Small-scale biodiversity needs randomness and productivity

By Casey Johnston

Biodiversity is the result of a marriage between randomness and productivity, according to a study recently published in Science. A researcher placed different levels of nutrient supplements in artificial ponds to see how they influenced the ecosystems that developed in them. He found that the most productive ponds turned out to be not only more diverse than less productive ponds, but that there were multiple routes to diversity, all of which involved a few random events.

At large scales, scientists know that a system's productivity directly correlates with its biodiversity—a fertile environment with lots of resources and nutrients allows many different species to flourish. But researchers have long been unsure if this relationship held for local environments, because their resource and nutrient profiles are so similar. For example, a jungle and desert provide obvious ways to compare the productivity-diversity relationship; two different puddles down the street from each other, not so much.

To get a better idea of how diversity and productivity work on a small scale, a scientist from Washington University in St. Louis divided 45 ponds into three groups, and fed each group different levels of nutrients to enhance productivity. Over the course of two years, he randomly introduced different species to the ponds, while allowing other plants and animals to colonize the sites on their own. The ecosystems ran for a total of 7 years.

He found that ponds with the highest levels of nutrients gave randomly introduced species a toehold that allowed them to thrive and fit into the ecosystem. Because of this, not only was each highly productive pond diverse, but the collection of species each pond supported was distinct from the other ponds.

At the other end of the productivity spectrum, random species introductions often didn't take in low-nutrient ponds. In the end, these held a far less diverse set of hardy species; the intermediate-productivity ponds had middling amounts of diversity. So even at small scales, productivity lets randomness flourish, preserving and creating diversity. Watch out for that axiom at your next company meeting.