When the tiny help the mighty: facilitation between two introduced species, a solitary ascidian and a macroalga in northern Patagonia, Argentina

Facilitation is recognized as one of the mechanisms by which nonnative species are integrated into new assemblages. The solitary ascidian Styela clava and the macroalga Undaria pinnatifida were introduced to San Antonio Bay, Argentina, with a couple of years of difference. We studied the occurrence pattern of both species in the area and tested the hypothesis that S. clava facilitates U. pinnatifida through a manipulative experiment. Our results clearly suggest a facilitation process between these species. The probability of finding U. pinnatifida is 30% higher in sites where S. clava is present and higher recruitment of U. pinnatifida occurred where S. clava is present than where it had been experimentally removed. Increased habitat complexity by the stalked ascidian S. clava can facilitate the establishment of U. pinnatifida by providing refugee from grazers, increasing surface for settlement, or providing greater access to light.