When plants and animals are introduced to a new region, they're sometimes so successful that they become pests, choking out or destroying the native species. In fact, it's estimated that invasive species constitute the second biggest danger facing the world's biodiversity, close behind that old favorite, habitat destruction.
So why do invasive species thrive in their new habitats? One theory is that they have an unfair advantage over the natives: They have to deal with fewer parasites and pathogens. The average animal species is plagued by 16 parasites, all of which have specially adapted to make its life miserable. Think of this as nature's system of checks and balances. Now, when that species migrates, it typically only brings three parasites along, and, on average, only four parasites in the new habitat will adapt to attack it.
Keep It In Check
So suddenly, the invading species has to contend with less than half of the diseases and parasites it had to deal with back on its home turf, and nature's regulatory system is thrown out of whack. For example, on the shores of Europe, parasitic barnacles keep the European green crab population in check by castrating many of them. When the green crab migrated to the shores of America, the parasite that kept it under control was left behind. This enabled the crab to flourish and, in the process, destroy native shellfish populations.
So one way to deal with invasive species might be to introduce their enemies to the new habitat as well. Of course, the danger here is that these parasites and pathogens might find the habitat's native species attractive and start attacking them, too.
"Conservation Biology: Parasites Lost" (Nature)