Morphological traits determine detectability bias in North American grassland butterflies

Abstract

Ecological research often includes observing and counting individuals to obtain a population estimate or index that can inform conservation, management, and policy. However, the ability to accurately estimate wildlife populations has always been hindered by bias, and researchers aim to overcome bias by using correction factors that calculate the relationship between observer counts and true counts. Accurate estimates of butterfly populations, for example, are especially necessary to inform management and policy, with over 40 species listed or proposed for listing under the United States Endangered Species Act. Researchers can utilize methods like line-transect distance sampling (LTDS) to help account for detection bias and calculate more accurate estimates, but species-specific traits or behaviors may influence survey effectiveness. We used LTDS to detect nearly 35,000 individuals of 33 species across five studies to calculate butterfly species' effective strip widths (ESW)—a type of correction factor—across grasslands in the Great Plains, USA. To better understand how species' traits influence detectability, we modeled the influence of species' morphological, life history, and behavioral traits on ESW. The average ESW was 5.42 m, but varied from 1.84 to 12.6 m. We found that morphology (size and color) impacted the ability of observers to detect butterflies, with larger and brightly colored butterflies detected farther away from observers compared to smaller and dull-colored butterflies. Additionally, observers were generally better at detecting individuals while they were flying and nectaring compared to resting. Surprisingly, species' life history and ecological traits did not help explain detectability differences. As conservation efforts continue to increase for butterflies, improved estimates of their population size will be necessary to evaluate management strategies and aid conservation decision-making for future policy. Future surveys need to consider butterfly size and color, adjusting weather protocols when necessary, to minimize and account for bias associated with butterfly species, especially if accurate population estimates are a study goal.