Bob Ricklefs, president of the American Society of Naturalists, spoke last night at the Evolution 2011 meetings. Although his talk was called “My life as a naturalist,” there was little biography or, indeed, natural history, though his theme was that natural history is essential to informing ecological theory.
I won’t go into all the details of Ricklefs’ talk, but wanted to point out that much of it was a critique of an ecological theory that has caused a lot of stir in the last decade: Steve Hubbell’s “neutral theory of biodiversity and biogeography.” This theory was made famous by his book:
I’d like to make this post short, for the “neutral theory” is complicated and, in truth, I don’t understand all the mathematics. It’s based largely on Hubbell’s famous work on tropical trees on Barro Colorado Island, in Panama, where he spent decades mapping distributions of the many individual trees (and the many species) that inhabit even a small patch of neotropical forest.
From his work, Hubbell proposed that in many communities (not just trees), species are ecologically equivalent to one another, and thus individuals in a group of related species can be regarded as simply selectively identical (i.e., “neutral”) entities whose abundance and identity drift around randomly over ecological and evolutionary time. (There’s an obvious parallel with the “neutral theory” of evolution, in which different alleles of a gene are selectively equivalent and are affected only by random processes.) According to Hubbell’s theory, it doesn’t matter whether a few square meters of forest is occupied by an individual of species X or Y: the ecological dynamics will be the same.
Any “patterning” of ecological communities,then, is simply an artifact of individuals reproducing and competing with each other as full ecological equivalents.
This flies in the face of years of ecological theory (supported by data) maintaining that species are not ecologically equivalent, but compete with each other based on differential use of resources. In his book, however, Hubbell claimed that assuming ecological equivalence of all species in a group (e.g., trees) could explain certain patterns of ecology, like species abundance curves showing that some species are very common and many others are quite rate. It turns out that, with certain assumptions, the neutral theory predicts abundance curves very similar to those seen in nature. The conclusion: since the “neutral” predictions match the data, related species must really be ecologically equivalent.
This theory always puzzled me. I’m not an ecologist, but I knew that there is plenty of evidence from nature that different but related species do use different resources and, though they compete, they are not ecologically identical. So how could a theory based on palpably false assumptions make accurate predictions about species distributions? My view was that this was simply a coincidence, but I stress again that I am not a trained ecologist and have watched the controversy as an outsider.
But Ricklefs is not an outsider: he’s deeply conversant with both theoretical ecology and natural history (he’s done a ton of field work), and so when he criticized the neutral theory in his talk last night, that was srs bzns.
He leveled several criticisms at the theory:
1. We know that species aren’t ecological equivalents. As I said, there are plenty of data showing that, for example, plant species inhibit each other’s growth in different ways: if you surround a member of species X with plants of species Y, the inhibition of growth is different from what you see when you surround it with members of its own species, X. Therefore the fundamental assumption that species are ecologically equivalent is empirically false. Ricklefs also cited a lot of work on birds showing ecological differences between species (Robert MacArthur’s warblers are a famous example) and differential competition.
2. The nice fit of Hubbell’s predictions to empirical data relies on making untested assumptions about the size of parameters. One of these is the rate at which new species arise. If you use other values of this parameter, which is of course unknown, the fit between real data and Hubbell’s predictions isn’t so good.
3. Species distributions change in regular ways along environmental gradients. Ricklefs showed distributions of tropical tree species across the Isthmus of Panama (Barro Colorado Island sits in the middle of a lake in the Isthmus). The array of tree species on the western (wet) side is very—and predictably—different from that on the eastern (dry) side. This is not expected if different tree species are ecological equivalents that respond identically to environmental differences.
4. Over huge ranges of space in similar environments, the distribution of species in sub-areas remains similar. If you look at patches of rain forest in similar environments thousands of kilometers apart, the distributions of tree species are pretty much the same. The neutral theory does not predict this: the great distances between those patches means that they should be attaining equilibrium more or less independently, and so shouldn’t show identical distribution of species’ abundance if all species are ecologically equivalent.
5. Attaining ecological/evolutionary “equilibrium” under the neutral theory requires billions or millions of years. Ricklefs said that the neutral theory predicts that distributions of species attain Hubbell’s equilibrium values only on times of the order of millions or even billions of years. Since no area of earth, including the tropics, has been undisturbed for anywhere near that long, the distributions of species we see cannot be those predicted by the neutral theory. There must be some other non-”neutral” explanation for species abundance curves.
Ricklef’s talk included other stuff, but I was most intrigued by these criticisms of a popular and fashionable ecological theory. I tend to agree with Ricklefs, though I’m not as qualified as he to judge these matters. He predicted that the “neutral theory of biodiversity and biogeography” will be dead and gone in a decade. I suspect he’s right. And I guess I find that judgment congenial, for it leaves species with an individuality that we’ve long appreciated as evolutionary biologists.