In 2011, the ecologist Ryan Chisholm was looking at tree census data from 12 different forests around the world. More than 4,000 species of trees grew in these places, their numbers rising and falling over the years. The pictures the numbers painted were of ecosystems where a species’ fortunes could change nearly overnight, on an ecological timescale. For instance, a small, glossy-leaved tree called Inga marginata had 400 individuals in a Panamanian forest plot in 2005; by 2010 it had nearly doubled its numbers.
In all 12 forests, however, one detail was particularly notable. The speed and magnitude of the changes didn’t look anything like what would have been predicted by one of the leading theories in theoretical ecology. Models based on that idea, called neutral theory, have shown that the distribution of species over the landscape can be explained using surprisingly simple inputs. But here the theory was breaking down. “You look at how big these fluctuations are,” Chisholm said. “And they’re just enormous. They’re so much bigger than what neutral theory would predict … Orders of magnitude bigger.”
When Chisholm gave a talk at the Smithsonian Tropical Research Institute in Panama, where he was a postdoc, he learned that other people had noticed the same thing. Whatever its successes, neutral theory did not model change well at all—even its estimates of how long it would take a species to go extinct could be tens to hundreds of times longer than the reality. A flurry of papers from various groups since then, including one by Chisholm and collaborators appearing last week in Ecology, look to answer the question: Can neutral theory be adapted so that it shows changes over time? And is it possible to link a beautifully simple model more closely with the complex messiness of biology without damaging the model?
In a neutral model, each individual in an ecosystem, regardless of the species, begins with the same fitness. As the years pass, metaphorical dice are rolled for each individual to decide whether it dies or reproduces. New individuals arrive from outside the community’s borders, and every now and then a new species arises. Over time, the results of these processes accrete in this pocket universe. The model includes nothing about the differences between species, or about niches where one species might thrive while another fails, or about whether a hurricane or drought has struck — it just posits a certain preprogrammed randomness. After a while, ecologists studying this pocket universe might push the pause button and see how the area has evolved. Depending on how they’ve set up the model, the ecologists could look at how many individuals there are of each species, for example, or how the species have arrayed themselves across the landscape.