Biodiversity Explained by Ignoring the Forest for the Trees

Brandon Keim Wired 25 Feb 10;

That biodiversity’s origin needs uncovering is surprising because the word seems to be everywhere. But scientists still don’t quite understand why one place has more species than another, or fewer.

The traditional explanation — every organism has its niche, competing not with other species but its own — sounds nice, but has holes. According to the tree study, that’s because ecologists haven’t looked for the right niches.

“We take this very complex, high-dimensional thing called the environment, and average out all the variation that organisms really require,” said Jim Clark, a Duke University biologist and author of the study, published Feb. 25 in Science. “Biodiversity is very much a niche response, but it’s just not evident at the species level.”

The central tenet of biodiversity science is that animals compete against their own kind, not against other species. Computer models of inter-species competition soon collapse, with rich diversity inevitably replaced by a few dominant species.

In the real world, that’s not what happens. Species seem to be sharing. So ecologists have developed a theory of niches: Every species has a particular specialty, a set of conditions for which it’s best suited. Some plants do well in shade, others in rocky soil, and so on.

This is true. However, it still doesn’t seem to explain biodiversity. Some ecosystems that are very poor in resources, and consequently don’t seem to have many niches, can still have a high species diversity.

“When you have thousands of species, it’s difficult to come up with ways to partition a limited set of resources or conditions,” said John Silander, a University of Connecticut ecologist who studies South Africa’s Cape Floristic region, a rocky scrubland with as much biodiversity as the Amazon rainforest. “People looking at niche differences always seem to come up short.”

Clark may have found the answer. He has spent the last 18 years studying trees in the southeastern United States and has assembled 22,000 detailed individual accounts, spanning 11 forests and three regions. For each tree, Clark has recorded its precise, on-the-ground (and in-the-ground and above-the-ground) exposure to moisture and nutrients and light, its response, and its proximity to other plants.

Ecologists usually aggregate this information, turning it into average. By going tree-by-tree, Clark found that there are, in fact, enough niches to go around. They’re filled when competition in a species drives individuals to fill them. Biodiversity — or, from another perspective, configurations of organisms that don’t need to compete against each other — is the result of this fierce race for resources.

The niches could only be seen at a fine-grained level, not in the coarse analyses typically used by ecologists. “We take environmental variation and project it down to a very small set of indices. Light becomes average light per year. Moisture becomes average moisture per year. It’s not just light and water and nitrogen — it’s variations of each of those things, in different dimensions,” said Clark.

“The approach he’s taken is marvelous. Nobody has looked at biodiversity in this fashion,” said Silander, who was not involved in the study. “He has the data needed to address the different hypotheses.”

Silander said the approach will likely be extended beyond the world of trees. Understanding the essential dynamics of biodiversity could improve ecosystem management, in applications from conservation to farming.

“It’s hard to find a place on Earth that doesn’t have some level of management going on,” said Silander. “We have to understand how species interact.”

“Ecologists spent a lot of time in the 20th century trying to find ways to reduce the complexity of natural systems so that we could understand them,” said Miles Silman, a Wake Forest University ecologist who was not involved in the study. “Clark has shown that the complexity that we were trying to reduce is very likely essential to understanding” biodiversity.