Variations in importance and intensity of competition underpin context dependency in the effects of an invasive seaweed on resident assemblages

Biological invasions are an important component of global biodiversity changes. Distinguishing between intensity and importance of competition may foster our understanding of the mechanisms underpinning context dependency in the ecological impacts of invasions. The importance of competition represents the effect of competition in regulating species performance in relation to that of the whole set of environmental drivers. In contrast, the intensity of competition estimates the difference in the performance of one or more target species between the presence and the absence of competition. We re-analyzed previously published data to evaluate how intensity and importance of competition from the invasive seaweed, Caulerpa cylindracea, on resident macroalgal assemblages changed following nutrient enrichment. Adding nutrients did not alter the intensity of competition from C. cylindracea on species richness, but decreased its importance. In contrast, both the intensity and importance of competition from C. cylindracea on community cover increased following nutrient enrichment. Thus, a greater impact of C. cylindracea at enhanced nutrient levels was not solely the result of increased intensity of competition, as could be concluded without taking into account that the performance of target organisms is generally impacted by multiple drivers. These results suggest that the importance of competition from exotic species depends on the extent to which native species are constrained by biotic interactions versus abiotic factors. Distinguishing between intensity and importance of competition seems crucial for assessing the ecological impacts of invasive species within a multiple stressor context.