BioEd Online

The first study to rigorously show how biodiversity improves water quality is published today in Nature¹. It offers proof that biodiversity helps ecosystems to withstand pressures such as pollution.

Brad Cardinale, an ecologist at the University of Michigan in Ann Arbor, used 150 artificial streams in his lab to examine how the number of algae species in a habitat affects the speed at which the pollutant nitrate is removed from the water. He found that in habitats containing a mixture of eight species, the organisms removed nitrate up to 4.5 times faster than they did in streams with just one species.

"Field studies in nature have shown that more-diverse ecosystems have lower concentrations of pollutants. This study shows that biodiversity can control a service vital to humanity, such as purifying water of a particular pollutant," says Cardinale.

"What makes the study unique is that it thoroughly unravels the impact of biodiversity in a greatly understudied system," says David Tilman, an ecologist at the University of Minnesota in St Paul.

Confronting complexity

Previous experiments into how biodiversity affects environments, often undertaken in grassland systems, have typically tried to isolate biodiversity-specific effects by keeping the habitats under study uniform². Cardinale, on the other hand, intentionally mimicked how streams naturally vary along their lengths, modelling the features — such as riffles, pools and floods — that he says allow diversity to matter.

The are hundreds of species of freshwater algae, but the eight that Cardinale chose to study are among the most abundant in North American streams. Some are adapted to areas in which the water flows quickly, whereas others thrive in low-flow habitats. Each species was able to establish its own niche in the model streams. "As the niches got filled up, the stream became a better biofilter for pollutants," says Cardinale.

When he removed the niche opportunities, making the stream habitats uniform, biodiversity no longer had an impact on nitrate uptake. Indeed, the biodiversity actually decreased, and a single species came to dominate each stream. Although some single species proved to be efficient at taking up nitrogen in these uniform streams, such conditions can produce other problems, including loss of erosion control and reduced replenishment of the water table, and can even create blooms of nuisance algae. "This work shows that environmental heterogeneity can't be left out of the equation," says Cardinale.

The biodiversity encouraged by the niches proved to matter even more than Cardinale had thought. Before starting the experiment he had expected that, as in previous studies³, efficiency of resource use would be maximized by the addition of the first three or four species of algae; after that, any further species would have no effect. But he found that the pollutant uptake increased in direct proportion to the number of species added to the stream, for all eight species.

"I don't think this research suggests that we need to conserve every species in an ecosystem, but it does raise an interesting question — how many species do we really need for water quality?" he says.

Extrapolate with caution

Many ecologists agree that Cardinale's experimental design and findings are impressive, but they mention a common caveat of lab studies — that the results shouldn't be presumed to exist in natural ecosystems.

"Generating a whole landscape in a single microcosm is a remarkable technical achievement — but the tricky bit is showing how this lab-based mechanism is relevant in a natural setting," says Jason Fridley, an ecologist at Syracuse University in New York.

Fridley notes that there is still a need to determine at what ecological scale lab findings are most relevant. "It has always been the elephant in the room," he says.

Cardinale admits that the linear relationship between diversity and increased uptake of nitrate may not extrapolate to field conditions. But he says that his work does show how dynamic and varying ecological conditions allow species to coexist.

And, he adds, his experiments underscore the mechanisms responsible for field observations — if species can exploit niches to divide the labour, they are more efficient as a team. "We have studies showing that more-diverse ecosystems have lower concentrations of pollutants — but we didn't have an explanation of why. To show why, we had to take a simplified system in the lab and control everything except diversity," he says.

"This study may be a useful stepping stone to get to the real breakthroughs — where experimental and observational approaches arrive at the same answer," says David Wardle, a plant ecologist at the Swedish University of Agricultural Sciences in Umeå.

To that end, Cardinale has been working on field-based stream studies, and is now planning projects to track the nitrate's ultimate fate — that is, whether it continues up the food chain or re-enters the stream when the algae die.

"We know that species diversity matters, but what we don't yet have is the data to tell us how many and which species to conserve," he says. Cardinale is hoping to gather that information through a working group that he is leading at the National Centre for Ecological Analysis and Synthesis at the University of California, Santa Barbara. "We're 5–10 years away from offering concrete answers," he says.