The Thorne’s hairstreak butterfly — the tiny insect only found in San Diego whose population has suffered from frequent wildfire — has taken a small step toward becoming an endangered species.

The U.S. Fish and Wildlife Service, the federal regulatory agency, agreed to evaluate the butterfly for listing under the Endangered Species Act, which comes as part of a settlement with the Center for Biological Diversity, an environmental group.

The decision, published Monday in the Federal Register, kicks off an in-depth evaluation to determine whether the butterfly warrants protection. If it’s listed, the federal government would have to take steps to ensure the small insect doesn’t go extinct.

The hairstreak feeds exclusively on the rare Tecate cypress tree, and its population is only known to exist around Otay Mountain, which has been hit twice by fire in the last seven years. In 2003, when the entire mountain burned over, some biologists feared the hairstreak had gone extinct. The insect’s habitat was also impacted by construction of a new segment of border fence.

Environmental groups have worked since 1991 to get the thumbnail-sized beige butterfly listed. The Center for Biological Diversity sued after the federal government rejected a 2004 request for listing. Internal agency documents the center obtained through open records laws showed that Fish and Wildlife Service biologists had recommended considering the butterfly for listing, only to have their recommendations overruled.

The review is expected to take a year.

— ROB DAVIS

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