Tropicalization of temperate reef fish communities facilitated by urchin grazing and diversity of thermal affinities

Aim: 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 sensitivity of fish communities to warming.

Location: Global temperate latitudes

Time period: Modern

Major taxa studied: Fishes

Methods: Community thermal affinity shifts related to habitat were assessed by simulating and comparing fish communities from 2,271 surveys across 15 ecoregions.

Results: 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.

Main conclusions: Our findings implicate urchins as drivers of ecological change, in part by affecting biological resilience to warming.