Butterflies with low-nitrogen host plants did not have more negative trends at UK transects with high nitrogen deposition in 1990-2024

We tested if UK butterflies whose larvae feed on low-nitrogen plants had more negative trends at monitoring transects with high nitrogen (N) deposition. We used 12,912 site-by-species abundance trends from 700 UK Butterfly Monitoring Scheme transects (1995-2024) and 44 resident species. Each species received the mean British Ellenberg N value of its main host plants. Each site received the CBED 1986-1995 total N deposition. A two-stage model estimated the deposition effect on site trends for each species, then a random-effects meta-regression related these effects to host-plant N. The hypothesis predicts a positive slope. The primary slope was -0.90e-3 (95% CI -1.98e-3 to 0.17e-3; species-permutation p = 0.11), opposite in sign. The independent periods 1990-2006 and 2007-2024 gave small positive slopes with intervals that included zero, so the sign did not repeat. Three latitude bands, NHx, NOx, later deposition, deposition change, other host-plant scores, a pooled two-way fixed-effects model and observation-bias checks (long or high-count series, extra site controls, site weights that match UK deposition shares) gave no hypothesis-consistent result. Sea-salt chloride, a negative control, gave a slope near zero. SOx gave the only nominal result (p = 0.02), also opposite in sign, and it does not stay significant after correction for six exposures. Thus, host-plant N preference did not predict which species declined more at high-deposition transects. Limits: observational data, non-random UKBMS sites, no public WCBS random-square indices, coarse 5 km deposition and 1 degree climate, few species, and possible N effects before 1990. The package has code, derived trends, site covariates, host-plant scores, results and figures.

nitrogen deposition · butterflies · UKBMS · Ellenberg N · host plants · air pollution · population trends · species traits · CBED · null result · observation bias