The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts

TLDR

Climate‑driven changes in biotic interactions can profoundly alter ecological communities, especially when foundation species are affected, as seen in marine systems where altered herbivory can trigger dramatic phase shifts from coral to macroalgae or from algal forests to barrens. The study proposes that tropical herbivores expanding poleward will drive a new phase shift away from macroalgal dominance in temperate reefs. This shift is enabled by poleward‑flowing boundary currents that create ocean warming hotspots, allowing tropical species to expand their range and increase grazing rates in temperate areas. Overgrazing by tropical herbivorous fishes has already been observed in Japan and the Mediterranean, and emerging evidence indicates similar events are occurring in other temperate regions with rising tropical fish presence.

Abstract

Climate-driven changes in biotic interactions can profoundly alter ecological communities, particularly when they impact foundation species. In marine systems, changes in herbivory and the consequent loss of dominant habitat forming species can result in dramatic community phase shifts, such as from coral to macroalgal dominance when tropical fish herbivory decreases, and from algal forests to ‘barrens’ when temperate urchin grazing increases. Here, we propose a novel phase-shift away from macroalgal dominance caused by tropical herbivores extending their range into temperate regions. We argue that this phase shift is facilitated by poleward-flowing boundary currents that are creating ocean warming hotspots around the globe, enabling the range expansion of tropical species and increasing their grazing rates in temperate areas. Overgrazing of temperate macroalgae by tropical herbivorous fishes has already occurred in Japan and the Mediterranean. Emerging evidence suggests similar phenomena are occurring in other temperate regions, with increasing occurrence of tropical fishes on temperate reefs.