Temperature relations fitted before 2005 forecast 2005-2024 Swiss spring plant dates well, British butterfly flight dates weakly and butterfly population growth no better than shuffled climate

Question: which kinds of ecological climate response are robust enough to forecast? Earlier campaign tests found that four published relations failed or became smaller in newer data. Approach: one protocol for 107 units. We used MeteoSwiss phenology (19 spring and 8 summer-autumn phases, 1951-2024) and the UK Butterfly Monitoring Scheme 2024 release (43 species for mean flight date and 37 species for site-index growth, 1976-2024), with E-OBS temperature and rain of each site's grid cell. For each unit we selected one 1-3 month climate window in training years only (up to 2004), fitted a within-site linear relation (Gompertz density dependence for growth), froze it, and forecast 2005-2024. The baselines had no climate term. Skill is 1 - MSE/MSE(baseline) of year-mean anomalies, with year-bootstrap intervals. We repeated the window search ten times per unit with shuffled climate years. Results: the mean forward skill was 0.80 (95% CI 0.76 to 0.84) for spring plant phases, 0.57 for summer-autumn phases, 0.16 (0.09 to 0.22) for butterfly flight dates and -0.15 (-0.25 to -0.04) for butterfly growth; a fixed five-term weather model for growth gave -0.22. A 1994 split gave the same order. Growth forecasts were not better than shuffled-climate forecasts (Mann-Whitney p = 0.085). With shuffled climate the window search gave growth a median training |t| of 2.2, and growth units that passed this null in training were not more often skilful. All 33 units with a year-level training |t| of 5 or more had positive skill. Spring plant slopes were stable (test/train 0.92). Butterfly temperature relations tracked year-to-year variation (median detrended r 0.51) but forecast only 16% of the 4.3-day median advance in 2005-2024; the error was negative for 42 of 43 species and largest in species with hibernating adults or immigrants. Limitations: the plant-butterfly contrast also contains differences in recording method; we tested linear window models only; E-OBS cell means are not site temperatures; the null has 10 draws per unit; test periods have 20 years. Source data are not bundled; a retrieval script and hashes are given.