THE BLOG
12/05/2008 05:12 am ET Updated May 25, 2011

Nobody Reads Nature: Why So Many Tree Species In The Amazon?

Today, we introduce a new feature, "Nobody Reads Nature," in which we try to convince you that ecology is really interesting, and that you should send us some money so that we can keep studying it.

The aphorism goes that you shouldn't miss the forest for the trees. But ecologists actually spend a lot of time trying to see the trees for the forest. More specifically, we spend a lot of time thinking about why some forests have more different kinds of trees than other forests. Or why Brazil has 2500 times more native tree species than Iceland does.

And we don't just think about questions like these. Sometimes we argue. Sometimes, after one too many strawberry wine coolers, it even comes to blows. At least that's what we've been told--we can't remember any of it.

One argument that has long simmered in ecological circles concerns why there are so many tree species in tropical rainforests. While these forests only cover 7% of the earth's land area, they contain roughly half of earth's biodiversity, including a lot of different types of trees. Theories to explain this phenomenon have been circulating for decades, and most of them have assumed that it has something to do with differences in the characteristics of the tree species themselves. Given that a tree has to make a living right where it is standing, it makes sense to specialize on a particular set of resources that are not being used by other nearby trees. Let's say you're a milk salesman. Your "resources" are customers who want a drink of milk. Would you rather set up shop next to another guy selling milk, or next to a guy selling cookies? You'd probably move in next to the cookie guy, because moving in next to the other milk guy = competing with him for the same set of milk drinkers = fewer customers. Similar competitive interactions occur among tree species in a forest, and different species come to occupy different niches. It is easy to see how, over time, this kind of competition would promote diversification.

Or maybe diversity arises because baby trees have a hard time surviving when they grow up next to mother trees, because the mother tree attracts things that eat trees, which gobble up all the baby trees. Again, over time, you'd expect to find that the baby trees that actually make it to adulthood were widely spaced throughout the forest. Another analogy: if we are Hamburglers and you are a hamburger factory, then we are going to be all up in your proverbial grill, eating all the little hamburglets you squeeze out. If, on the other hand, you take your hamburglets and spread them all over the damn place--some in the dog park, some in the bathtub, some under your mother's pillow--we're going to have a hard time finding them all. Although we'd probably find the ones under your mother's pillow. Anyway, the hamburglets that we can't find stand a better chance of growing up into proper hamburgers.

These theories make good sense, and a great many ecologists have spent a great deal of time trying prove or disprove them. Everyone was happy. But a few years back, an ecologist named Stephen P. Hubbell rocked the ecological world by suggesting that maybe the diversity of tropical forests really had nothing to do with differences between tree species. Maybe, Hubbell proposed, differences between species don't really matter in determining their success. Maybe all those tree species are, for all practical purposes, interchangeable, and their success or failure has more to do with random events and complicated math than with their characteristics per se.

There followed seven more years of refined scientific argumentation, counter-argumentation, and drunken conference-hall brawls.

Then along came our friend Nathan Kraft of Berkeley, who addressed this issue in Science last week. Nathan hasn't resolved the debate, obviously. But he has made an important contribution to the subject in an asskicking paper that, frankly, we wish we had written ourselves.

Kraft and his co-authors made use of a 25-hectare plot (think 25 football fields) in which every tree with a stem bigger than 1 cm in diameter is mapped. That's a mind-boggling 150,000 trees, representing more than 1,100 species. Within that big plot, they looked at the characteristics of the trees occurring within each 20 x 20 m piece (625 in total).

They then generated a "null expectation" of what one should find in each piece of each plot, assuming that species characteristics don't particularly matter. Scrabble™ players will appreciate this approach: pretend that tree species are letters, and that each species is weighted in proportion to its occurrence, as Scrabble™ letters are weighted in proportion to their commonness in English words. Then just draw species randomly from a bag. If the collection of species that you get from a random draw matches what you observe in nature, then you can assume that the processes generating pattern in nature are also essentially random, or at least so complicated that they generate patterns that we can't distinguish from random ones. If what you observe in nature is significantly different from what you get in your random bag draws, then you must conclude that some non-random process is operating--i.e., that species, unlike Scrabble™ letters, have some traits that prevent their being plucked together in the same random draw.

The finding? In many cases, the observed patterns in nature were indeed different from those expected under the assumption of randomness. In many cases, the species occurring in a single 20 x 20 m subsection displayed a broader distribution of traits (for example, the shape of their leaves, the size of their seeds, etc.) than would be expected by random chance. In other words, species traits do matter, and some process is making sure that very similar species tend not to occur side-by-side.

As to what exactly that process might be, Kraft et al. only speculated. So there is much more work to be done in Ecology Land--please send money right away.