Cascading effects of global warming: evidence for the impact of range-expanding thermophilic species invasion on resident species

Global warming is affecting marine biodiversity in a multitude of ways, promoting several direct and indirect effects. Across the Mediterranean basin, rising temperatures are greatly favouring the northward expansion and invasion of thermophilic predator species, that may impact the populations of resident prey. As a model example of this phenomenon, two species are considered: the thermophilic annelid Hermodice carunculata, a generalist predator which is expanding northward; and its prey Parazoanthus axinellae, a boreal zoanthid which is particularly sensitive to the increasing temperature. With the hypothesis that the spreading of H. carunculata may affect the P. axinellae population, monitoring activities were conducted in the Capo Carbonara Marine Protected Area (NW Mediterranean) in 2020 and repeated in 2021. Moreover, the interaction between the two species was investigated under controlled conditions in the laboratory. A significant correlation between the increasing abundance of H. carunculata and the decreasing cover of P. axinellae was observed in the field. Laboratory experiments showed that the consumption of P. axinellae by H. carunculata increased significantly after exposing P. axinellae to a thermal stress and was concentrated on zoanthids showing signs of necrosis due to the heat shock. Considering the ongoing global temperature increase, the predation by H. carunculata could not constitute the only cause of the observed reduction of P. axinellae: it can be seen as an element of a cascading effect of global warming that could further affect the zoanthid populations, which are already suffering from rising temperatures.