PS 80-204
Developing estimates to inform the management of an endangered butterfly: Field-based comparison of double-observer and distance sampling approaches
The estimation of animal abundance is a first step in assessing a species’ status and setting conservation priorities. 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 resource management. The two methods commonly used to estimate abundance and account for detection probabilities for butterflies are mark-recapture (capture-recapture) and distance sampling from line transects. Both methods have limitations in their application in assessing rare species. Although seldom used in lepidopteran research, the double-observer method has the ability to produce density estimates with relatively few detections and therefore could prove useful to resource managers of very rare butterflies. We used the Bartram’s scrub-hairstreak, a butterfly species recently proposed as endangered, as a case study to compare the efficacy of double observer and distance sampling methods to estimate abundance of a rare species. To do this, we first delineated all potential habitat on Big Pine Key, FL using a systematic grid-based method. We then walked a stratified random sample of 50m long transects collecting both double observer and distance data for each butterfly detected. We completed one island-wide survey each week during the 2013 flight period.
Results/Conclusions
We completed 16 island-wide surveys in 2013, detecting a total of 59 Bartram’s scrub hairstreaks. Using the program DISTANCE, we estimated peak Bartram’s density during the 2013 flight season to be 14 ± 4.4 butterflies per hectare, with an estimated detection probability of 0.3 (95% conf. interval 0.26-0.41). We analyzed our double-observer data in R using the multinomPois procedure in the UNMARKED package and estimated a similar peak density of 13 ± 4.5 butterflies per hectare, and detection probability for both observers of 0.67 ± 0.12. Both distance sampling and double observer methods (181 ± 57 and 169 ± 59, respectively) seem to produce comparable results in estimating densities of Bartram’s Hairstreak butterflies and could be used for other rare species as long as assumptions of the methods can be met. Our findings offer land managers of this species a grid-based approach to delineate habitat, and two alternative methods for determining densities and daily abundances based on site-specific needs and available resources. In a broader context, these data provide support for the use of double-observer methods for applications in rare butterfly surveys.