Climate Warming Is Causing an Increasing Dominance of Smaller Moth Species

Aim To test whether spatial trait–temperature relationships predicted by ecogeographic rules (Bergmann's rule and the thermal melanism hypothesis) can be used as space‐for‐time substitutions to anticipate temporal changes in community morphology under climate warming. Location Finland. Time Period 1993–2021. Major Taxa Studied Geometrid moths (Lepidoptera: Geometridae). Methods We analysed 29 years of standardised moth monitoring data spanning a 1200 km latitudinal gradient in Finland. For each moth community, we quantified mean wingspan and pigmentation as thermal morphological traits. We examined spatial relationships between community‐mean traits, latitude and temperature and assessed temporal trends in these traits in relation to long‐term changes in temperature. Results Across space, site‐level community‐mean trait composition over the study period showed that moth communities in colder northern regions were dominated by larger and darker species, whereas warmer southern communities comprised smaller and lighter‐coloured species, consistent with both ecogeographic rules. Through time, communities shifted towards smaller mean wingspan over the study period, consistent with the spatial expectations described by Bergmann's ecogeographic rule, while mean pigmentation exhibited no consistent directional change, contrary to expectations from the thermal melanism hypothesis. Main Conclusions Our results provide mixed support for space‐for‐time substitution in predicting climate‐driven changes in community morphology. While spatial patterns in wingspan successfully anticipated temporal shifts consistent with Bergmann's rule, spatial gradients in pigmentation did not translate into temporal change. These findings indicate that the predictive power of ecogeographic rules depends on the trait considered, and highlight both the promise and limitations of space‐for‐time approaches for forecasting climate‐driven reassembly of insect communities, particularly at high latitudes.