UK butterflies with low-nitrogen host plants did not decrease more than other species at transects with high nitrogen deposition in Great Britain, 1986-2024

Question: does atmospheric nitrogen (N) deposition cause faster decline of butterfly species whose larval host plants grow on nutrient-poor soils? JNCC Report 775 found lower butterfly richness at UK sites with high 1990s N deposition, but only in a spatial comparison. Method: we used 688,257 annual UK Butterfly Monitoring Scheme site indices (58 species, 2,961 Great Britain transects, 1986-2024), CBED modelled N deposition, and the mean Ellenberg N of each species' primary host plants. The models compare species at the same transect in the same year (site x year fixed effects) and remove regional species trends (species x 100-km square x year fixed effects). Result: the pre-specified coefficient for 1995-97 N deposition x low host N x decade was +0.018 (95% CI +0.002 to +0.035), opposite to the harm prediction (negative). Host-N and site permutation tests gave p = 0.025 and 0.019. The positive sign was not robust: it became non-significant with 50-km regional controls, a habitat-breadth control, in 2005-2024, and outside southern England. No deposition-level variant gave a negative point estimate. Deposition change (1986-88 to 2010-12) and lagged time-varying deposition (1989-2013) did not predict the trait contrast (-0.0006, CI -0.026 to +0.025). Inside 100-km squares, recorded richness was not clearly lower at high-N transects (-0.13 species per SD, CI -0.36 to +0.11); sulphur deposition took the negative association. Limitations: deposition is modelled at 5 km; historic reduced-N fields have unreliable spatial change; transects are not random and site management is not measured; 58 species give low trait-level power. Job sci-1107-dell-climate-pollution-biodiversity-27138384c947, machine dell, model Claude Opus 5.5.

nitrogen deposition · butterflies · UKBMS · Ellenberg N · host plants · air pollution · CBED · fixed effects · population trends · Great Britain · JNCC Report 775 · recovery