Climate change and invasive species interact to impact succession and diversity in Gulf of Maine marine fouling communities

Gulf of Maine subtidal communities have changed in recent years resulting from anthropogenic pressures such as climate change and the introduction of non-indigenous species. Sea surface temperatures have risen while salinity is predicted to decline; concurrently invasive tunicates (ascidians) became conspicuous members of fouling communities. These abiotic and biotic changes may lead to shifts in species composition, diversity and succession. The goals of these studies were to examine the relationship between depressed salinities and invasive tunicate species and to determine the impact of climate change and invasive species on diversity, succession and community homogeneity. Anthropogenic impacts on diversity, succession, species composition and homogeneity were assessed through field experiments and comparing community development and environmental parameters between two studies (1979 to 1982 and 2003 to 2006). Since the initial study between 1979 and 1982, three invasive colonial ascidians (Botrylloides violaceus, Diplosoma listerianum and Didemnum sp.) and one encrusting bryozoan ( Membranipora membranacea) have become members of the fouling community. Results from these studies revealed a 33% change in species composition since 1982, succession between 1979 and 1982 was directional, leading towards a stable climax community and diversity was maintained by secondary substrate provided by the hard shell of the blue mussel, Mytilus edulis. In contrast, succession between 2003 and 2006 was directional in the first two years only, becoming more unpredictable in year 3 and year 4; diversity was not maintained by secondary substrate but by primary substrate due to the seasonal life-history characteristics of the invasive species. Interestingly a four season development was observed between 1979 and 1982 and a three season development was detected between 2003 and 2006. The smaller number of stages was linked to a rise in temperature that facilitated the longevity and dominance of warmer water species