Geographic variation in non-native species richness and propagule supply for coastal marine ecosystems in North America

Over 300 non-native species of invertebrates and algae are known to have established self-sustaining populations in coastal estuaries of the United States, and the non-native species richness (NNSR) varies two orders of magnitude among estuaries. Various factors may cause the observed spatial variation, including differences in propagule supply, but their relative contributions remain largely unexplored. We explored the relationship between NNSR and propagule supply, focusing specifically on shipping as a supply mechanism. On a national scale, commercial shipping is a dominant source of non-native species that have colonized estuaries, contributing strongly to an increase in detected invasions in recent time. U.S. ports receive > 100,000 ship arrivals per year that can transport marine organisms on the hulls and in ballast water. We estimated how much of the observed variation in NNSR among estuaries can be explained by differences in number of ship arrivals, wetted (underwater) surface area, and ballast water discharge volume, as proxies for ship-mediated propagule supply. Our results suggest that the magnitude of propagule supply alone explains little of the observed variation among estuaries. Thus, although invasion success (establishment of new populations) is expected to increase for any one site with propagule supply, it is apparent that spatial variation of invasion magnitude includes strong effects of source and/or recipient systems. Current analyses are now explicitly testing for effects of source and recipient region attributes on NNSR.