Considerable geographic variation exists in the extent of biological invasions, across local to global scales. The importance of evolutionary history for invasion dynamics was noted by Darwin, and recent studies are beginning to consider how biodiversity of source regions can influence the probability of invasions. Paired bodies of water with different evolutionary histories, but connected in recent time by canals, provide a strong comparative framework for testing mechanisms that affect patterns of invasions. For example, Panama provides an exceptional opportunity to test for invasion asymmetry across two tropical oceans at similar latitudes. The opening of the Panama Canal in 1914 created an aquatic corridor and new shipping route connecting the Atlantic and Pacific basins. The resulting large influx of commercial ships over the past century suggests Panama may be a potential hotspot for tropical marine invasions and provide insight into the ecological and evolutionary mechanisms shaping patterns of invasions. We used standardized quantitative comparisons and settlement plate experiments to test the hypothesis that invasion across the Canal is asymmetrical, with more invasions in the less diverse Pacific compared to the Atlantic. We also tested whether predation, a suspected mechanism shaping invasion, limits the abundance and distribution of species on the Atlantic and Pacific sides of the Canal.
Total species richness of sessile marine invertebrates (bivalves, bryozoans, tunicates, barnacles, hydroids and polychaete worms was higher in the Atlantic compared to the Pacific. However, introduced species richness was higher in the Pacific (18 species) compared to the Atlantic (11 species). We detected an asymmetrical exchange of species across oceans; about 30% of the introduced species on the Pacific were from the Atlantic compared to only 9% in the opposite direction. While the number of introduced species was higher on the Pacific, almost half occurred at only one site, whereas invaders were more evenly distributed in the Atlantic. Further, predators reduced the total cover of shared introduced species by almost 50% in the Pacific but did not have a similar effect in the Atlantic. This pattern was predominantly driven by the reduction of dominant introduced species by macropredators. These results are consistent with the idea that high diversity in source regions and ecological interactions between species shape patterns of marine invasions.