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

Abstract

Aim: Global declines in structurally complex habitats are reshaping both land-and seascapes in directions that affect the responses of biological communities to warming. Here, we test whether widespread loss of kelp habitats through sea urchin overgrazing systematically changes the sensitivity of fish communities to warming. Location: Global temperate latitudes.Time period: Modern. Major taxa studied: Fishes.Methods: Community shifts in thermal affinity related to habitat were assessed by simulating and comparing fish communities from 2271 surveys across 15 ecoregions. Results:We found that fishes in kelp and urchin barrens differed in realized thermal affinities and range sizes, but only in regions where species pools had high variability in the thermal affinities of species. Barrens on warm temperate reefs host relatively more warm-affinity fish species than neighbouring kelp beds, highlighting the acceleration of tropicalization processes facilitated by urchin grazing. In 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 ecological resilience to warming.

Marine protection enhances the resilience of biological communities on temperate rocky reefs

“…The absence of keystone species, such as top predators in protected zones, allows for an increase in the density of species like sea urchins (Costello et al, 2022). These sea urchins, functioning as herbivores, prompt a pronounced reduction in the coverage of marine algae, potentially exerting adverse effects on the structural composition of the biological community (Costello et al, 2022; Schuster et al, 2022). The complexity of ecological systems suggests that various interacting factors may influence the outcomes, and our study, with its existing data, does not allow us to definitively identify a singular cause.…”

“…The absence of keystone species, such as top predators in protected zones, allows for an increase in the density of species like sea urchins (Costello et al, 2022). These sea urchins, functioning as herbivores, prompt a pronounced reduction in the coverage of marine algae, potentially exerting adverse effects on the structural composition of the biological community (Costello et al, 2022;Schuster et al, 2022).…”

Quantifying the ecological consequences of climate change in coastal ecosystems

“…This process, known in low-to mid-latitudes as tropicalisation, in combination with the closely related process of deborealisationthe loss of cool-associated species from highlatitude placesresults in the continual reassembly of biotic communities in coastal waters (McLean et al, 2021). When tropicalisation involves the arrival of herbivores, seagrasses and macroalgal habitat can be rapidly transformed (Vergés et al, 2016(Vergés et al, , 2022Schuster et al, 2022;Santana-Garcon et al, 2023), reshaping entire ecosystems and their functioning (Peleg et al, 2020). But other arriving taxa can have equally profound impacts (de et al, 2022).…”

Taxa‐dependent temporal trends in the abundance and size of sea urchins in subtropical eastern Australia

“…In temperate ecosystems, high sea urchin abundance can result in phase shifts from macroalgae dominated to sea urchin and coralline algae dominated systems, termed ‘barrens’ (Eklöf et al., 2008 ; Filbee‐Dexter & Scheibling, 2014 ; Ling, 2008 ; Ling et al., 2019 ; Pederson & Johnson, 2008 ; Steneck et al., 2002 ). Although sea urchins can also be abundant along subtropical transition zones, where they are hypothesised to accelerate topicalization through grazing (Schuster et al., 2022 ), the population ecology of sea urchin species with different thermal affinities is not well understood.…”