Patterns of distribution of the invasive alga Asparagopsis armata Harvey: a multi-scaled approach

Human activities are contributing to the homogenization of the world's biota by transporting thousands of species to areas outside their native range. Some of these species can lead to profound changes in the structure and function of natural ecosystems in areas where they are introduced, with dramatic economic and ecological impacts. The red alga Asparagopsis armata is considered an invasive species in Europe, but can be particularly abundant in the Azores (NE Atlantic), where there is virtually no information on its ecology. Here, we investigate the patterns of spatial distribution and impacts of A. armata in the Azores, as a first step to understand its potential spread. Adopting a hierarchical design, we surveyed the distribution of A. armata, and associated assemblages, at a range of spatial scales (from islands (10s of km apart), locations (kms apart), sites (10s of m apart) to quadrats (m apart)). Results showed that the abundance of A. armata varied significantly among locations and that there was substantial variability in its abundance among islands, locations and quadrats and little variability among sites. At the scale of locations, the abundance of A. armata did not correlate with any of our predictor variables (sea urchin density, Latitude, Longitude and sea temperature). However, at the quadrat scale, there was a significant and negative correlation between the abundance of A. armata and diversity of associated macrophytes, as well as, with the abundance of the conspecific A. taxiformis and the abundance of most of the remainder macroalgal groups. The potential role of biotic and abiotic factors in generating the observed patterns is discussed. This study further highlights the suitability of the analytical tools used here to examine patterns of distribution over a range of spatial scales and its applicability in the field of aquatic invasions.