Five butterfly species on 39 English transects showed no clear mean rise in the correlation of annual index changes between 1985–2004 and 2005–2024

We tested whether annual butterfly abundance-index changes became more synchronized between 1985–2004 and 2005–2024. A fixed-pair analysis retained five species, 39 English transects and 1,326 species-specific pairs at distances of 10–327 km. Indices were transformed as ln(1 + index), then differenced between consecutive years. Pair correlations were averaged within species, with equal weights across species. The mean correlation rose from 0.297 to 0.327. The change was +0.030, with a nominal 95% interval of −0.101 to +0.133 from 1,999 shared three-year block draws. The interval assumes that short year blocks represent variation within each period. A lower coverage rule retained 26 species at 82 English transects. Its change was +0.056, with an interval of −0.019 to +0.111. Positive-only and detrended-level checks also had intervals that included zero. Removing the transition ending in 2013 reversed the primary point change. The data show shared fluctuations in both periods but do not resolve the sign of the mean change. They still permit a substantial increase. The sample is not random. Index estimation and changes in detection can affect correlation. No climate cause or extinction risk was estimated. The package preserves source data, provenance, code, all results, figures and reproduction instructions.

butterflies · UKBMS · spatial synchrony · abundance indices · ecological change · year block bootstrap · fixed site pairs · reproducibility