Habitat Size Determines Productivity/Diversity Relationship, Ecologists from Pitt, University of Chicago Reveal in March 28 Edition of Nature

March 29, 2002

Increased Productivity Doesn't Always Translate into Increased Biodiversity

PITTSBURGH—The size of the habitat is crucial to determining the relationship between the biodiversity and the individual productivity of species, according to a study by scientists at the University of Pittsburgh and the University of Chicago. The study was published in the March 28 edition of Nature.

Jonathan M. Chase, an assistant professor of community ecology in Pitt's Department of Biological Sciences, and Mathew A. Leibold, an associate professor in the Department of Ecology & Evolution at the University of Chicago, studied wetland plants, both submerged and emergent, and wetland animals—amphibians and invertebrates such as insects and snails—near the Kellogg Biological Station run by Michigan State University. Chase says he found similar patterns at the University of Pittsburgh's Pymatuning Laboratory of Ecology near Linesville, Pa.

"Ecologists have long recognized that species diversity in a particular site is affected by many factors, but that primary productivity is one of the most important," says Chase. "Primary productivity is often determined by available nutrients, light, rainfall, and the like. However, there is a lot of confusion and controversy about just how diversity varies with productivity." Ecologists define primary productivity in flora as the efficiency of collecting carbon, in the form of carbon dioxide, through photosynthesis.

Chase and Leibold found that in small habitats, the productivity/diversity relationship followed a hump-shaped pattern, where species diversity increases at first, then declines as the primary productivity increases. In larger habitats, the researchers found that the relationship followed a steady increase, with no subsequent decline.

Researchers define a small-scale habitat as being a small pond, for instance, and a large-scale habitat as a series of ponds in a watershed, which could be as large an area as a county or state.

"This 'scale dependence' happens because small communities become more different from one another as productivity increases, and thus the entire diversity of the large scale increases, even though the diversity on the small scale decreases," Chase explains. "Our work was in wetlands, but similar results are likely to hold in a variety of different types of ecosystems. In addition, our work might help to explain large patterns such as the increases in species diversity as you move to the tropics."

The natural inclination to always associate higher productivity with more diversity is not the case, and Chase says that several recent studies of small-scale habitats do not support that assumption.

"For example, if one species is very productive, it might be able to 'steal' all the resources, and biodiversity would be very low," Chase says. "At very low productivity, there are very few species. At intermediate productivity, more species can exist, but at high productivity, a few species can use all of the resources, and biodiversity gets low again. However, we found things are different when you look at a different spatial scale, like a whole watershed of several ponds. Here, diversity increases consistently as productivity increases.

"So, the main question is, 'How can diversity go down at high productivity at the small scale, but continue to increase at the larger scale?' We found that it could do this because the types of species that lived in different high productivity ponds were highly variable," Chase continues. "In some ponds, a group of species lived together, but in nearby ponds, an entirely different group of species lived together. So, if you add the two groups up, you get high biodiversity, even though in a single pond, there was low biodiversity. This clarifies the dilemma of why diversity goes down with increasing productivity in small-scale habitats, but goes up in large-scale ones."

Chase says that his work with Leibold holds several implications for understanding the effects that humans have on the environment.

"Many human actions, such as fertilizer runoff from farmer's fields, increase the amount of nutrients that enter wetlands. Our work helps us to understand what the effects of these actions might have for species diversity of plants and animals within single ponds and within entire watersheds," Chase says.

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