June 24, 2003 CHAMPAIGN, Ill. -- High in the canopy of a Neotropical Panamanian forest, researchers have discovered that birds, especially native ones during the rainy season, protect trees by reducing the numbers of leaf-eating insects.
The finding -- being published this week on the Online Early Edition of the Proceedings of the National Academy of Sciences -- was a mild surprise, said researchers at the University of Illinois at Urbana-Champaign. That birds help crops and low-lying plants in temperate forests by devouring insects had been known. However, many scientists had theorized that the rich diversity of life in tropical forests would diffuse any significant contributions by birds.
Worldwide, but especially in Neotropical forests, bird populations are declining amid forest fragmentation as areas are cleared for ranching, farming or housing. The yearlong research project, the first to study bird-arthropod interactions in a Neotropical forest canopy, was carried out by Sunshine A. Van Bael, a doctoral student in animal biology at Illinois and a research fellow at the Smithsonian Tropical Research Institute.
"We found that birds were indirectly defending trees by consuming their herbivore pests," said Van Bael, who studied bird activity on 44 canopy branches of three tree species on 18 individual trees and 38 saplings (younger ones under the canopy).
During the rainy season (May to December), the number of chewing arthropods is 90 percent higher than in the dry season. Native tanagers, including the blue-gray, crimson-backed, plain-colored and white-shouldered, and native vireos, particularly the lesser greenlet, dined extensively on caterpillars easily visible on the tops of leaves.
The 31 species of birds observed -- while preferring smooth to spiny caterpillars -- ate enough to drastically reduce damage to leaves in comparison to damage done to leaves in specially built exclosures where access by birds was limited.
Where the birds could not reach, the densities of leaf-eating insects were much higher. Average damage levels by the end of the rainy season increased by 86 percent where the foliage was not reached by the birds.
Van Bael's research was done using a high-rise canopy crane that was built in 1990 by the Smithsonian Tropical Research Institute to study tree physiology. There are now nine cranes being used for forest-related research around the world. The crane allowed for repeated observation of branches where foliage was inaccessible or accessible to birds.
"This research, because of good design and controls, lets us make solid inferences. This is a good example in that it shows the ecological services that birds provide for the health of the forest," said Jeffrey D. Brawn, a professor in the department of natural resources and environmental sciences and department of animal biology at Illinois. "This is another piece of evidence suggesting that if we tinker too much with our forests, there will be adverse effects. If I were growing trees for a living, I'd want some birds around."
Brawn and Scott K. Robinson, a professor of animal biology, were Van Bael's faculty advisers and co-authors of the PNAS paper, which will appear later in a print issue of the journal.
"Birds clean herbivores off of trees," Van Bael said. "This is important because we need our trees, especially in the tropics, to be healthy. They provide us with oxygen and sequester carbon emissions, protecting us from global warming. Understanding how animals interact with trees will help us keep the forests healthy."
The research also considered the impact of birds on leaf-eating insects under the canopy and by migratory temperate-zone birds, including Tennessee, bay-breasted and chestnut-sided warblers, which begin arriving in Panama in August. The migratory birds ate large numbers of insects off the leaves in the late rainy season. In the dry season (January-April), when insect densities are lower, migratory birds eat fruit that they can reach on the trees.
Below the canopy, significant reductions of insect populations on the accessible leaves of lower level vegetation were not observed during the insect-laden rainy season.
The three tree species that were involved in the study were Anacardium excelsum, Cecropia longipes and Cecropia peltata.
Van Bael returned to Panama in late May to begin postdoctoral research at the Smithsonian Tropical Research Institute. In a project beginning in September and funded by the National Zoo's Migratory Bird Center, she will evaluate plantations of cacao plants (chocolate beans), as potential habitats for migratory birds, which have been losing their habitats to deforestation in the region.
Cacao production is being encouraged to create buffer zones between forests and clear-cutting for farms, and, Van Bael said, to help reduce erosion and subsequent damage to mangroves and coral reefs along the coast. Cacao plants grow readily in the shade of forest canopies.
"Herbivore activity itself isn't a huge problem, but fungal disease, often carried by arthropods, is the biggest challenge facing cacao production," she said. By possibly attracting more migratory birds, she added, their numbers on the plantations could be increased and they would have a job to do.
"The findings of our PNAS study give strength to the argument that natural controls of insect and arthropod populations are going to be advantageous," Brawn said. "If one wanted to get rid of pests with pesticides, it would be very expensive. The use of birds would be cheaper and done naturally."
The study also provides scientists with baseline knowledge that may prove useful for forest management as climatic changes alter the interactions between plants and animals, Van Bael said.
The study in PNAS was funded by grants to Van Bael from the U.S. Environmental Protection Agency STAR Fellowship and Smithsonian Institution Fellowship programs, American Ornithologists' Union, American Natural History Museum and the University of Illinois at Urbana-Champaign.
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