Species travelling across oceans from one place to another has always happened in the past. Our wanderings around the world by ship has increased the problem by providing additional opportunities for animals to find new homes and that has changed significantly over the past 250 years. Sometimes the problem of invasive species 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.
The problem is even worse now there are millions of tonnes of plastic floating in our oceans.