April 7, 2004

Diversity begets diversity: Experiments confirm ecological tenet

In contrast with traditional coffee farming practices, some coffee growers have cleared forests to grow coffee in monoculture.

ANN ARBOR, Mich.—In discussions of biological diversity, ecologists long have maintained that variety in one part of an ecosystem, say plants, leads to variety at other levels, such as insects that feed on or make nests in those plants. It's a logical assumption, but one that has been difficult to test.

In the April 9 issue of Science, University of Michigan researchers report results of an experiment with twig-nesting ants showing that diversity does indeed beget diversity, but apparently not for the reason ecologists have always thought.

"The reason was thought to be very straightforward: If you have different resources, then different types of organisms will use those resources," each finding its own, specialized niche, said Ivette Perfecto, associate professor of Natural Resources, who collaborated on the work with graduate student Inge Armbrecht and John Vandermeer, professor of ecology and evolutionary biology. "But we found that the ants we studied prefer diverse resources for the diversity per se, not because different species of ant are using different resources."

Ants preying on the coffee berry borer, one of the main coffee pests in Colombia.

The experiment took place in a coffee plantation in Colombia, where ants are known to nest in hollow twigs in the leaf litter. The point was to test whether twigs collected from a diverse mix of shade trees, typical of traditional coffee farms, would harbor a more varied assortment of twig-nesting ants than twigs that came from a single species of shade tree. The researchers cut similar-sized twigs from living trees, let them dry for a month and bored holes in the twigs. Then they placed the twigs in mesh bags, in either single-species or mixed-species groups of eight twigs per bag. The bags were placed on the ground in a grid pattern that alternated single species and mixed species bags, and the ants nesting in the bagged twigs were surveyed twice over a five-month period.

"More ant species and more ant colonies were found in the diverse twigs, either because they chose them more frequently or because they survived better in them," Armbrecht said. "The striking part is that the ants we found were not specialists occupying different 'compartments' or niches." In other words, the context in which the twig was found—single species or mixed species—seemed to matter more to the ants than the species identity of the twig.

A sample of twigs used in the experiment.

The explanation for the results still has the researchers scratching their heads. One possibility is that ants, which are known for their chemosensory abilities, perceive the mixtures as somehow different and more appealing than all-the-same environments. It's also possible that the mix of different tree species somehow influences the way twigs retain water and decompose, which may in turn affect the variety of bacteria, fungi and small insects making up the food web that ants utilize.

What is clear, said Perfecto, "is that the whole is more than the sum of the parts. It sounds like a cliché, but we have demonstrated it to be true."

The results have implications for coffee growing and for conservation in general, said Armbrecht, who has completed her studies at U-M and is on the faculty at Universidad del Valle in Cali, Colombia. In traditional coffee farming, coffee bushes are grown under a variety of native trees. But in recent years, many traditional plantations have been altered to produce higher yields. In the most extreme cases, forests have been cleared, and sun tolerant varieties of coffee now are grown on virtually treeless slopes; other growers plant economically valuable trees, such as cacao, to shade coffee plants.

A typical rural landscape in the region of Colombia where the study was conducted. Coffee plantations with different amounts of shade are interspersed with cattle pastures.

"For coffee growing, this study illustrates the need to promote diversity of shade trees in coffee plantations," because of the potentially widespread effects on biodiversity throughout the ecosystem, Armbrecht said. That's probably important for keeping the coffee plantation healthy—some species of ants prey on coffee pests—but the effects are even more far-reaching, Vandermeer said. "There's a growing body of literature suggesting that tropical agricultural ecosystems, in addition to natural preserved areas, are important repositories of biodiversity."

The broader conservation message, Armbrecht added, is that "reduction of biodiversity at one level, because of human activities, might release a cascade of catastrophic consequences for biodiversity at other levels. This, from the precautionary point of view, means that all levels of biodiversity should be protected simultaneously."

The researchers hope their findings will spur other scientists to do similar experiments in both natural and managed ecosystems.

"We think this opens up a whole new world of interpreting biodiversity," said Vandermeer, "and we hope that other people will pick up on it."

Related links:

Ivette Perfecto

John Vandermeer

Inge Armbrecht's research group at Universidad del Valle

The Biodiversity Center

"Biodiversity . . . What is it?"

Contact: Nancy Ross-Flanigan
Phone: (734) 647-1853
E-mail: rossflan@umich.edu

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