Of detectability and camouflage: evaluating Pollard Walk rules using a common, cryptic butterfly

Abstract

Estimating distribution and abundance of species depends on the probability at which individuals are detected. Butterflies are of conservation interest worldwide, but data collected with Pollard walks—the standard for national monitoring schemes—are often analyzed assuming that changes in detectability are negligible within recommended sampling criteria. The implications of this practice remain poorly understood. Here, we evaluated the effects of sampling conditions on butterfly counts from Pollard walks using the Arctic fritillary, a common but cryptic butterfly in boreal forests of Alberta, Canada. We used an open population binomial N‐mixture model to disentangle the effects of habitat suitability and phenology on abundance of Arctic fritillaries, and its detectability by sampling different conditions of temperature, wind, cloud cover, and hour of the day. Detectability varied by one order of magnitude within the criteria recommended for Pollard walks (P varying between 0.04 and 0.45), and simulations show how sampling in suboptimal conditions increases substantially the risk of false‐absence records (e.g., false‐absences are twice as likely than true‐presences when sampling 10 Arctic fritillaries at P = 0.04). Our results suggest that the risk of false‐absences is highest for species that are poorly detectable, low in abundance, and with short flight periods. Analysis with open population binomial N‐mixture models could improve estimates of abundance and distribution for rare species of conservation interest, while providing a powerful method for assessing butterfly phenology, abundance, and behavior using counts from Pollard walks, but require more intensive sampling than conventional monitoring schemes.

Butterfly foraging is remarkably synchronous in an experimental tropical macrocosm

“…minimum body temperature) [7]. For instance, in field conditions, noon can be too hot for butterflies to forage, and some days might be too cold for their activity [14,24]. Abiotic minima are particularly important in determining daily foraging patterns in limiting environments, such as alpine and arctic systems, where a variety of adaptations evolved to facilitate butterfly activity [7,8,34].…”

“…Abiotic conditions determine whether butterflies can forage—see, e.g. the importance of thermal balance and wind speed for butterfly activity [5,7,8,14,15]. Conversely, biotic limits are usually less important in determining butterfly diel activity.…”

“…light and temperature), confusing interpretations. There seems to be temporal trends in activity even after controlling for abiotic conditions [14], but evidence of endogenous timing mechanisms remains rare [9]. Meanwhile, abiotic conditions impose a strong selection on butterfly behaviour [7,8,19,20], suggesting that butterfly activity might be mostly mediated by physiological constraints.…”

Does a short Pollard walk transect capture butterfly and bee diversity? A test to inform pollinator monitoring and community science initiatives

“…Most data collected via monitoring programmes assume constant detection within recommended sampling conditions and suffer from imperfect detection, which affects estimates of species occurrence and abundance, and therefore of beta diversity patterns (Riva et al, 2018, 2020). It is likely that longer transect lengths would, to some extent, increase the accuracy of biodiversity estimates due to a higher sampling effort.…”

“…This is due to many reasons, including site selection bias (diverse sites overrepresented in monitoring programmes; Buckland & Johnston, 2017), spatial coverage bias (e.g. most sites sampled near roads; Didham et al, 2020), and imperfect detection (distribution and abundance of rare and/or hard to detect species often underestimated; Riva et al, 2018, Riva et al, 2020). Following these considerations, expanding ongoing monitoring programmes to cover larger spatial extents, at a higher spatial resolution, and for more taxa seems necessary to resolve when, where, and why we are experiencing insect declines.…”