Thanks again for the comments on my previous two posts about eugenics. As a novice blogger, I was surprised by their focus. I expected comments about the past--the historical significance of the eugenics movement--but instead the future dominated, with assorted speculations about the possible futures that genetic engineering could bring to our species. By coincidence, I've been thinking about the future as well, but from a different angle, thanks to a pair of papers in press at Trends In Ecology and Evolution. Instead of introduced genes, they're interested in introduced species.
Before humans came on the scene, animals and plants had a much harder time moving to new places. Unless they were birds or windblown spores, they couldn't cross oceans to new continents or islands. They had to wait for a land bridge like Panama to emerge, offering a path to a new habitat.
Then humans started moving species around. When Polynesians spread across the Pacific, for example, they brought pigs and rats in their canoes. As canoes gave way to tankers and airplanes, the traffic in species took a steep climb. Harold Mooney at Berkeley has called this new arrangement the New Pangaea. In a sense, we've created a single supercontinent in which animals and plants can mingle across its length and width.
In one of the Trends papers, Julian Olden of Colorado State University and his colleagues wonder about what life on New Pangaea is going to be like. (At the Trends in Ecology and Evolution web site you need a subscription to get access to the full text, but you can check Olden's publications page.) Olden and co. see a pretty grim picture, although they leave open the possibility for some bright spots.
As more species shuttle to new homes, the authors predict that diversity will suffer. The damage will come at many different levels:
--Within a single species, for example, one population may acquire combinations of genes not shared by other populations. This is the key ingredient for making new species, but it's also important for letting the old species survive--when catastophes strike (droughts, fires, etc.), the genetic variation can act as an insurance policy, allowing some members of the species to survive. According to the Olden paper, in the New Pangaea many species will become more uniform genetically. Cutthroat trout, for example, have been stocked all over the world from a single American population. The newly arrived trout breed with native subspecies, merging with them and blurring their distinctiveness. Captive breeding and genetic modification may make the problem worse.
--Just as there will be less variation within species, there will be less variation between species. Closely related species often still retain the ability to interbreed. In many cases, the only thing keeping them from hybridizing is being physically separated. Bring them together, and they'll really get together. The hybrids will mate with the members of the parent populations, and gradually the two species will merge.
--In other cases, the biological invaders will simple drive species extinct through competition. That's already happening now. Think of zebra mussels, which have devastated the diversity of native shellfish in the Great Lakes and surrounding rivers.
The result of all this loss of diversity, according to the Olden paper, is that nature is going to get homogenized to a level that may have never been seen before in the history of life. Some scientists say that we're now leaving the Holocene Era and entering the Homogenocene.
It's hard to say how nature will change on New Pangaea. There some evidence that ecosystems get more vulnerable to droughts and other calamities when they lose diversity. The network of connections that keeps it intact becomes simpler, and thus easier to tear down. New Pangaea may even affect the future of evolution itself. With less genetic variation, species may be less likely to adapt to a change in the climate or some new predator. It may become harder for new species to form. For one thing, so many once-isolated populations are hybridizing with invaders. For another, there will be no refuge from competition, where populations can experiment and find new ways of making a living.
It's a mistake to react to papers like this one by collapsing in complete apocalyptic despair. For one thing, other scientists have looked at the same heap of evidence and drawn different predictions. Dov Sax and Steven Gaines, two biologists from UC Santa Barbara, show in another Trends paper in press (pdf here) that invasions can add diversity to a region, not just take it away.
We're not just talking about just adding zebra mussels to the plus column. Invaders that hybridize with native species don't always destroy diversity--as I mentioned the other day, hybrids can be the source of new species. And the invaders themselves evolve, as they adapt to their new home, in some cases potentially becoming entirely new species themselves. Sax and Gaines show some impressive data in their paper. On oceanic islands, for example, plant diversity has actually doubled over the past few thousand years, even when you take into account the species that have become extinct.
If biological invasions were the only force acting on diversity, some researchers even predict that the diversity of New Pangaea should ultimately rebound to pre-human levels. Michael Rosenzweig, a leading evolutionary ecologist at the University of Arizona, summarizes his argument in this paper, starting on page 10. (But see this opposing view from Michael Collins at the University of Tennessee.)
It would be an even bigger mistake to use the work of Sax and Gaines to say that everything is just fine and that any concern about the future a symptom of eco-freak hysteria. Biological invasions may or may not cause an overall loss of species over the long term. But that's not the only force that's shaping the New Pangaea. The space available for species is also shrinking, as forests, prairies, and other habitats contract. That means that the global level of biodiversity will shrink. And so while we may be dealing some species a winning hand at the moment, they'll be inheriting a world that suffered serious ecological damage--which may make their victory a Pyrrhic one.
We, of course, are the biggest winners of all on New Pangaea, having put ourselves on every continent. The big question for us is what we're winning. To me, it's a question that's far more important to our future than whether a trip to the doctor's office will add 10 points to your unborn child's IQ.