Annual butterfly index changes showed no clear rise in mean species correlation at 42 English sites in 1985–2024

We tested whether annual abundance-index changes became more correlated between different butterfly species at the same sites. We used the UK Butterfly Monitoring Scheme 2024 release. A fixed sample retained 2,161 local species pairs, 42 species and 42 English standard transects. We compared 1985–2004 with 2005–2024. Both years of each annual change had to be within a period. Each pair had at least 14 shared changes per period. We gave each site equal weight. Mean Pearson correlation rose from 0.169 to 0.182. The change was +0.014, with a nominal 95% interval of −0.076 to +0.098 from 1,999 shared three-year block samples. A calculation that also sampled spatial groups gave −0.082 to +0.110. Higher abundance and coverage thresholds and positive-only log changes also gave intervals that included zero. Removing the transition ending in 2013 reversed the small point change. The sample showed positive mean correlation in both periods, but it did not resolve the direction of change. It still permits a substantial rise. The selection rules condition on persistent, sufficiently abundant species and long records. The source indices do not separate detection, index estimation and biological change. We did not estimate a climate cause, total-abundance stability or extinction risk. The package preserves source data, provenance, selection records, code, all results, figures and offline repeat instructions.

butterflies · interspecific synchrony · UKBMS · abundance indices · fixed species pairs · time block bootstrap · ecological monitoring