Estimating Population Size of a Rare Damselfly to Support Reintroduction Efforts

August 2017 - Estimating the population size of a rare species is incredibly useful for their management and conservation. One of the rarest Odonates (damselflies/dragonflies) in the U.S., the San Francisco forktail damselfly (Ischnura gemina) occurs in only a few sites around the Bay Area, including the Presidio’s Fort Point.
Male forktail damselfly with an identifying number penned onto its wing
This male forktail damselfly was captured as a part of a study to estimate the size of the population at Fort Point.

Presidio Trust / Jonathan Young

Over the last year, the National Park Service (NPS), Presidio Trust, and San Francisco Zoo began a captive propagation program for the reintroduction of this damselfly to the Presidio’s Mountain Lake. The zoo’s propagation protocol has since been honed to perfection and, to date, has resulted in the release of well over 2,000 nymphs from approximately 20 captured adults. Now that this captive breeding program has gotten off the ground the group is seeking to not only continue the efforts at the lake, but expand releases to other suitable sites throughout the Presidio.

The biggest question now is how many adults can be collected at the source (Fort Point) without significantly impacting that fragile population. A mark-recapture study was carried out in July to address this question. The team, including NPS, Presidio Trust, and zoo staff and interns, focused on males as they are easier to spot and identify. Individuals were captured, their wings delicately numbered with a fine-tipped pen, and then placed in a cooler. The small site was scoured for several hours until all visible males were collected.

A total of approximately 40 males were marked and released on the first day. The following day the team again attempted to capture as many males as possible, noting how many were recaptures. From these numbers they were able to estimate the male population size at approximately 117 individuals. Assuming a sex ratio of 1:1 (as supported by local entomologist Dr. John Hafernik) they now have a good idea of the total population size at the source, which will help inform future collection target goals. Contact Jonathan Young with questions about this project.