Invaders in waiting? Non-equilibrium in Southern Hemisphere seaweed distributions may lead to underestimation of Antarctic invasion potential

Abstract

Bioinvasions pose a major threat to global biodiversity. Correlative Ecological Niche Models (ENMs) can be a valuable tool to identify invaders and invasion sites. However, in cases when species are in non-equilibrium with their native environment (i.e. do not fill their niche), correlative approaches have limited power and invasions lead to shifts of the realized niche. In recent years, several new seaweed species have been reported in Antarctica. It is impossible to unequivocally identify which of these species are truly non-natives, however, here, we provide literature-based evidence that seaweed species have been introduced to Antarctica. Under this assumption, we reconstruct pre- and post-introduction niches of these species, calculate relative niche sizes and overlap between pre-Antarctic and Antarctic sites, and evaluate increase in niche size due to inclusion of Antarctic habitats. In seven species, the absolute occupied temperature range is dramatically enlarged, with minimum sea surface temperature (SST) being 2-5°C lower than in the pre-Antarctic ranges. In all species except one, summer SST is 5-20°C lower than in the pre-Antarctic ranges. As a result, several species’ niches increase dramatically. We hypothesize that species from the Southern Hemisphere do not cover their whole abiotically suitable range due to lack of settling substrate in cold-water regions while species from the Northern Hemisphere tend to fill their niches to a greater degree due to higher connectivity between tropic and polar regions along coastlines. Thus, while correlative ENMs for Northern Hemisphere species will probably be successful in predicting Antarctica as a suitable habitat, such models will likely be insufficient to do so for Southern Hemisphere species. From a precautionary standpoint, we argue that not only species from climatically matching regions pose an invasion threat for Antarctica, but that also species from other, climatically non-matching regions, might be potential invaders. In light of higher connectivity of the Antarctic continent with other continents this finding significantly increases invasion risk for Antarctica.