Beyond the tropics: ocean warming, shifts in species interactions and the tropicalisation of temperate reefs

Adriana Vergés (1), Hamish Malcolm (2)

1 UNSW Australia, School of BEES, NSW 2052, Australia,, @adriatix

2 NSW Department of Primary Industries, Coffs Harbour NSW 2450, Australia,

Climate-driven changes in species interactions can profoundly alter ecological communities, particularly when they impact foundation species. In marine systems, changes in plant-herbivore interactions can lead to the loss of dominant habitat forming species such as corals when tropical fish herbivory decreases, or to declines in algal forests when temperate urchin grazing increases. Emerging evidence from Japan and eastern Australia indicates that ocean warming is causing a novel type of phase-shift in tropical-temperate transition zones, whereby tropical and subtropical herbivores are overgrazing the canopy forming algae that typically dominates shallow temperate reefs. In eastern Australia, we use a ten-year (2002-2012) video dataset that encompassed a 0.6°C increase in mean sea surface to quantify patterns of kelp-herbivore interactions in a tropical-temperate transition zone. Kelp was present on 70% of surveyed reefs in 2002, declined through time and completely disappeared from 2010 onwards. Simultaneously, grazing incidence on kelp increased steadily from <10% to over 70% in the years preceding kelp disappearance. The proportion of tropical and herbivorous species in fish communities increased as kelp declined. Experimental video evidence shows transplanted kelp is quickly consumed within hours by tropical rabbitfish (Siganidae) and drummers (Kyphosidae) in reefs from where it has disappeared, where large densities of grazing surgeonfish dominate consumption of algal turfs. Our results suggest that climate-mediated increases in herbivory pose a very significant global threat to kelp dominated ecosystems and the communities they support.