Tropical rainforests are the most diverse and complex natural communities on the planet, but whether that complexity makes them more stable or more fragile than other ecosystems is hotly debated among ecologists. Now an elegant field experiment conducted with the help of Earthwatch teams at La Selva, Costa Rica demonstrates indirect effects on a tropical forest food web following the introduction of a new top predator.
Dr. Lee Dyer, a professor at Tulane University, and Dr. Deborah Letourneau of the University of California, Santa Cruz, reported on the field experiment in a recent article in Ecology Letters. The article explores "diversity cascades," or indirect changes in a food web, among invertebrates associated with the tropical shrub, Piper cenocladum.
"This is one of the first solid examples of a terrestrial diversity cascade," said Dyer, principal investigator of the Earthwatch-supported Rainforest Caterpillars project. Although diversity cascades have been identified in aquatic food webs, which are much simpler, their presence in more complex terrestrial systems has been controversial. "It clears up some questions about how rainforests respond to perturbations, or disturbances, which is important given the huge losses of diversity that continue to occur in these forests."
The shrub Piper cenocladum hosts a diversity of invertebrate animals, from symbiotic ant colonies (Pheidole bicornis) that protect the plant from herbivores to other insects, mites, worms, and crustaceans that feed on plant matter as well as each other. The complex food web found in and on each individual shrub is a microcosm of the larger community of the rainforest, and served as the subject of Dyer and Letourneau's experiment.
The researchers introduced a new predator, the clerid beetle Tarsobaenus letourneauae, to half of their sample of 80 shrubs. When they removed the shrubs 15 months later, Earthwatch teams helped them count the approximately 43,188 invertebrates living on them, representing more than 50 species.
Dyer and Letourneau found that the invertebrate communities living on shrubs with the introduced predator were significantly altered, the result of direct and indirect effects (diversity cascades). These findings are consistent with the hypothesis that the complex relationships of rainforest communities are vulnerable to disturbance.
"On the one hand, the high diversity and complexity of tropical rainforests is thought to make them especially stable," said Letourneau. "On the other, rainforest biodiversity is thought to make it fragile, like a house of cards, where the removal or addition of a single card could cause a major disruption. The presence of diversity cascades supports the characterization of tropical rainforests as vulnerable ecosystems whose integrity may be threatened by the erosion of complex interactions among species."
Earthwatch teams continue to support Dyer's work in Costa Rica, focusing primarily on the ecology of caterpillars that are among the most voracious herbivores in the Piper shrub system. Their research stands to tease apart the complex interrelationships that are essential to understand for more effective conservation of tropical rainforests.
For more information, see Dyer, Lee A., and Deborah Letourneau. "Top-down and bottom-up diversity cascades in detrital vs. living food webs." Ecology Letters, (2003) 6: 60-68.
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