Rare Damselflies in Distress: Scientists Work to Sustain an At-Risk Species in the Presidio

October 2021 - This year has been a roller coaster for scientists keeping tabs on the Bay Area’s most at-risk insect—the San Francisco forktail damselfly. They’ve had some good news—the first population estimate in five years revealed stable numbers despite the drought—but also been confronted with diminishing water levels threatening the species' remaining stronghold like never before.

Like their dragonfly cousins, damselflies have long, slender, often-colorful abdomens. But they’re smaller and more dainty than dragonflies, their eyes don’t touch, and when they’re resting, they fold their wings vertically along their bodies. San Francsco forktails are a joy to spot, in their shades of iridescent black, green, and blue (males), and gold (females). In decades past, they fluttered over year-round seeps and streams from San Mateo to Marin Counties. These days, San Francisco forktails may not exist beyond the Presidio of San Francisco, where their future is also uncertain due to droughts and sea level rise. The seep-fed pool that is their main home is at the bottom of a steep bluff, right by the road leading to Fort Point. It’s maybe two dozen feet from San Francisco Bay, and barely above sea level.

This species can handle water that isn’t the cleanest, but salt water or no water? Not so much. Year-round fresh water is key for the damselflies’ survival. Adults live and breed above the surface for only around 30 days. For their lifecycle to continue, they need water so they can lay their eggs in submerged plants. The eggs hatch into aquatic nymphs, which eventually climb out and molt, emerging as adults, and so on, month after month, from March to November. Over the winter months, it is the aquatic nymphs that sustain the population until the spring. If just one generation can’t reproduce at any point during the year, the chain is broken.

As the drought stretched on throughout the summer, Presidio and Golden Gate National Recreation Area staff kept a careful eye on the loss of open water by Fort Point. They trimmed back some dense vegetation and completed a sediment and topographic survey. They considered options like bringing in water from elsewhere as a last resort. Luckily, the seep bounced back and staff didn’t need to take more urgent action this year. Meanwhile, the topographic survey will help identify locations where sediments can be removed to help maintain the sparsely vegetated, open water habitat that the damselflies prefer.

In early summer, park researchers also did a population study of San Francisco forktails just like one they did in 2017. The goal was not only to see how they were handling the dry conditions, but to see how many adults the park’s San Francisco Zoo partners could capture for captive breeding.

They thought they’d find a smaller population, but in fact they found an estimated 102 males, very close to the 117 estimated previously. After an unsuccessful attempt to reintroduce the damselflies into Mountain Lake, park and zoo scientists began focusing on reintroducing them to a seep at West Crissy Bluff three years ago. They were able to proceed with reintroductions throughout the summer. So far, the damselflies released there seem well on their way to becoming a self-sustaining population. Researchers may do a population study there next year to be sure.

Even if the Fort Point damselflies can withstand more droughts and the West Crissy Bluff population takes hold, that is still just two small, still-vulnerable populations. So what is next for the San Francisco forktail? When those two populations are stable, scientists hope to look beyond the Presidio for more suitable places to reintroduce this special Bay Area resident.

For more information

• Contact Presidio Wildlife Ecologist Jonathan Young or Golden Gate Aquatic Ecologist Darren Fong
• Check out this amazing new San Francisco forktail segment from KQED’s Deep Look!

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