Spatial and temporal dynamics of ascidian invasions in the continental United States and Alaska

Species introductions have increased dramatically in number, rate, and magnitude of impact in recent decades. In marine systems, invertebrates are the largest and most diverse component of coastal invasions throughout the world. Ascidians are conspicuous and well-studied members of this group, however, much of what is known about their invasion history is limited to particular species or locations. Here, we provide a large-scale assessment of invasions, using an extensive literature review and standardized field surveys, to characterize the invasion dynamics of non-native ascidians in the continental United States and Alaska. Twenty-six non-native ascidian species have established documented populations on the Pacific, Atlantic and Gulf coasts (spanning 25–57°N). Invader species richness is greatest for the Pacific coast (19 spp.), followed by the Atlantic (14 spp.) and Gulf (6 spp.) coasts, and decreases towards higher latitudes. Most species (97 %) expanded their range after initial introduction, although the direction and latitudinal extent of secondary spread varied. Temporal analyses, based on literature reported first records and repeated field surveys, show an increase in recorded non-native ascidians at continental, regional, and local scales. Our results underscore that non-native species continue to establish and spread, and the transfer of biofouling organisms on underwater surfaces of vessels is an active and potent vector that remains largely unmanaged. More broadly, we suggest that ascidians provide a tractable and important indicator group for evaluating invasion dynamics and management strategies.