Distance sampling and the challenge of monitoring butterfly populations

Summary1. Abundance indices generated by the UK Butterfly Monitoring Scheme (UKBMS) have been influential in informing our understanding of environmental change and highlighting UK conservation priorities. Here, we critically evaluate the standard ‘Pollard Walk’ methodology employed by the UKBMS.2. We consider the systematic sampling biases among different butterfly species and biotopes using distance sampling. We collected over 5000 observations on 17 species using distance sampling at 13 study sites in England and Wales. We fitted detection functions to explore variation in detectability among species and sites.3. Our results suggest that around one‐third of individual butterflies in the Pollard Walk box were missed. However, detectability varies markedly among species and sites. We provide the first species‐specific estimates of detectability for converting Pollard Walk data into population densities. A few species show no drop off in detectability and most require only a modest correction factor, but for the least detectable species, we estimate that 3/4 of individuals are not recorded.4. Much of the variation among sites is explained by substantially higher detectability among sites in England than in Wales, which had different recorders. Biological traits have only limited explanatory power in distinguishing detectable vs undetectable species.5. The variation in detectability is small compared with the variation in true abundance, such that population density estimates from the Pollard Walk are highly correlated with those derived from distance sampling.6. These results are used to evaluate the robustness of the Pollard Walk for comparisons of abundance across species, across sites and over time. UKBMS data provide a good reflection of relative abundance for most species and of large‐scale trends in abundance. We also consider the practicalities of applying distance sampling to butterfly monitoring in general. Distance sampling is a valuable tool for quantifying bias and imprecision and has a role in surveying species of conservation concern, but is not viable as a wholesale replacement for simpler methods for large‐scale monitoring of multispecies butterfly communities by volunteer recorders.