More Productive, Less Diverse

Discover, September 30, 1994

Nobody, says University of Arizona ecologist Michael Rosenzweig, is going to come up blind with a program to save biodiversity without understanding why it’s here and how it is maintained. And one of the problems that is stumping Rosenzweig and other biodiversity researchers these days is a strange pattern that keeps popping up wherever they look, from the deep seafloor to the Negev Desert. The pattern is this: When organisms get more of what they need, more nutrients, sunlight, and water, their diversity rises–but only up to a point. After that point, more resources mean more productivity–the rate at which plants grow, making food for animals–but less diversity. It makes absolutely no sense, says Rosenzweig. It’s loony.

The conventional wisdom that prevailed until recently made a lot more sense. It held that higher productivity invariably led to more diversity. The reasoning was simple. Ecosystems are inhabited by species running the spectrum from common to rare. In an unproductive habitat, animals will find less food, and so there will be fewer of each species. Below a certain population, the less common species are easily wiped out by catastrophes like floods or fires, lowering diversity. A productive habitat, on the other hand, is a banquet with room at the table for everyone.

Rosenzweig first began to question the conventional wisdom in the early 1980s, when he did a survey of rodents in Israel with ecologist Zvika Abramsky of Ben-Gurion University. The survey showed that, as expected, diversity rose with productivity, but only up to a certain level. Then it fell. Grasslands that got two feet of rain a year had fewer species of rodent than places in the Negev that got only six inches. We published it as a curiosity, Rosenzweig recalls. We thought maybe Israel is a special place.

But as Rosenzweig delved further into the matter, he found that Israel was not special at all. As early as the1960s, deep-sea biologists had discovered that the abyssal plain of the Atlantic Ocean–one of the most food-poor environments on Earth, where animals subsist on scraps of organic matter falling from the sunlit sea surface–had a greater diversity of species than the far more productive shallow waters near coastal areas. The greatest diversity of all occurred at intermediate depths, on the continental slope, where the productivity was also intermediate. Rosenzweig started collecting other such surveys and doing more of his own, and everywhere he looked–among large mammals in Africa or birds in South America–he found the same rise and fall of diversity he had first seen among rodents in Israel. Increasing productivity always led eventually to a loss of diversity. It’s only been in the last five years that we’ve realized the pattern exists around the world, says Rosenzweig. At some fundamental level we ecologists had failed to see something here.

University of Minnesota researcher David Tilman first saw the pattern in a mathematical model. Ecosystem modelers usually like to make their work easier by spreading resources smoothly. When Tilman took the more realistic step of spreading them unevenly–less nitrogen in the soil here, more water there–he found that his model generated a greater diversity of plants. Since each plant species could excel at some ratio of resources, he says, many different species could divide up the ecosystem and coexist happily.

A patchy distribution of resources, Tilman thinks, is a key to high diversity–and that’s the distribution of resources you get when the overall level of resources is neither very high nor very low. At each extreme the resource distribution is more uniform, and so intense competition, instead of coexistence, is the rule. In very unproductive habitats, Tilman explains, all the plants are limited by a single resource, nitrogen. On the other hand, in an extremely productive ecosystem, there’s so much biomass that little light gets through the canopy. So now the plants are all competing again for a single resource– light. Tilman has some experimental evidence to back up this theory: for the past 11 years he has fertilized plots of Minnesota prairie grass, and as his theory predicts, the most heavily fertilized plots have lost the most diversity. In those plots there is so little light reaching the ground–as well as so much plant litter–that the seedlings of some species cannot survive.

Different animal groups sometimes compete for the same resources, too, and Rosenzweig thinks that may help explain why their peak diversity occurs at different levels of productivity. Around Tucson, for instance, where Rosenzweig lives, rodents like kangaroo rats and pocket mice compete with ants for seeds.The rodents do well in the desert, with 12 species thriving. But where the desert gives way to more productive grassland, the rodent diversity drops off. It may be, Rosenzweig suspects, that ants are so much better at consuming the more abundant food that they drive out some of the rodents. To test this hypothesis, Rosenzweig is boosting the productivity of an experimental plot of desert; he has set up feeders that dispense millet seeds to rodents and ants alike. He expects the success of the ants to cut down the species diversity of the rodents in the vicinity of the feeders.

Neither he nor Tilman nor anyone else is anywhere near a general explanation of why biodiversity peaks at intermediate levels of productivity. Tilman’s patchy-resource theory, for instance, was partially contradicted by his own experiment: when he fertilized plots of prairie unevenly, they actually became less diverse. On the other hand, everyone seems to agree that at high productivity, biodiversity does decline. That has disturbing implications.

When thinking about the loss of biodiversity, we often visualize chain saws in the rain forest. But the link to productivity suggests we can also destroy diversity in a subtler way. The fossil fuels we burn and the cattle we raise on high-protein grain–which increases the nitrogen content of their urine–are constantly spewing nitrogen into the atmosphere. More than 30 million pounds of it rains back down to the ground each year. We are thus fertilizing the entire planet, and presumably we are making its ecosystems more productive, as Tilman did with his plots of prairie. Our experiments show where the North American prairie will be 20 years from now, Tilman says. We can expect significant loss of diversity. We can’t save species on land if whole communities are changed, and that’s the direction we’re going.