Aquatic invasion patterns across the North Atlantic.

Biological invasions are a major driver of biodiversity loss and socioeconomic burden globally. As invasion rates accelerate worldwide, understanding past invasion dynamics is essential to inform predictions of future invaders and impacts. Owing to a high diversity of pathways and current biosecurity gaps, aquatic systems near urban centres are especially susceptible to alien species establishments. Here, we compiled and compared alien species lists for three different aquatic recipient regions spanning the North Atlantic: Chesapeake Bay, Great Lakes-St. Lawrence River and North and Baltic Seas. Each system is a major trade centre, with a history of invasions, and characterized by a strong natural salinity gradient. Our goal was to compare the alien species across systems, to test for similarities in the taxonomic composition and geographic origin as well as species overlap among the three regions. We selected specific macroinvertebrate, algal and fish taxa for analysis, to control for uneven taxonomic and biogeographic resolution across regions. Cumulatively, we identified 326 individual alien species established in these aquatic systems, with the North and Baltic Seas most invaded overall (163), followed by Great Lakes-St. Lawrence River (84) and Chesapeake Bay (79). Most invasions were from Ponto-Caspian, Eurasian, Northwest Pacific, Northwest Atlantic and North American origins, and mostly comprised Arthropoda, Chordata, Mollusca and Annelida. However, origins and taxonomies differed significantly among destinations, with Ponto-Caspian species particularly successful invaders to the North and Baltic Seas then Great Lakes-St. Lawrence River, but less so to Chesapeake Bay. Nevertheless, approximately eight-tenths of invaders established in only one region, indicating disparate invasion patterns and a high potential for future aliens to accrue from increasingly diverse source pools and pathways. These results support biosecurity strategies that consider a broad range of geographic origins and taxonomic groups to limit the translocation, arrival and spread of alien species worldwide.

Salinity tolerance and geographical origin predict global alien amphipod invasions.

Invasive alien species are driving global biodiversity loss, compromising ecosystem function and service provision, and human, animal and plant health. Habitat characteristics and geographical origin may predict invasion success, and in aquatic environments could be mediated principally by salinity tolerance. Crustacean invaders are causing global problems and we urgently require better predictive power of their invasiveness. Here, we compiled global aquatic gammarid (Crustacea: Amphipoda: Gammaroidea) diversity and examined their salinity tolerances and regions of origin to test whether these factors predict invasion success. Across 918 aquatic species within this superfamily, relatively few gammarids (n = 27, 3%) were reported as aliens, despite extensive invasion opportunities and high numbers of published studies on amphipod invasions. However, reported alien species were disproportionately salt-tolerant (i.e. 32% of brackish-water species), with significantly lower proportions of aliens originating from freshwater and marine environments (both 1%). Alien gammarids also significantly disproportionally originated from the Ponto-Caspian (20% of these taxa) when compared with all 'other' grouped regions (1%), and principally invaded Eurasian waters, with translocations of salt-tolerant taxa to freshwaters being pervasive. This suggests habitat characteristics, alongside regional contexts, help predict invasibility. In particular, broad environmental tolerances to harsh environments and associated evolutionary history probably promote success of aliens globally.

Do alternative resources dampen functional responses of native but not alien gammarids?

While aquatic invasive predators are among the most impactful trophic groups, we lack the understanding of whether alternative food resources mediate adverse predatory effects and stabilize native prey communities. Here, we use comparative functional responses to examine the influence of alternative food resources (Fucus sp.) on predator-prey interaction strengths from three gammarid crustaceans, with one native (Gammarus locusta) and two existing and emerging invasive (Gammarus tigrinus, Pontogammarus maeoticus, respectively) species, towards larval chironomid prey. All gammarids exhibited Type II functional responses, irrespective of the presence of alternative seaweed disks. Fucus sp. disks significantly reduced predation rates overall; however, significant reductions in maximum feeding rates (i.e., functional response magnitudes) were only evident in the native species and not for the two invaders. Our results thus may suggest that alternative resources dampen the predatory interaction strength of native but not invasive alien species, concerning these three study organisms. This potentially exacerbates the impacts of invasive predators relative to natives in diverse communities. Studies should increasingly consider alternative resources when quantifying ecological impacts of current and future invasive alien species compared with natives.