Thursday, March 8, 2018

Alpha diversity and beta diversity

At today's journal club meeting, we discussed Alexander Pyron's opinion piece We don't need to save endangered species - extinction is part of evolution. I mentioned it in passing before and still think that his core argument, which is also reflected in the title, is logically equivalent to saying that murder is okay because all humans are going to die of natural causes one day anyway. But reading his piece more thoroughly than before, I now notice a few other, um, problems. The highlights:
Species constantly go extinct, and every species that is alive today will one day follow suit. There is no such thing as an "endangered species," except for all species.
What weirds me out here is the lack of a phylogenetic perspective in a piece written by a systematist - species are discussed as individuals that pop out of thin air and then disappear again. Of course, in the very long run every species will one day go extinct when the sun expands and boils off the oceans. But until then, in the time frame that Pyron discussed, no, not every species will go extinct, quite a few of them will diversify and survive as numerous descendant species, as did the ancestor of all land vertebrates or the ancestor of all insects in the past. They thus become effectively immortal (until, once more, the sun explodes anyway, etc.).
Yet we are obsessed with reviving the status quo ante. The Paris Accords aim to hold the temperature to under two degrees Celsius above preindustrial levels, even though the temperature has been at least eight degrees Celsius warmer within the past 65 million years. Twenty-one thousand years ago, Boston was under an ice sheet a kilometer thick. We are near all-time lows for temperature and sea level ; whatever effort we make to maintain the current climate will eventually be overrun by the inexorable forces of space and geology.
This is sadly a classic of climate change denialism. Yes, there was change in the past too, but there are some major differences. One is the rate of change - the impacts we are having are coming much faster than most natural changes (excepting e.g. meteorite strikes and similarly sudden events), so that animals and plants have less of a chance to migrate or to adapt than they had in past cycles of warm and ice ages. Second, they have even less of a chance to migrate because we have fragmented their available habitats by putting roads, towns, croplands and pastures into their way. Third, past changes did not affect a highly urbanised human population of more than seven billion people; the potential of global change producing catastrophic results even just for us is much greater now than when we were just a few million widely dispersed hunter-gatherers. So yes, it is true that we cannot freeze the status quo in place forever, but I think we would do well to slow the rate of change as far as possible.
Infectious diseases are most prevalent and virulent in the most diverse tropical areas. Nobody donates to campaigns to save HIV, Ebola, malaria, dengue and yellow fever, but these are key components of microbial biodiversity, as unique as pandas, elephants and orangutans, all of which are ostensibly endangered thanks to human interference.
I just don't even. What is the logic here? "Nobody cares about conserving diseases that horribly kill us humans, so we should not care about conserving harmless pandas either?" How does that follow?
And if biodiversity is the goal of extinction fearmongers, how do they regard South Florida, where about 140 new reptile species accidentally introduced by the wildlife trade are now breeding successfully? No extinctions of native species have been recorded, and, at least anecdotally, most natives are still thriving. The ones that are endangered, such as gopher tortoises and indigo snakes , are threatened mostly by habitat destruction. Even if all the native reptiles in the Everglades, about 50, went extinct, the region would still be gaining 90 new species -- a biodiversity bounty. If they can adapt and flourish there, then evolution is promoting their success. If they outcompete the natives, extinction is doing its job.
And this is perhaps what frustrates me most, because while this is not an uncommon argument against biosecurity measures one would expect a biologist to know about different types of biodiversity instead of confusing them. To explain more clearly what is going on, consider the following diagrams. First, we have three areas, roundland, squareland, and hexagonland, with two endemic species each.

Then humans recklessly move species between the areas, allowing them to invade each other's natural ranges. It turns out that three of the species are particularly competitive and prosper at the cost of the other three, driving them to extinction.

Now there are three types of diversity to consider. The first is alpha-diversity, which means simply the number of species in a given place. As we see it has gone up by 50% in all three areas, from two to three species. Yay, more diversity! This is what Pyron proudly points at in Florida.

What is lost, however, is beta-diversity or turnover, that is the heterogeneity you observe as you move between areas. It was very high originally, as every area had its unique species, but now it has been wiped out entirely. Beta-diversity in the second diagram is precisely zero. Under the first scenario a squarelander can go on a holiday trip to roundland and admire the unique flora of that part of the world; under the second scenario they will travel to roundland and merely see the same few weeds that they have growing in their own front yard back home. And the endemic plants of hexagonland have all gone extinct, a 100% loss of that area's irreplaceable evolutionary history.

(Note that beta-diversity would also be zero if all six species survived everywhere. But that is clearly not a realistic assumption, as it would require each area to have such a high carrying capacity that they should each have evolved more than two species to begin with. We would not expect that all the plant species of the world could survive next to each other in, say, Patagonia, even if they were all introduced there.)

Finally, in our example global diversity has of course also been reduced, by 50%. So yeah, great to have more alpha-diversity in Florida, but does that make up for a massive net loss in both beta-diversity and global diversity? The argument seems rather misguided.

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