Global declines in structurally complex habitats are reshaping both land and seascapes in directions that may change how biological communities respond to warming. Here, we test whether the widespread loss of kelp habitats through overgrazing by sea urchins changes fish community structure in directions that systematically alter warming sensitivity. We use simulations and comparisons of communities from 5996 sites across 19 ecoregions to test for thermal diversity shifts related to habitat. We find that the realized thermal affinities and ranges of fishes from kelp and urchin barrens differ, but only in regions with high initial response diversity. Fish communities in warm-temperate barrens host relatively more warm-affinity species than neighbouring kelp beds, highlighting that urchin herbivory can exacerbate tropicalization processes. By contrast, relatively cool-affinity species colonize cool-temperate barrens and explain apparent lags with ocean warming in these locations. Evidently, urchins are agents of ecological change with implications for climate resilience.

Global declines in structurally complex habitats are reshaping both land and seascapes in directions that affect biological communities’ responses to warming. Here, we test whether widespread loss of kelp habitats through sea urchin overgrazing systematically changes warming sensitivity of fish communities. Community thermal affinity shifts related to habitat were assessed by simulating and comparing fish communities from 2,271 surveys across 15 ecoregions. We find that fishes in kelp and urchin barrens differ in realized thermal affinities and range sizes, but only in regions where species pools have high variability in species’ thermal affinities. Barrens on warm-temperate reefs host relatively more warm-affinity fish species than neighbouring kelp beds, highlighting acceleration of tropicalization processes facilitated by urchin herbivory. By contrast, proportionally more cool-affinity fishes colonize barrens at high temperate latitudes, contributing to community lags with ocean warming in these regions. Our findings implicate urchins as drivers of ecological change, in part by affecting biological resilience to warming.