Estimating abundance for three endangered Florida Keys butterflies

Background/Question/Methods

Rigorous abundance estimates can be used to establish baselines, set recovery targets, and assess management actions, all of which are essential aspects of evidenced-based natural resource management. Unfortunately, for many endangered species these estimates do not exist and management decisions are based largely on anecdotal information. This is particularly true for rare butterflies, largely due to the time investment needed to collect rigorous data as well as the difficulty in applying standard abundance estimation methods to rare butterfly species. We develop the first quantitative abundance estimates for three rare butterflies in the Florida Keys: Miami blue (Cyclargus thomasi bethunebakeri) – US endangered, Bartram’s scrub-hairstreak (Strymon ascis bartrami) – US endangered, and sawgrass skipper (Euphes pilatka klotsi) – US candidate. For Miami blues we adapted distance sampling from point transects (a method typically used for birds) for use with butterflies, for Bartram’s scrub-hairstreaks we compared density estimates derived from double observer and traditional distance sampling methods, and for sawgrass skippers we used traditional distance sampling methods. In addition to densities, we estimated total potential suitable habitat for all three species.

Results/Conclusions

We detected three periods of peak abundance for Miami blue butterflies: Spring 2012, Summer 2012, and Summer 2013. All three periods were similar; peak daily densities ranged from 590 (95%CI: 500-702) to 680 (95%CI: 574-806) butterflies ha^-1. Total potential habitat for Miami blues was 18 hectares. In summer 2013 we estimated peak daily Bartram’s scrub-hairstreak density to be 12 ± 3 butterflies ha^-1 with distance sampling and 13 ± 4 butterflies ha^-1 with double observer method. These butterflies occupied 13 ha of suitable habitat. We surveyed sawgrass skipper flight seasons in Spring 2013 and both Spring and Fall 2014. We estimated butterfly density ha^-1 to be: 8.7 (95%CI: 5.4 – 13.9), 12.5 (95%CI: 9.11 – 17.1), and 12.9 (95%CI: 8.6 – 19.2), respectively. Potential sawgrass skipper habitat included 409 hectares. These are the first estimates of potential habitat and density for all three butterflies. Because of their rarity, each species requires some type of habitat management. Our results will allow land managers to better target management actions where they are most necessary and to assess the effectiveness of actions. Additionally, our estimates provide a baseline against which future surveys can be compared to assess long-term trends in each butterfly’s status.