Science - March 30, 2006

Nature likes uniformity

Why are there so many different species of trees in a rainforest and hundreds of species of phytoplankton in the ocean? The ecologists Marten Scheffer and Egbert van Nes think they have a new answer to an old question: it pays to be similar to your competitor.

The rich variety of species in nature has puzzled biologists for a long time. Scientists have traditionally sought the answer in the concept of ‘niches’. Each species does best in its own territory, and the existence of so many different ‘niches’ in nature explains why there are so many different species.
But this explanation is not entirely satisfactory. In the rainforest, for example, there are lots of different species of trees that are all quite similar to each other. The same height, the same nutrient requirements: the same niche, you’d think. So why is it that, during the course of evolution, not just one species has risen to dominate over all others?

Aquatic ecologists Professor Marten Scheffer and Dr Egbert van Nes have come up with computer models that they believe provide an answer to the question. They published their findings this week in the American journal Proceedings of the National Academy of Sciences (PNAS).

The scientists made a computer simulation of nature. They started with a simple model in which species of different sizes compete with each other. The smaller the difference in size, the bigger the ‘problems’ the species have with each other. Scheffer: ‘It’s a simple model, but it is a reflection of what happens in the world. Seedeaters of the same size eat approximately the same food, and therefore compete with each other. But a very big seedeater and a very small one will not interfere much with each other because they eat different seeds.’

Scheffer brought evolution into the equation by allowing species the possibility to evolve a little away from their nearest competitors. ‘You would think that you’d end up with a few specialised species: one big, one middle-sized and one little one, for example. But this is not the case. What we see is that groups of species that resemble each other are left over.’

So you get a group of big species, a group of middle-sized ones and so on. And this is also the pattern often that emerges in nature. Scheffer cites examples in his article: water beetles in Europe, phytoplankton in Dutch lakes and birds on the American prairies. The distribution does not seem to be logical, but it is what happens, explains Scheffer. ‘It seems that there are two ways in which species can co-evolve: by being sufficiently different from each other or by being similar to each other.’

The ecologists think they have found the explanation for the evolutionary phenomenon at least with their model calculations. ‘It’s self-organisation. We are convinced that we have come up with a new explanation. It is not easy to understand intuitively, nor is it a simple mathematical calculation. But that is the reason that no one has come up with the idea earlier. These simulations require considerable computational capacity.’

Not everyone is convinced by Scheffer’s arguments. The scientific journal Nature refused to publish the article. ‘We submitted the article to Nature first, and extensive correspondence followed. I think we responded to all criticisms, but apparently we did not convince all the reviewers. The editor in chief did ask us to let him know if we publish. Maybe they’ll do an editorial commentary on it.’ / KV

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