Monitoring Butterfly Abundance: Beyond Pollard Walks

Jérôme Pellet,Jason T Bried,Patrick O Heer,David Parietti,Daniel Cherix,Antoine Gander,Raphaël Arlettaz

doi:10.1371/journal.pone.0041396

Abstract

Most butterfly monitoring protocols rely on counts along transects (Pollard walks) to generate species abundance indices and track population trends. It is still too often ignored that a population count results from two processes: the biological process (true abundance) and the statistical process (our ability to properly quantify abundance). Because individual detectability tends to vary in space (e.g., among sites) and time (e.g., among years), it remains unclear whether index counts truly reflect population sizes and trends. This study compares capture-mark-recapture (absolute abundance) and count-index (relative abundance) monitoring methods in three species (Maculinea nausithous and Iolana iolas: Lycaenidae; Minois dryas: Satyridae) in contrasted habitat types. We demonstrate that intraspecific variability in individual detectability under standard monitoring conditions is probably the rule rather than the exception, which questions the reliability of count-based indices to estimate and compare specific population abundance. Our results suggest that the accuracy of count-based methods depends heavily on the ecology and behavior of the target species, as well as on the type of habitat in which surveys take place. Monitoring programs designed to assess the abundance and trends in butterfly populations should incorporate a measure of detectability. We discuss the relative advantages and inconveniences of current monitoring methods and analytical approaches with respect to the characteristics of the species under scrutiny and resources availability.

Highlights

Assessing species abundance is a fundamental requirement in ecology and conservation [1,2]
Butterflies and day-flying moths are widely believed to be reliable sentinels of environmental variation and human disturbance, with changes in distribution and abundance mirroring landscape, habitat and climate change [6,7,8,9,10,11]. Both species richness and species abundance estimates for butterflies usually rely on fixed-route transects
Individual detectability in M. dryas was as low as 10% whereas in I. iolas it was almost ten times higher (97%)

Summary

Introduction

Assessing species abundance is a fundamental requirement in ecology and conservation [1,2]. As it is impractical to survey all taxa, conservationists typically focus on groups that are expected to reflect broader biodiversity patterns, ecological change, or ecosystem integrity This ‘‘coarsefilter’’ approach typically relies on plants or vertebrates as surrogates or umbrellas for insects and other invertebrates, but evidence is mixed on whether indirect conservation of invertebrates is effective (e.g., [4,5]). Butterflies and day-flying moths are widely believed to be reliable sentinels of environmental variation and human disturbance, with changes in distribution and abundance mirroring landscape, habitat and climate change [6,7,8,9,10,11] Both species richness and species abundance estimates for butterflies usually rely on fixed-route transects (or Pollard walks, see [12,13,14]). These counts are aggregated for each site (e.g., using the sum of the average weekly counts) to produce a species-specific abundance index [16]; sometimes the maximum count is used as the index

Methods

Results

Conclusion