Evaluating differences in density estimation for central Iowa butterflies using two methodologies

Abstract

The Pollard-Yates transect is a widely used method for sampling butterflies. Data from these traditional transects are analyzed to produce density estimates, which are then used to make inferences about population status or trends. A key assumption of the Pollard-Yates transect is that detection probability is 1.0, or constant but unknown, out to a fixed distance (generally 2.5 m on either side of a transect line). However, species-specific estimates of detection probability would allow for sampling at farther distances, resulting in more detections of individuals. Our objectives were to (1) evaluate butterfly density estimates derived from Pollard-Yates line transects and distance sampling, (2) estimate how detection probabilities for butterflies vary across sampling distances and butterfly wing lengths, and (3) offer advice on future butterfly sampling techniques to estimate population density. We conducted Pollard-Yates transects and distance-sampling transects in central Iowa in 2014. For comparison to densities derived from Pollard-Yates transects, we used Program DISTANCE to model detection probability (p) and estimate density (D) for eight butterfly species representing a range of morphological characteristics. We found that detection probability among species varied beyond 2.5 m, with variation apparent even within 5 m of the line. Such variation correlated with wing size, where species with larger wing size generally had higher detection probabilities. Distance sampling estimated higher densities at the 5-m truncation for five of the eight species tested. At this truncation, detection probability was <0.8 for all species, and ranged from 0.53 to 0.79. With the exception of the little yellow (Pyrisitia lisa), species with median wing length <5.0 mm had the lowest detection probabilities. We recommend that researchers integrate distance sampling into butterfly sampling and monitoring, particularly for studies utilizing survey transects >5 m wide and when smaller species are targeted.

Keywords: Butterfly; Detection probability; Distance sampling; Pollard-Yates; Program Distance; Wing length.

Figure 1. Correlation of butterfly detectability and wing length.

Species-specific butterfly detection probabilities for eight Iowa butterflies were derived from distance sampling at a 5-m truncation. The resulting detection probabilities correlated (r = 0.91) with species’ median wing length (mm) (Opler, Lotts & Naberhaus, 2010), used as a proxy for size.

Figure 2. Butterfly density estimates from two survey techniques.

Species-specific butterfly densities (number/ha) are compared for eight Iowa butterflies. Densities were derived from Pollard-Yates line transects and transects incorporating distance sampling, with data truncated to observations ≤ 2.5 m, 5.0 m, and 50 m. Surveys were performed in Iowa in 2014. Vertical bars depict 95% confidence intervals for distance sampling estimates.