Butterfly index concentration showed no clear median change for 27 species on complete British transects, 1995–1999 to 2015–2019

We tested whether butterfly abundance indices became more concentrated among complete monitored transects. We used the UK Butterfly Monitoring Scheme 2024 release. The main panel has 27 species and 132 transects. Each species has records from the same 30–76 transects in 1995–1999 and 2015–2019. We compared shares of length-adjusted site indices from different years. The median concentration change was −13.36%. The central 95% of 2,999 shared spatial and year resamples was −26.68% to +5.66%. This range does not establish a median change. Twenty-one species had a lower point estimate. A comparison of 2000–2004 with 2020–2024 gave +2.12% on a different complete panel, with a range of −17.35% to +17.39%. The period result also changed with the eligible sites. The sample has 131 English transects and one Welsh transect. It cannot establish a trend across Britain or a change in occupied area. The package has unchanged source files, provenance, all code, figures, and checks. A fresh run matched all 67 numeric output files.

butterflies · UKBMS · species distributions · spatial concentration · survey coverage · reproducibility