Research Shows Why More Species Are Better For Ecosystems

In a paper published in the Jan. 24 issue of the journal Nature, University of Maryland biology professor Margaret Palmer and doctoral student Bradley J. Cardinale show that when several species of caddisfly larvae live together in a stream, they get more food from the stream and, as a result, are likely to be more productive than when only a single species inhabits the same area.

The study is one of the first to look at aquatic species in the growing controversy over biodiversity loss and the potential importance of conserving different species in an ecosystem.

"Our research supports findings from other studies that show when you decrease the diversity of species in an ecosystem, the ecosystem becomes less productive," said Palmer. "What's really exciting about our work is that we were able to show why this happens. We found that species sometimes help each other capture food. When you lose a species, the others may eat less and become less productive."

The Maryland project is one of the first to explore biodiversity with animals rather than plants. The lifestyle of the caddisfly, an insect found in streams around the world and replicated by fly fishermen in fishing lures, lends itself well to this type of study. The larvae live in little cases attached to rocks. They build silk nets that capture and filter food from the water. The larvae are large enough, at up to three quarters of an inch long, that they can be dissected and examined to see how much food ends up in their guts.

Palmer and Cardinale, along with adjunct faculty member Scott Collins, gathered caddisfly larvae from natural streams and put them in streams they had constructed in the lab, where they could control the environment. In some of the lab streams, they put only a single species. In other streams, they combined several species.

"The pattern of water flow around the caddisflies totally changed when we mixed the species," said Palmer. "Water flow was faster, and more food particles were delivered to the larval capture nets than in the single species streams."

"It appears that placing species together, each with a different size filtration net, created a form of physical complexity that altered the flow of water near the stream bed and allowed the whole community to capture more food," said Cardinale.

"When species help each other capture food, it is an interaction ecologists call facilitation," said Palmer. "We found that increasing the diversity of species in a stream increases the likelihood of facilitation. Studies with plants have suggested this happens, but because it's much harder to measure food capture in a plant, research has been unable to prove it. Based on our results, we can now hypothesize that in any ecosystem where food is delivered passively, such as by wind or water, facilitative interactions that are maintained by a high species diversity could cause that ecosystem to perform better."

The team's findings raise new concerns about the ecological consequences of species extinction. "Between 17 and 50 percent of the species now on the planet are [e1]predicted to be lost as a result of human activities," Cardinale said. "Our study shows that ecosystems with more species are more efficient and that as species vanish, the ecosystems we rely on so heavily will become less productive."

Funding support for the research was provided by grants from the National Science Foundation and the Chesapeake Bay Foundation.