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Abstract
Butterflies may serve as indicators of biodiversity trends, but for this purpose reliable methods of monitoring their distribution and abundance are essential. We discuss advantages and disadvantages of the currently used methods and suggest potential refinements, based on methodological advances achieved in other organisms. While assessing butterfly distribution, it is vital to account for imperfect species detection at investigated sites. This can be achieved through conducting repeated presence-absence surveys within a single season, and analyzing data with statistical models that estimate detection probability and site occupancy by a species. Transect counts, predominantly used for assessing butterfly abundance in monitoring programs, are cost-effective and easy to implement, but less reliable than mark-release-recapture sampling frequently applied for the same purpose in research studies. Deficiencies of transect counts stem from the fact that they do not account for individual detection probability and temporal fragmentation of butterfly populations, i.e., the situation in which just a small fraction of individuals belonging to a single generation is present on any day of a season. Consequently, transect counts can only yield relative abundance indices, which presumably correlate well with daily butterfly numbers, but not necessarily with their seasonal population sizes. Possible refinements to transect counts that would allow the estimation of individual detection probability include double observer or double zone approaches. In contrast, finding an effective way to estimate longevity (a measure of temporal fragmentation) with transect counts seems impossible. Instead, efforts should be made to evaluate how variation in longevity affects transect-count results.