Everyday our lives seem to be getting more and more complex; there is more to remember, more to do, and more to worry about. On the other hand, we are making many of our ecosystems simpler and simpler. After logging we tend to replant forests with just one type of tree. On farms we usually plant a field with just one type of crop. And, for our lawns we generally pluck, dig, and spray to make sure that just one type of grass is growing there. Ecosystems are also being simplified as a result of extinctions.

A common simplified ecosystem.	Another, very common, simplified ecosystem.

For some years now, many ecologists have pointed out that studies suggest that less diverse ecosystems are often not as healthy as diverse ones. These ecologists have warned that if an ecosystem loses a whole group of organisms that all perform similar jobs, the ecosystem will function less reliably. Recently, researchers from the University of Minnesota’s Cedar Creek Natural History Area have gone a step further, taken a closer look, and found that diversity may even be quite important within those functional groups.

A functional group is a group of species that all play related roles in an ecosystem. If, for example, an elementary school were an ecosystem, you could classify all teachers – regardless of which grades they were teaching – as one functional group. In many experiments that examine how diversity plays a role in ecosystem functioning “people have used functional groups as a way to simplify the world,” said Dr. Peter Reich, lead author of the new study that appeared in the Proceedings of the National Academy of Sciences.

In many of the diversity experiments in the past, only functional groups were taken into consideration. In the rest, both individual species and functional groups were considered, but ecologists were unable to distinguish which of the two was influencing their results. “We know that all deciduous trees aren’t the same, we know that all legumes aren’t the same, but they are statistically confounded,” elaborated Dr. Reich. The Cedar Creek scientists found no past experiments that actually separated the diversity of species in an ecosystem from the diversity of functional groups in an ecosystem, and they decided it was time to do so. They posed the question, “are the characteristics by which we define functional groups important enough or strong enough to make these generalizations?”

The researchers chose to study four functional groups that are considered important in grasslands: nitrogen-fixing legumes, non-legume forbs (flowering plants that aren’t grass), C3 grasses and C4 grasses (C4 grasses photosynthesize differently from C3s, and C4s do better in hot weather). From each functional group they selected four species that were either native to the area or had existed in the area for a while and had become firmly established.

An example of a legume, Vicia villosa, or hairy vetch (not used in experiment)	It’s not in bloom yet, but common milkweed, or Asclepias syriaca, is a forb (not used in experiment)

The experiment was composed of three mini-experiments. In the first, the researchers grew between one and four species in grassland plots, but all of the species that were together in a plot had to come from the same functional group. In the second experiment, each plot contained four functional groups and either 4, 9, or 16 different species. In the third experiment, the researchers planted four species in each plot, but they could be from 1, 2, 3, or 4 functional groups.

Out in the field the experiment looked like six large, round patchwork quilts. Some squares were colorful, some green, some were overflowing and some were sparse. Each circle had a white border made of pipes. The pipes around half of the circles blew out extra carbon dioxide – causing the air within the circles to have twice the normal level - while the others blew out plain air. Some of the plots had extra nitrogen added to them while some of them were left with ambient levels of nitrogen. The elevated CO2 and added nitrogen were at levels that experts predict we will reach by the year 2050 as a result of greenhouse gasses and nitrogen deposition from industry.

A patchwork of plants. Different squares have different levels of diversity.	The pipes in the background are pumping carbon dioxide into the air over the plots.

All of these combinations together allowed the researchers to measure how increasing numbers of functional groups or increasing numbers of species affected the health of the plots. In addition, they could observe how increased functional group diversity or species diversity affected plot health under conditions that might exist 50 years from now. Because plants use both CO2 and nitrogen, experts hope that plants may be able to cushion the impact of those environmental changes for a while, and the actual extent to which plant communities could respond may be related to their diversity.

The results of the study were quite interesting. As other experiments have shown, having more functional groups seemed to lead to healthier, more productive plots. But, having more species also seemed to lead to healthier, more productive plots, even when all four of the functional groups were already represented. Independently of one another, both the diversity of the functional groups and the diversity of the species showed a positive effect on the plant communities, and these findings extended to the plots with elevated CO2 and elevated nitrogen. Plots with higher functional group or species diversity appeared better able to respond to these conditions than the plots with lower diversity.

Dr. Reich claimed he wasn’t really surprised by the experiment’s results. “Since functional groups are real, and species are different, it makes sense that they both matter,” he said.

Any combination of three functional groups was more productive than combinations of two or just one, which suggests that no single functional group can take the credit for causing all of the increase in productivity that was seen when all four groups were present. It appears that there was no single super-productive plant species either. Calculations suggested that the species diversity increased the productivity in plots through complementarity.

Complemenarity means that as the plants evolved together and competed against one another, the species - even very similar ones - found individual niches where they were able to work the best. Some plants have very shallow roots and some have deep roots. Some plants photosynthesize better on a hot day while some require a cooler day. The more of the many possible niches in an ecosystem that are used, the more productive one might predict it would be as a result of the species complementing each other.

The experiment from Cedar Creek suggests that any kind of biological diversity could be beneficial in an ecosystem. However, many ecologists would say, and the researchers from Cedar Creek concede, that the results from this experiment (along with many other diversity experiments) cannot be directly translated into nature. After all, natural systems aren’t randomly assembled like the experimental plots were. Yet, the results are still worth taking into consideration, especially considering our current trend of simplifying ecosystems. It is not certain if natural systems would respond in the same way as experimental systems to changes in diversity . “But,” Dr. Reich added, “my hunch is that these diversity effects would still be there… though they’re somewhat offset by people’s choice of species or the natural species that have won out.”