Shifts in the abundance and location of species with warming have been documented from the tropics to the poles. Given that such shifts lead to community restructuring with the potential for changes in trophic interactions, it is presently unknown whether habitat protection will promote resilience to global change drivers including warming and invasive species. We take advantage of a natural experiment to test for biodiversity responses in protected and fished temperate reef communities in southeast Australia that have been regularly surveyed over a 21 year period. During this observation window, the region has undergone rapid warming and invasion by a range-extending urchin (Centrostephanus rodgersii) that grazes macroalgal beds and forms extensive rock barrens. We used a generalized additive mixed-effects modelling framework to contrast responses in the community thermal index (CTI, the average realized temperature niche for all species present), invasion status, and different functional groups (trophic level: animals and structural group: algae) at sites with and without protection.
Results/Conclusions
Overall, a regional tropicalization has occurred in the brown macroalgae and fishes, particularly so in the sessile invertebrates, detected as an increase in the CTI. The tropicalization signal is due to increases in the local richness and abundance of warm-affinity species, and with increases in the urchin. Protection from fishing, however, strongly influenced the structure of these transitional communities, including colonization by the urchin. Protected communities are distinguished by gaining canopy-forming algae and resisting colonization by small foliose algae, low trophic level warm-water fishes and the urchin. By contrast, herbivorous and omnivorous invertebrates have declined in protected areas and communities where carnivorous fishes and lobsters are most abundant. Our findings thus implicate the role of biological interactions in affecting how communities transition with warming, and highlight the potential for predation to not only decelerate range expansions, but also accelerate range contractions.