Invasive seaweeds may functionally compensate for the expected loss of endemic temperate species in the fast-warming Mediterranean Sea

Background and Aims Climate change and bioinvasions can cause dramatic shifts in community composition followed by alterations to ecosystem functioning and services. Traditionally invaders are considered as having mostly negative impacts on the environment. However, it has recently been suggested that thermally adapted species introduced to climate change hotspots might potentially compensate for native loss, thus preserving their ecosystem functions. To test this, we investigated the eco-physiology of two dominant invasive seaweeds of the Levantine shallow reefs, Lobophora schneideri and Galaxaura lessepsiana, and then compared them with the endemic, thermally sensitive Gongolaria rayssiae.Methods We followed structural (i.e. percentage cover and biomass) and physiological traits at different ambient conditions. Ex situ metabolic rates were assessed at ambient seawater temperature and under thermally manipulated conditions through thermal performance curve experiments. Based on the estimated physiological performances, we obtained thermal habitat suitability projections under predicted future scenarios for the Levantine basin.Key Results The invaders' calculated annual inorganic carbon uptake was 1.5-2 times higher than that of the endemic G. rayssiae, mostly related to their high biomass year-round, while the native is fully branched only between late winter and early summer. Furthermore, the gross primary productivity thermal optima of L. schneideri and G. lessepsiana (30.7 and 31.7 degrees C, respectively) were higher than that of G. rayssiae (24.5 degrees C). Habitat suitability modelling indicated a likely replacement of the native distribution by the invasive species, with potential variation between the two Levantine invaders.Conclusions Some of the reef metabolic functions such as oxygen production and carbon uptake, possibly lost with the extinction of the native species due to warming, could be maintained by dominant invasive thermophilic species. These results highlight the importance of investigating and understanding the complex role of non-indigenous invasive seaweed species in compensating and regulating ecosystem metabolic processes and services, otherwise potentially lost in tropicalized environments.