Food traffic

Researchers develop new conceptual tool for describing ecological communities

Ever since Charles Darwin wrote one of the first descriptions of a food web — outlining who eats whom — in 1838, biologists such as Rockefeller scientist Joel E. Cohen, Ph.D., Dr.P.H., have been studying patterns of ecological communities of species living together. In these ecological communities, big animals usually eat smaller animals, and small animals typically eat still smaller animals or plants (which are consumed by some big animals, too). Moreover, large predators tend to be rarer than small prey species.

But, for the most part, studies of species abundances and body sizes have not been directly linked to food webs. The Abby Rockefeller Mauzé Professor at the Rockefeller University, Cohen has teamed up with two other scientists to present a new conceptual tool for describing ecological communities that combines the traditional food web with data on species abundances and body sizes.

Cohen’s and his colleagues’ research, published in the Feb. 18 issue of the Proceedings of the National Academy of Sciences, reveals some surprising relationships and provides new insights about previously documented ecological patterns. The research describes a community of fish and plankton in tiny Tuesday Lake, in the Upper Peninsula of Michigan.

One startling finding reported in the paper is that the amount of biomass in each category of body size was roughly constant across different body size categories. Biomass is simply the weight of living material in a particular species or size class of species. Since the biomass remained roughly constant, the weight of the few, heavy fish roughly equaled the weight of the more numerous large zooplankton lower in the food web. Those zooplankton, in turn, roughly weighed the same as the still more numerous, smaller zooplankton further down the chain, and so on.

The journal article is the first scientific report linking such a pattern to a food web, says Cohen. He and his colleagues presented the data in various graphical representations, each useful in a different way.

In the journal paper, Cohen, Tomas Jonsson, Ph.D., a former postdoctoral researcher in Cohen’s lab who is now at the University of Skövde, Sweden, and Stephen Carpenter, Ph.D., of the University of Wisconsin, analyzed data from Michigan’s Tuesday Lake. If further research confirms the tool’s generality beyond the one ecological community studied, then ecologists will be better able to say just how rare a particular big, fierce animal ought to be—or explain deviations from the expected pattern.

The data were collected in the mid—1980s by Carpenter, an ecologist who studies and manipulates aquatic communities. Carpenter’s data on Tuesday Lake met a need in Cohen’s research, because the information combined body size and species abundance numbers with a food web.

When the community of species occupying the upper layer of the water in the small lake was thoroughly documented in 1984, about 56 species were present. About half were microscopic plants called phytoplankton that drift in the water. Most of the rest were zooplankton, small animals that eat the phytoplankton and, in the case of a few of the largest zooplanktors, eat other zooplankton. Finally, three species of small fish occupied the pinnacle of the Tuesday Lake food web.

From the extremely numerous phytoplankton to the far less numerous fish, the number of individuals per species spanned 10 orders of magnitude. Thus, the commonest species was 10 billion times more numerous, per cubic meter of lake water, than the rarest. The body size varied even more, by 12 orders of magnitude from largest to smallest.

But since the rare species tend to be large and the common species to be small, when abundance and average body size are multiplied together to get biomass for a species, the numbers are roughly constant from species to species.

This finding was determined when Cohen and his coauthors graphed each species’ average body mass against its numerical abundance. The result, when plotted on logarithmic scales on both axes, is a nearly straight line from rare, heavy species to common, light species. Many data points, each representing one species, are close to the fitted line; a few are farther off the line.

“I’m interested in explaining the deviations,” Cohen says. For example, a species far above the trend line is one with, on average, a larger population than its average body size alone would predict. That leads to scientifically testable questions: Why is the species more abundant than expected? Is the productivity of the species on which it feeds greater than expected, and if so, why is that?

Cohen, who heads Rockefeller’s Laboratory of Populations, hopes to find more data sets that incorporate average body size and abundance with food web relationships, so he can test if these relationships are more general.

“Ideally, we would like to create communities in the lab,” he says. “You can yank out one thing and see how other species react.”

In fact, this sort of manipulation was done at Tuesday Lake, where researchers removed almost all the fish in 1985 and replaced them with an equal weight of a larger, predatory fish, the largemouth bass. The bass then ate all the remaining little fish of the original three species. Carpenter and colleagues then collected food web, body mass, and abundance data again in 1986.

On analysis, Cohen and Jonsson found that although species changed between the two sampling years, ecological patterns remained almost the same. “The manipulation,” he says, “had remarkably little effect on the relationship between biomass and abundance. The natural history of the system changed, but not the ecology.”

“It was as if you had a different cast of characters acting out the same play,” he says.

Indeed, Cohen’s basic research on ecological communities relates to his applied work with a completely different cast of characters: research to understand and reduce the transmission of the trypanosome that causes Chagas disease in Latin America. There a human household with resident dogs, cats, chickens—and bugs that transmit the infection of Chagas disease by feeding on all the warm—blooded vertebrates—may be viewed as an ecological community with its own food web. Understanding how to describe such systems better may suggest better ways to intervene to prevent transmission of infection.