Image: Courtesy of JEF HUISMAN DIATOMS, such as Asterionella [above], are often pitted against green algae in the lab under different conditions to study how species compete for limited resources. When silicon is scarce, the algae proliferate and drive the diatoms the way of the dodo. If phosphorus is limiting, the diatoms outcompete the algae, which are phosphorus fumblers. When silicon and phosphorus are both scarce, diatoms and algae both prosper.

Jef Huisman of the University of Amsterdam and Franjo Weissing of the University of Groningen in the Netherlands remember the phone calls and e-mails that arrived from fellow theorists when their findings appeared in the journal Nature in 1999. "Oh, how stupid we were!" their colleagues exclaimed. "We never thought of this possibility!" Using a simple trick, the pair had exposed a wild and unforeseen side of a set of equations that govern a famous model in ecology¿one that represents how competing needs for shared resources sort species (not unlike contestants on Survivor) into winners and losers.

Researchers had assumed that the equations always enacted the same scenario, which on a computer unfolds as asymptotic curves crossing the screen. In other words, adept species multiply while unfit species dwindle, until the number of species matches the diversity of scarce resources or sinks below it. The model put a theoretical limit on biodiversity¿which Huisman and Weissing's results shattered. Indeed, they showed how the very same competitive circumstances could send opponents into cyclical and chaotic swaps of ascendancy.

More recently the duo has discovered that when some adversaries square off, the instabilities eventually end and a few specialized winners emerge. But there's a catch: who survives depends on a "fractal"¿a curve so detailed that no measurement guarantees an accurate forecast of the winners. "The fittest," they write in the May issue of American Naturalist, "can be as unpredictable as a roll of the dice."

These results have ecologists rethinking the very nature of competition among species and the origins of biodiversity. "We thought we had some understanding of what the limits on species diversity are," says James Grover of the University of Texas. "They [Huisman and Weissing] show things might be quite a bit more complicated than we thought."

Not-So-Model Behavior

It's easy to see why the model has proven so attractive for so long. Consider phytoplankton¿the typically microscopic organisms of oceans and lakes that, by harvesting sunlight, support almost all life in the water via the food chain. Every species needs sunlight, everyone needs CO₂, all need the same nutrients, and where one cell floats looks as good a place as any other. Wouldn't just one of these species do? Why are there so many? Is no one here the fittest?

A contest in a beaker among two kinds of phytoplankton, a green alga and a diatom illustrates how the model equates limiting resources with a habitat's biodiversity. As conventionally staged in the lab, hoses circulate nutrients and CO₂ at concentrations that a researcher specifies. When silicon is scarce, the algae proliferate and drive diatoms the way of the dodo. If phosphorus is limiting, the diatoms outcompete the algae, which are phosphorus fumblers. When silicon and phosphorus are both scarce, diatoms and algae march toward modest prosperity together.

Image: Courtesy of JEF HUISMAN PHYTOPLANKTON from Lake Kinselmeer in the Netherlands is dominated by filamentous cyanobacteria but contains many other varieties as well. In fact, the average tablespoon of lake or ocean water contains around 50 times more species of phytoplankton than traditional ecological models would predict.

Comments