“For the last ten years it’s been a fairly hot topic in ecology,” says Dr. Janneke HilleRis Lambers, an ecology researcher for the University of Minnesota. She is speaking about biodiversity, and she is absolutely right. The question of what difference is made in an ecosystem by how diverse the organisms there are caused what was characterized by journalists as “an acrimonious dispute” and a “full-blown war” between opposing camps of ecologists.
It has been a lively academic debate – peppered with occasional accusation and name-calling – a question at the center of which was whether an ecosystem becomes progressively more productive the greater the diversity of species it contains, as some experimental results had suggested was the case.
It couldn’t really be denied that these experiments showed a relationship between diversity and productivity, but critics argued that this was most likely due to the fact that at higher levels of diversity, it is more likely that an experimental system will contain a single super productive and very competitive species. This species will push the less productive species aside and make it seem like the whole system was more productive than a system where only less productive species were residing. This explanation is called the ‘sampling effect.’ On the other hand, the supporters of the original experiments proposed that in more diverse systems, different species may find different niches to work in, and as more niches are filled the entire system can become more productive. This explanation is called ‘complementarity.’
By the beginning of the 2000s when Dr. Lambers came to research at Cedar Creek Natural History Area, a hotbed of biodiversity research in central Minnesota, the issue had been hashed and rehashed. “It seemed like everything had been done,” she thought. But as it so often turns out in science, this was not the case.
In 2001 the journal Science published a review of what was known at the time about how biodiversity affects the way ecosystems function. The review concluded that though some consensus had been reached about the fact that biodiversity often matters, there were still many questions left to answer.
“That’s when I realized nobody had taken a look at what the different species are doing,” says Dr. Lambers. Nearly all previous biodiversity experiments had examined how entire systems had performed at different levels of diversity. So, Dr. Lambers, with a group of other researchers, chose instead to watch how individual species reacted at different levels of diversity, in hopes of clarifying what sorts of mechanisms cause the phenomenon of increasing productivity with increasing diversity.

An experimental plot at Cedar Creek	Looking at individual plants – do their differences matter?

The study, the results of which have been published in the August, 2004 edition of the Ecology Letters with Dr. Lambers as the lead author, took a look at 14 species of grassland plants growing in experimental plots at Cedar Creek. Each of the 168 total plots had a randomly chosen combination of plant species, and the plots also had different levels of diversity – some plots contained only one species, others contained four or eight, for example. The researchers first measured which species became more productive as plots became more diverse, a phenomenon called overyielding, and which species became less productive as plots became more diverse, called underyielding. If a plant overyields, it probably means that it finds it easier to compete against other species than to compete against its own species, whereas for underyielders the opposite is true.
The researchers next posed some questions about the overyielding species. The questions, says Dr. Lambers, were “related to what there has been controversy about, and directly related to previous criticisms.” They were also based on knowledge of the characteristics of the experimental systems.
The results of the study seem to show that critics have been, in the words of Dr. Lambers, “both right and wrong.”
As the concept of the sampling effect outlines, the plants that were the most competitive for the most limiting resource in the grassland, nitrogen, were generally the plants that overyielded. And, when C4 grasses (the very most competitive) were in a plot, the other plants (except legumes who can get their own nitrogen from the air) tended to be less productive, most likely because the C4 grasses used up most of the nitrogen for themselves. But, in order for these results to be evidence of the sampling effect, the overyielding plants would also have had to have been the most productive on their own, and they weren’t! Plus, the overyielders should have been replacing underyielders over time, but there was no evidence of this either. Something was allowing the less competitive plants to persist instead of disappearing.
There was evidence for complementarity as well. Most of the overyielding plants showed some salient differences in characteristics. For example, some were warm-season growers and some were cool-season growers, which means their timing could help maximize the use of resources. Getting to grow in a plot with legumes made many of the plants more productive, while taking away the legumes made the same plants less productive. This suggests that the presence of legumes made more nitrogen available for the others. A particular leaf disease that is dependent on diversity, on the other hand, did not seem to factor into what was causing plants to be more or less productive. However, there are still many other possible outside factors, along the lines of pathogens or herbivores, that could be allowing overyielders and underyielders to coexist long term, the paper concludes.

Nitrogen fixing legumes helped other plants succeed.	A monoculture of bunchgrass

Out of these results that support both sampling effects and complementarity comes a distinct message. “What’s really clear, at least for me, is that it’s not going to be that simple,” states Dr. Lambers. “It’s probably going to be a lot of factors that contribute.” She notes that in different circumstances, different mechanisms might be responsible for allowing higher diversity ecosystems to be more productive. In the paper, the researchers point to another experiment that was conducted at Cedar Creek at the same time. In this experiment nitrogen was not a limiting resource, and the list of what overyielded and what underyielded was distinctly different. It would be nice if the context were always the same, says Dr. Lambers, as it would make results easier to analyze, but “circumstances can change.”
Human activity, especially over the last century, has had a very significant impact on biodiversity. We have modified many ecosystems to be less diverse, caused many species to go extinct, and moved foreign invaders into ecosystems. Studies on biodiversity can provide important information on what steps we may wish to take to preserve ecosystems we find valuable (for any reason, be it ethical, aesthetic, or functional) and the way they function.
Dr. Lambers says, “I feel like in a small way that the results from my study are the types of things that will help science advisors to think and talk about what to do in making policy.” Recognizing how complex the interactions of species are and how complicated the issue of biodiversity is could be quite as important as finding actual answers about how they work. This way scientists, citizens, and policymakers may come to realize that there aren’t simple solutions to how we can preserve earth’s biodiversity either.

A polyculture