Abstract
<jats:p>Aim Detecting how long-term climate warming reshapes ecological communities requires moving beyond species turnover to examine how the climatic space available for community assembly itself becomes reorganized. We tested whether sustained warming restructures moth assemblages through a contraction and displacement of occupied thermal space, a mechanism capable of driving reassembly without conspicuous compositional change. Location Serchio Valley, central Italy (Bagni di Lucca, 200 m; Monte Prato Fiorito, 800 m) Time period 1978–1980 and 2022–2024, spanning four decades of documented regional warming. Taxon: Moths (Lepidoptera, Noctuoidea); 314 species. Methods Assemblages were sampled across the full flight season (April–October) using a standardised protocol, with 20-day intervals between occasions. We integrated three complementary approaches: community ordination (NMDS, PERMANOVA, PERMDISP); kernel-based characterization of occupied thermal space coupled with niche-overlap tests (Schoener's D, equivalency and similarity tests) and species-level ridge-penalized logistic regressions projecting occurrence under a +2 °C scenario. Climatic predictors were drawn from ERA5 reanalysis data. Results Community composition underwent a directional, non-random reorganization, with historical and recent assemblages clearly separated at both sites. This shift was accompanied by a pronounced displacement and contraction of the occupied thermal space (Schoener's D = 0.458 overall; equivalency and similarity tests p < 0.001), with contemporary communities confined to a narrower, warmer thermal envelope, even where species richness remained broadly stable. Species-level projections revealed non-random filtering: the strongest predicted declines concentrated in univoltine taxa associated with cooler conditions, whereas species with broader ecological amplitudes and flexible life-histories persisted or expanded. Main conclusions Long-term warming reduces the dimensionality of viable climatic space, selectively retaining species able to occupy a progressively narrower and warmer thermal envelope. This reshapes community structure before widespread turnover or local extinctions become apparent, meaning substantial ecological reorganization can remain invisible to approaches focused solely on species identity.</jats:p>