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

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.

Long-term empirical evidence of ocean warming leading to tropicalization of fish communities, increased herbivory, and loss of kelp

“…It confirms, for the east coast of Australia, the global model of herbivore "tropicalization" of temperate seaweed communities proposed by Vergés et al (16). Using a 10-y video dataset, field experiments, and a simulation model, we demonstrate that increases in the proportion of tropical herbivores and an overall intensification of herbivory led to the loss of ecologically and economically important kelp forests in a warming tropical-temperate transition zone.…”

“…Climate projections estimate that ocean isotherms will continue to shift poleward at a rate seven times faster in the 21st century than the 20th century (43). As tropical and warm-temperate herbivores respond to these isotherm shifts worldwide (16,44), climateinduced increases in herbivory are emerging as a new threat of global proportions to valuable algae-dominated temperate reefs and the important ecosystem functions they support. (Fig.…”

“…In contrast, at lower latitudes near the warm edge of distribution of many kelp species, deforestation has been generally linked to oceanographic anomalies in temperature, nutrients, or salinity (14,15). Recently, however, the range expansion of tropical herbivorous fishes has been proposed as a new and potentially important driver of kelp deforestation (16). A recent review identified 80 species of tropical and subtropical herbivorous fishes expanding their ranges into temperate reefs in association with polewardflowing boundary currents around the globe (16).…”

“…Recently, however, the range expansion of tropical herbivorous fishes has been proposed as a new and potentially important driver of kelp deforestation (16). A recent review identified 80 species of tropical and subtropical herbivorous fishes expanding their ranges into temperate reefs in association with polewardflowing boundary currents around the globe (16). The impacts of these warmwater herbivores can be much greater than those of native species, as documented in the eastern Mediterranean (17,18).…”

Trends and frontiers for the science and management of the oceans

“…In many cases, changes in environmental stress are driving marine organisms in search of more hospitable environments. A particularly striking phenomenon is the 'tropicalisation' of temperate marine ecosystems that sit just outside tropical latitudes [25]. Unlike tropical coral reefs, temperate rocky reefs are often dominated by large fl eshy seaweeds.…”