
Sources:
http://www.mip.berkeley.edu/essig/endins/xerces.htm
http://www.xerces.org/Xerces.htm
*http://www.wholeearthmag.com/ArticleBin/421.html
and several other webpages with the same information