SAN FRANCISCO (BCN)

Ecologists Friday afternoon continued their re-introduction of the Variable Checkerspot butterfly to the Presidio in San Francisco, a city known for the number of butterflies that have gone extinct there, an ecologist said.

At noon, about 300 butterfly larvae were released at El Polin Spring near MacArthur Avenue following a collection this morning on San Bruno Mountain in San Mateo County.

The goal of officials of the trust is to release 1,500 larvae by Monday and wildlife ecologist Jonathan Young said he thinks they'll easily meet that goal.

Scientists have released 1,000 since Monday.

"It's a new era in the Presidio," Young said because so many opportunities exist for re-introducing extinct species.

San Francisco is known for the number of butterflies that have gone extinct in the city. The city is famous for being the last place where the Xerces Blue butterfly was seen before it went extinct, according to Young.

Xerces Blue disappeared in 1941, according to the University of California at Davis.

The last Variable Checkerspot seen in the Presidio was in 1978, Young said.

It's functionally extinct in San Francisco because even though it exists in the Laguna Honda area, the genetic diversity is so small that "it's just a matter of time" before the population disappears, according to Young.

In the Presidio, the Variable Checkerspot is also being reintroduced behind The Presidio Landmark apartments, on the western end of the park.

Ecologists are releasing 750 larvae in both locations. Young said the hope is that the butterflies that emerge from around the apartments will fly into and populate the Lobos Creek area.

"We're hoping they will spread throughout the park in suitable habitat," Young said.

The Variable Checkerspots start emerging from their cocoons in mid-April but may start earlier this year, Young said. The lifespan of the butterfly is only about 15 days.

Young is hoping that a couple hundred will be flying around each Presidio location when they all finally emerge.

The habitat for the Variable Checkerspot was wiped out when the Presidio was a U.S. Army post from 1846 to 1994, when it became a national park.

Now there is plenty of habitat to sustain a long-term healthy population, Young said.

Some previously extinct animals have been reintroduced recently to the Presidio.

Several hundred three-spine stickleback fish were introduced to Mountain Lake in April 2015 and Western pond turtles were released into Mountain Lake in September 2015.

"There's a whole suite of species that no longer exist," Young said.

Ecologist plan to release Variable Checkerspot larvae each year for the next two to three years until the population can sustain itself.