Monday, September 23, 2013

Invasive species: a perspective

Gaillardia, a species originally native to Washington state, was brought to the Outer Banks of North Carolina sometime within the last century. It turned out to be an aggressive invader which has blanketed the dunes up and down the islands with flowers. It is now an accepted part of the landscape. Photo collection of the author.

In the last post, we examined a single microscopic version of species invasion. Today we're going to look at the question from a much larger perspective.

 First of all, the context. When you look around you, you see bugs, fish, birds, dogs, and so on. There are also larger creatures, of course; but if you look at the question in any detail, you'll find out that bugs outnumber them by a wide margin. Insects and arthropods, and other words, win when it comes to the numbers, as far as what we can see around us.

 Human beings tend to discuss species diversity in terms of larger, visible organisms. When big animals like rhinos are in danger of becoming extinct, alarm bells go off. A rule of thumb is the smaller the animal, the less fuss people make about it if it disappears. An exception for that rule can be made in cases where small animals form huge swarms or flocks, such as the passenger pigeon. Even here, though, size (the flock) basically matters; things we can't directly see don't worry us much, at least when it comes to species diversity.

 The greatest amount of species diversity by far, however, exists at the microscopic level. Bacterial and other microscopic lifeforms outnumber life forms we can see by what is probably many, many trillions to one. What that means is that there are so many more of them that their combined biomass easily eclipses all other life forms.  (For more detailed information on this, read this article by Stephen Jay Gould.) This has been known to biologists for quite some time: the dominant life form on Earth is bacteria. Human beings are an insignificant afterthought, in terms of numbers.

 The point is that if mankind killed off, say, 80 or 90% of all the visible organisms on earth—that is, I mean, organisms that could be seen with the naked eye—it would be a tiny, tiny fraction of the total biodiversity of all species on the planet, because of the unimaginably immense diversity found at the microbial level.  This may be small comfort to naturalists and environmentalists; but it is a scientific fact. The loss of diversity we currently see is deplorable, but it is only deplorable to us. From the perspective of the planet, such things have been seen many times, and they have always worked out rather well in the end.

Extinctions of up to 95% of all life forms on earth are not unheard of; we know of at least 5 major extinction events from the fossil record, each one of which was (at least so far) immeasurably worse than the current extinction events taking place because of the actions of human beings. In each case, life rebounded, and the earth completely repopulated itself with an impressive and even incredible variety of new species. The last major extinction event, the Cretaceous extinction event, was what made it possible for mammals to take over many of the niches in the biosphere formerly occupied by dinosaurs, and eventually led to the rise of man.

 The lesson to be learned from this is that extinction events, no matter what causes them, are anything but irrevocable, and that the biosphere is well equipped to recover from them. It turns out that even a tiny reservoir of species diversity will rebound and repopulate the planet.

The issue with invasive species crowding out and even extincting native ones is thus of little or no concern on geologic time scales. The process is, moreover, absolutely natural. To give an obvious example, the Hawaiian islands originally rose from the sea as barren lumps of lava. Every single plant and animal you now find in Hawaii arrived there as an invader of one kind or another. Successive waves of creatures competed with one another to form the rich habitats we see today; but each one of them, when it first arrived, disrupted and dislodged other species. The implication, in fact, is that a significant number of species on every continent first arrived there as foreign invaders.

So the process of species invasion is an entirely natural one. The fact that human beings have accelerated it is of obvious concern only to human beings who are negatively impacted by it; the loss of native species can be seen, from an overall perspective, as incidental and unworthy of control or management. While we may be alarmed at the loss of a single interesting or unusual species in our own time scales, that is, in the context of recorded history, nature takes these things into account and recovers from them over the long run. It will, assuredly, compensate for man's activities in one way or another, given enough time. This means that the majority of biologists and conservationists occupied with the idea of attacking problems of individual invasive species, and individual species loss, are probably misguided.

While the argument can well be made that habitat diversity is better for overall habitat health and the health of all species in general, the idea that we have any actual control over habitat diversity at this point in time is something of a fantasy. The forces of economic development are pushing forward relentlessly, in nearly completely autonomous actions that can't be constrained by bureaucrats, biologists, or small groups of fanatics arguing against them. Things, in other words, are bad and going to get worse, from any pragmatic point of view. But they will only be worse from the point of view of human beings. Nature, which has a nearly infinite reservoir of resiliency to it, will remain supremely indifferent to these temporary events.

 This may sound like a pessimist's point of view; but, as I've always pointed out, pessimist is a word foolish optimists use to describe pragmatic realists. While we can make some difference in terms of species and habitat preservation, we can not make every critical difference, and we are certainly far from making all the difference that's necessary. Not everything, in other words, can be saved; and many more pragmatic biologists are beginning to recognize this. In a situation of this kind, triage in which habitat is preserved, rather than individual creatures, becomes the preferred method of conservation.

 And we will discuss this in the next post.

Of interest for today: the first and perhaps most dramatic event of speciation on earth was the cambrian explosion. This article sheds some new light on why it took place.

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