It's well-established that the more species that thrive in a habitat, the better it is at weathering a variety of events from floods to drought to fire. Now, an international study with strong ties to the University of Minnesota is shedding new light on the effect of an increasingly common human-caused disturbance -- the addition of nutrients such as nitrogen, phosphorus, and potassium -- on a wide range of grassland ecosystems around the world.
Building on research published by internationally renowned U of M ecologist David Tilman more than three decades ago, a new study led by Stan Harpole, a professor at the Helmholtz Centre for Environmental Research in Leipzig, Germany and a former student of Tilman, published today in the journal Nature, shows that adding multiple nutrients decreases plant diversity in complex ways that go beyond simply crowding out the competition's sunshine.
In the early 1980s, Tilman developed what has since become fundamental theory of biodiversity based on experiments at Cedar Creek Ecosystem Science Reserve. Tilman found that Limitations in the amounts of the various things plants need, such as nutrients, water or sunlight, are what allow species to co-exist, since each one can specialize in getting by with something in short supply.
Through a collaboration with colleagues around the world associated with the Nutrient Network, an international network of grassland research sites, Harpole gathered data from the same set of experiments on 45 study sites across six continents. Plots contained anywhere from 13 to 103 plant species. The results transcend any single set of environmental conditions.
Harpole, along with a half dozen other researchers, co-founded the Nutrient Network just over a decade ago. Study co-authors Elizabeth Borer and Eric Seabloom, both professors in the University of Minnesota's College of Biological Sciences, also co-founders, coordinate the network. The network's more than 100 members collaborate on global-scale studies that look at various aspects of nutrient cycle to better understand human impact on grassland ecosystems.
The study provides valuable perspectives for protecting ecosystems, adds Borer. "It gives us insights into the mechanism as to what supports biodiversity that then allow us to make management decisions."
Harpole notes that in addition to raising additional theoretical questions about the mechanisms behind the decrease in diversity, the research offers guidance into how we might preserve biodiversity as ecosystems face multiple threats from human action.
"Ecological systems are extremely complex," says Harpole, "and we need to grapple with a number of interacting factors that affect their diversity and functioning if we wish to be able to understand and predict the effects of multiple global changes."