Most introduced species apparently have little impact on native biodiversity, but the proliferation of human vectors that transport species worldwide increases the probability of a region being affected by high‐impact invaders – i.e. those that cause severe declines in native species populations. Our study determined whether the number of high‐impact invaders can be predicted from the total number of invaders in an area, after controlling for species–area effects. These two variables are positively correlated in a set of 16 invaded freshwater and marine systems from around the world. The relationship is a simple linear function; there is no evidence of synergistic or antagonistic effects of invaders across systems. A similar relationship is found for introduced freshwater fishes across 149 regions. In both data sets, high‐impact invaders comprise approximately 10% of the total number of invaders. Although the mechanism driving this correlation is likely a sampling effect, it is not simply the proportional sampling of a constant number of repeat‐offenders; in most cases, an invader is not reported to have strong impacts on native species in the majority of regions it invades. These findings link vector activity and the negative impacts of introduced species on biodiversity, and thus justify management efforts to reduce invasion rates even where numerous invasions have already occurred.

An invasive benthivorous fish, the Eurasian round goby (Neogobius melanostomus) is abundant throughout the lower Great Lakes -St. Lawrence River system. We examined the round goby's potential to alter benthic communities on cobble substrates in the upper St. Lawrence River. During the summers of 2008 and 2009, macroinvertebrates and benthic algae were sampled across sites with varying goby densities. Archived data from various sites in 2004-2006 (prior to invasion) were available for comparison. Macroinvertebrate community composition varied significantly among samples grouped into categories based on goby density and time since invasion. Macroinvertebrate diversity and dominance by large-bodied taxa declined with increasing goby density. Surprisingly, dreissenid biomass did not vary consistently with goby density, in contrast to studies in the Great Lakes. The biomass of all non-dreissenid taxa was negatively correlated with increasing goby density across sites and over time at three of four sites. Negative effects were most pronounced on the biomass of gastropods. Benthic algal biomass increased with goby density across sites, suggesting a trophic cascade driven by the impacts of gobies on gastropods and other algivores. Our study highlights the potential ecosystem impacts of an expanding goby population in a large river.Résumé : Le poisson envahissant, consommateur de benthos, le gobie à taches noires d'Eurasie, Neogobius melanostomus, est abondant dans l'ensemble du système des Grands Lacs inférieurs et du Saint-Laurent. Nous étudions le potentiel qu'a le gobie à taches noires pour modifier les communautés benthiques sur les substrats pierreux dans la région amont du SaintLaurent. Durant les étés 2008 et 2009, nous avons échantillonné les macroinvertébrés et les algues benthiques dans un ensemble de sites présentant des densités diverses de gobies. Il existe des données en archives pour plusieurs sites en 2004-2006 (avant l'invasion) pour servir de comparaison. La composition des communautés d'invertébrés varie significativement dans les différents échantillons regroupés en catégories d'après la densité des gobies et le temps depuis l'invasion. La diversité des macroinvertébrés et la dominance des taxons de grande taille diminuent en fonction de l'augmentation de la densité des gobies. De manière inattendue, la biomasse des dreissénidés ne varie pas de manière cohérente en fonction de la densité des gobies, comme elle le fait dans les Grands Lacs selon certaines études. Dans trois de quatre sites, la biomasse de l'ensemble des taxons moins les dreissénidés est en corrélation négative avec l'accroissement des gobies dans les divers sites et les différentes périodes. Les effets négatifs les plus prononcés s'observent sur la biomasse des gastéropodes. La biomasse des algues benthiques augmente en fonction de la densité des gobies dans les différents sites, ce qui laisse croire à une cascade trophique causée par les impacts des gobies sur les gastéropodes et les autres consommateurs d'algues. Notre étude soulign...

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Aim  The transport of organisms in ships’ ballast tanks is a dominant vector for aquatic invasions worldwide. Until recently, efforts to manage this vector have overlooked the potential transport of invertebrate resting stages in the residual waters and sediments within emptied ballast tanks, i.e. NOBOB (‘No Ballast On Board’) tanks. The resting stages (statoblasts) of freshwater bryozoans are often buoyant and locally abundant and thus can be taken up easily during ballasting operations. They are also resistant to extreme environmental conditions and can generate new colonies after being dormant for decades; as such, they would likely remain viable propagules after lengthy transport in ship ballast tanks. This study quantified the occurrence of freshwater bryozoan statoblasts in ballast tank sediments of transoceanic ships.Location  North American Great Lakes.Methods  We quantified the frequency of occurrence, abundance and diversity of bryozoans (as statoblasts) in residual sediment samples taken from 51 NOBOB tanks of 33 transoceanic ships visiting the Great Lakes from 2000 to 2002.Results  Our study identified 11 species, comprising nearly 12% of the total number of freshwater bryozoans known worldwide. These include two exotic species unrecorded in the Great Lakes (Fredericella sultana and Lophopus crystallinus), an exotic species already established in the region (Lophopodella carteri) and three cosmopolitan species (Plumatella casmiana, P. fungosa and P. repens). Our estimates suggest that a ship with NOBOB tanks may carry up to 106 statoblasts.Main conclusions  The discovery of species unrecorded in the Great Lakes and the potentially large numbers of statoblasts being transported in ship ballast tanks indicate a significant risk of new species introductions. Furthermore, the presence of cosmopolitan species and an exotic species already established in the Great Lakes suggests the strong possibility of cryptic invasions via the introduction of exotic genotypes.

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