Species travelling across oceans from one place to another has always happened in the past. However, our wanderings around the world by ship has increased the problem by providing additional opportunities for animals to find new homes. However, sometimes the problem can be catastrophic. The American Signal crayfish in the header image is wiping out the British White Clawed crayfish because it carries a virus against which its UK cousin has no defence.
An Issue Out of Control
Unfortunately the shear quantity of floating plastic debris in the oceans has meant that the transportation opportunities for alien species is increasing at a pace and this is a major threat to biodiversity. The small marine animals that hitch rides across the ocean are typically non-mobile and spend most of their lives attached to rocks, plants, shells, and other surfaces. The alien invaders include, but are not limited to bryozoans, barnacles, polychaete worms, hydroids, crabs and molluscs. Nature was able to cope with the rate of change before plastic started speeding things up. It is unlikely that it will manage to cope in the plastic age if it continues. You can read more here.