Have you heard of a gifted survivor called the Brown Tree Snake? Boiga Irregularis, as it is known, reached Guam about 50 or so years ago, probably as an accidental passenger in American military planes. Since then, in the absence of predators and in the presence of plentiful prey, all utterly unprepared to deal with this new predator that arrived so abruptly, it has driven several of the island’s indigenous bird species to extinction.
In Out of Eden, Burdick goes in search of these invasive species. The result of that labour is rather good. Nature writing can be predictable. A mere catalogue of cases where invasives are creating chaos (I should probably mention that invasives are the largest threat to biodiversity today after habitat destruction). But burdick is after larger, more philosophical questions.
As you move through the book, the questions keep changing. The Brown Tree Snake is indeed one of the best known instances of invasives. But is it representative? All invasives do not end up as wildly successful as Boiga Irregularis. Even if, given that they hail from crowded, competitive ecosystems, and might therefore fare well in island ecosystems (which have more niches available, and are less competitive), that doesn’t mean they will decimate the local species. How do invasions work anyway? Do they happen at a species level, or are they a larger phenomenon than that, with one ecosystem gradually replacing another?
There has always been species invasion. A bird blown off course by a storm. A python, too dim to hope it will make landfall, wrapped around a log bobbing through the sea. But the process has accelerated in recent times. Take ships. They have carried ballast water across the world in their holds. When the time came to take on cargo, they dumped that ballast, containing all sorts of species from the original port. Settlers travelling to colonise new lands invariably lugged their favorite species to make their new homes more familiar, more welcoming.
There is a huge, gnarly, man-made question lurking around here. How does one even begin to quantify the extent of invasions? When one is studying an environment, what does one compare it to? The wake of human maritime history, writes Burdick, washed over marine biological history so thoroughly, and so long ago, that it is impossible now to envision what the seascape might have looked like without us.
The answer lies in distinguishing between alpha and beta diversity, he concludes towards the end. All invasives may not annihilate local species. there might indeed be a jump in the number of species in a location. That is alpha diversity. Beta diversity is the relative diversity among such locations, between, say a new york harbour and the san francisco bay. It is, as he says, a fancy measure for homogenisation.
The same damn species everywhere.
How does nature work? How should one visualise it? Is it a finely tuned machine, like cogworks or the insides of a watch, liable to grind to a halt if too many loose screws are tossed in? Or is an ecosystem instead like an airplane: remove some critical rivets – the native species integral to its structure – and the entire infrastructure crashes to the ground? Some scientists refer to these rivets as “keystone species”, evoking less an aircraft than a vaulted cathedral. Or perhaps an economic analogy is more apt: an ecological free market of producers and consumers, all competing for limited natural resources, all buying, stealing, or otherwise exchanging the nutritional equivalent of energy vouchers. Is it a machine, an edifice, an organism, a community-watch programme, an international bank?