University of Minnesota
September 19, 2012
A diverse assemblage of plants, like this one at Cedar Creek Ecosystem Science Reserve, produces more vegetation and performs "ecosystem services" better.
Plants form the strongest networks when there are lots of species
By Deane Morrison
One of the biggest joys of science comes when nature hands you a complete surprise.
It happened recently to two of the U's most prominent ecologists and their research colleagues, who uncovered the functional equivalent of social networks in ordinary grassland plants.
Those networks, destroyed when land is cleared for agriculture, hold hope for treating some of the world's biggest ecological ills. The existence of complex plant relationships leaped out from data on how high biodiversity—large numbers of species—boosted yields of vegetation and became even an even stronger force with time.
The two studies, led by Regents Professors David Tilman of the Department of Ecology, Evolution and Behavior and Peter Reich of the Department of Forest Resources, were published in the journals Proceedings of the National Academy of Sciences and Science.
The more the merrier
A range of studies by Tilman, Reich, and others has centered on the question of whether it is beneficial to have lots of species in our vegetation. They work with prairie grasslands, using them as model systems to learn about the effects of diversity across vegetation, from natural prairies and rain forests to agricultural areas and lawns.
"Theory suggests that mixtures of species should do better than each alone," says Reich. "[This is] because [all] species differ in how, when, and where they acquire carbon, nutrients, and water from their environment and how well they do this in wet versus dry conditions, or cool versus hot periods."
Humans demand a lot from tended areas, especially agricultural fields. We want abundant yields of food, and we also want vegetation to store carbon; hold water and nutrients in soil; provide beauty, shade and animal habitat; and perform other "ecosystem services." Studies have shown that areas of higher plant diversity do all these things better.
In the current studies, performed at the University of Minnesota's Cedar Creek Ecosystem Science Reserve, the researchers measured the amount of biomass produced by plots with biodiversities ranging from one to 16 plant species. They noted how the biomass density tracked with biodiversity vs. other variables, such as amounts of added nitrogen fertilizer; watering; and the presence or exclusion of deer and insect herbivores.
• The level of biodiversity affected biomass production at least as much as any other factor. For example, plots with 16 species had about 2.5 times the biomass of those with one species; this effect was stronger than adding water or nitrogen.
Leading the field
The studies described here are the two longest-running and most comprehensive studies of biodiversity and ecosystem function in the world. Other U of M researchers involved were professor Sarah Hobbie and postdocs Forest Isbell and Kevin Mueller, all of the Department of Ecology, Evolution and Behavior. Hobbie and Reich are also fellows in the Institute on the Environment.
• After several years, diverse plots became more sensitive, meaning that the loss of even one species caused a larger drop in biomass. Meanwhile, the impact of fertilizer treatment—the second biggest factor—declined.
"We never imagined when we started studying biodiversity that we'd see the effects we found," says Tilman. "Moving from one to 16 species was slightly better for productivity than adding the amount of nitrogen that a farmer would add to a typical corn crop. That was a major surprise."
Every species counts
Past work at Cedar Creek has shown that just having more species means better health and productivity for plots. But that has raised the question of whether species exhibit redundancy, meaning that one can substitute for another, and thus removing one or a few will have minimal effects.
The fact that diverse plots became more sensitive to the addition or subtraction of species over a 15-year period answered that question.
"This argues against the idea of lasting redundancy among species," says Reich. "Instead, it appears that in diverse assemblages, each species plays its own part in increasing biomass production.
"This means that simplification of diverse ecosystems is a threat. And that we need to learn to use diversity as a tool, and promote it in systems as different as native forests and grasslands, as well as in agriculture."
The findings imply that different plant species come to rely on each other and miss the loss of any of their "partners," the researchers say.
They liken the situation to a diverse sports team, where each member has a specialty. At first, one or two members can be lost because the team hasn't learned to work together. But after they do, they become more dependent on one another's special skills, and it does matter if one or two are lost.
Can biodiversity losses be minimized?
Land being converted to agriculture poses the biggest threat to biodiversity, Tilman says. Of Earth's 8.5 billion hectares of usable land, 3.5 billion are used for grazing animals and 1.5 billion for crops. Much recent clearing of diverse areas like rain forests has had harmful effects (loss of soil fertility and releases of carbon dioxide from burning forests), but not enough good ones—primarily food production—to offset them.
But much less rain forest would be cleared if the agriculture that replaced it produced more food, using better seeds and the right amounts of fertilizers and water.
"[Cleared] land could be as much as four times more productive on average," Tilman notes. "If [intensive agriculture] is done right, it could prevent a billion hectares being cleared in the next 40 to 50 years."