Behavioural shifts of Diadema savignyi and their impact on coral communities around Rapa Nui

Abstract

A complex system of feedbacks between species and their surroundings often generates ecosystems at relatively stable states. Perturbations in such a system can lead to regime shifts especially in cases of bistability. Here the behavioural shifts of the sea urchin Diadema savignyi were first determined empirically at Rapa Nui. After which scenarios were created in a cellular automaton model to test whether these shifts were important to consider in the ecosystem, namely by testing the impact of herbivory pressure on the total covers of corals and algae. In bare dominated microhabitats, a doubled foraging rate as well as spatial aggregation were observed for urchins. These behaviour switches did not lead to evident changes in the model. However a scenario in which urchins also forage on coral recruits did show a decrease in coral cover as urchin density increased. Empirically around the island it was proposed increased pressures from human activities or wave exposure reduced the recruitment rates of corals to such an extent that urchin grazing may have led to barren seascapes at certain sites.