Rare forktail damselflies introduced at Presidio’s Mountain Lake

Adult forktail damselflies will be abundant around the Presidio’s Mountain Lake by spring if the larvae that an ecologist planted in the north arm of the body of water grow to maturity. Adult forktail damselflies will be abundant around the Presidio’s Mountain Lake by spring if the larvae that an ecologist planted in the north arm of the body of water grow to maturity. Photo: Marianne Hale, Handout Photo From San Francisco Zoo Photo: Marianne Hale, Handout Photo From San Francisco Zoo Image 1 of / 1 Caption Close Rare forktail damselflies introduced at Presidio’s Mountain Lake 1 / 1 Back to Gallery

A tribe of young insects now populates Mountain Lake in the Presidio, adding new wildlife to thrive among the fish, frogs, turtles and mussels already transplanted into what was once a mud-filled habitat for ugly weeds and abandoned koi pets.

On a rainy Friday afternoon, Presidio ecologist Jonathan Young dumped a plastic bucketful of water holding more than 1,000 tiny larvae of rare forktail damselflies into the shallows of the lake’s north arm, where they will mature through stages until March, and then emerge from their watery nursery to skim over the lake as colorful inch-long adults.

Similar to dragonflies but smaller and extremely rare, the San Francisco forktails are known to live at only one place in San Francisco now: a small patch of watery lowland at Fort Point, on the rocky edge of the bay, where they are threatened by the breaking waves and every storm surge.

“That population is doomed there,” Young said.

Which is why San Francisco Zoo intern Susanna Ngoi went to the Fort Point site a month ago and collected four male and four female adult forktails, and watched them as the females laid more than 2,000 eggs to hatch into larvae.

The San Francisco forktails were once officially listed as endangered, but they were delisted when they were found again briefly in two other locales.

“The species is so rare that I practiced how to collect and breed them first with three generations of Pacific forktails, another species that are more common and that already live at the zoo,” Ngoi said. “That way I could develop my really safe protocol for insect husbandry with the San Francisco forktails.”

Ngoi bred the forktails at the San Francisco Zoo under the watchful eye of Jessie Bushell, the zoo’s conservation director.

The females laid “lots and lots of eggs,” Ngoi said, and one female alone produced anywhere from 200 to 1,000 eggs at a time. The eggs quickly developed into tiny larvae so small she needed a microscope to observe them, but they grew quickly, so by Friday each of the larvae Young released in Mountain Lake was at least large enough to be visible.

The insect larvae are voracious, and so are the adult forktails, Young and Ngoi said. They thrive on small fish, baby tadpoles and zooplankton. But they have predators in Mountain Lake’s birds, turtles and spiders, and even their larger relatives, the dragonflies. The two are members of the same order of insects called the Odonata.

Friday’s introduction of damseflies to Mountain Lake, where Ohlone Indians once fished and the early Spanish explorers once camped, marked the latest stage in the lake’s restoration by the Presidio Trust.

The site just north of the Inner Richmond District has long been a victim of the city’s growth. It is polluted by fumes from cars heading to the Golden Gate Bridge and a dumping ground for refuse, including unwanted pet goldfish. Presidio officials began dredging it and cleaning its shores four years ago.

Now its water is certified as drinkable, the Presidio’s scientists have replanted dozens of native aquatic plants, and the lake’s long-gone animal species — the fishes called three-spined sticklebacks, the California chorus frogs, and the rare Western pond turtles — have been restored to the water. The first batch of freshwater mussels called California floaters are maturing in baskets in the lake, to be planted on the bottom when they’re ready, Young said.

David Perlman is The San Francisco Chronicle’s science editor. Email: dperlman@sfchronicle.com