People are still ironing out the definitions used in invasive species ecology. Specifically, what constitutes an "invasive" species is a subject of contention among scientists and policy makers alike. Some contend that an invasive species has to have been introduced by humans, either accidentally or intentionally.
Of the species that are naturally dispersing into new habitats on a regular basis, there are very few that can do so on a continental scale. (One example that comes to mind is the Sea Beans, a group of plants whose water-dispersed seeds can cross oceans.) Now comes this story from the BBC about marine creatures hitchiking across the ocean on trash. For thousands of years, organisms like mollusks and corals have attached to natural debris that floats on ocean currents. But now surveys show that more than half of the debris found near the Antarctic is man-made. Not only is there a lot more debris than is natural, but it is even easier for organisms to survive on the more-durable artificial materials. So, if one of these species starts spreading aggressively, is it invasive or not? (Story tip from The Daily Grist)