Species migration and what makes an invasive species

As I have mentioned, I have been enjoying The Next Great Migration by Sonia Shah. And the book continues to surprise me.

After blowing up some long-held conventional wisdom about how humans came to colonize some of the more obscure parts of the globe in Chapter 7, Chapter 8 made me think long and hard about invasive plant and animal species.

Foe example, the assumption among biologists and ecologists, and certainly among laymen like myself, is that invasive species = "BAD". Period. The name itself says so, and there are whole university courses aimed at "invasion biology". But just how black and white is this issue?

I confess I had always wondered how scientists establish whether a particular species is native or not. Doesn't it depend on when you are comparing it with? Does native mean pre-colonial? Pre-human? Pre-ice-age? Go back far enough and we arrive at the single continent of Pangaea, and then how do we think of native habitat? The species that live in a certain area are constantly changing, and always have done, as rock core samples and fossil records show with great aplomb. 

And the more we research, using GPS trackers, genetic analysis and other technologies, the more - and the more improbable - changes and routes and methods of transportation of plant and animal species we discover. 

And some of them elude us still, such as the highland tamarind trees of Réunion and the almost identical koa trees of Hawaii, 18,000 kilometers away, with no obvious means of transportation between the two. To take another example, the camel is usually considered a Middle Eastern animal, but, in fact, the camel family originally evolved and attained its greatest diversity in North America; it is currently most diverse in South America; and it only actually occurs in the wild in ... Australia!

Another issue, though, is whether what we choose to describe as "invasive" species are wholly and necessarily bad. Recent research suggests that only 10% of newly-introduced species establish themselves, and only 10% of those (i.e. 1% of original number) flourish in ways that can threaten existing resident species. And a surprisingly small number of those actually displace (i.e. cause exinctions in) local species. Indeed, some studies have shown that newcomers can actually INCREASE biodiversity (this one I find a little difficult to believe, and Ms. Shah suggests that the study in question was rejected by Nature periodical (suppressed, she hints at, because of its radicalness, although Nature is not normally so shy and tentative).

Furthermore, she suggests that the economic and ecological benefits of wild migrants are not always included in cost-benefit analyses. For example, zebra mussels are considered wholly bad, but they filter water very effectively and provide food for fish and waterfowl (and it is argued, less convincingly, that they cannot be held responsible for the collapse of native clams, which were struggling anyway).

Other papers have questioned whether European purple loosestrife, which has become so ubiquitous in North America, actually "kills wetlands" or "creates biological deserts", as is commonly claimed. And it is undeniably pretty, and that must count for something, right?

Not all of these arguments seem totally convincing to me, although they are apparemtly based on real papers by real biologists and ecologists. But it's certainly food for thought. Some of our long-held beliefs and presuppositions may be starting to crack.