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Multi-species herbivore outbreak follows El Niño drought in Panama


Plant-eating insects inhabit all forest ecosystems, but sometimes their numbers explode, resulting in massive tree defoliation. In the October issue of the Journal of Tropical Ecology researchers from the Smithsonian Tropical Research Institute (STRI) associate a severe moth outbreak with drought conditions following the 1997-1998 El Niño Southern Oscillation (ENSO) event in a dry lowland forest near Panama’s Pacific coast. If ENSO events become more common, repeated herbivore outbreaks might alter forest species composition.

"Although we regularly monitor insect populations from STRI’s Canopy Crane system, we were actually tipped off about the outbreak by the crane operator, who complained that his parked car was covered with insect droppings," explains STRI post-doctoral fellow, Sunshine Van Bael.

Moth larvae devoured 250% more leaf material than usual even as researchers were setting up an experimental protocol to monitor herbivores and leaf damage on twenty tree species reachable from the gondola on the arm of a 42 m tall modified construction crane in Panama City’s Metropolitan Park. The crane, one of two in Panama and nine worldwide, permits scientists to ascend into the treetops to study previously inaccessible aspects of forest dynamics and species diversity.

Dr. Annette Aiello, Staff Scientist at STRI noted smaller outbreaks at five other Pacific coast sites. Wondering if this was a country-wide phenomenon, entomologists at STRI’s research station on Barro Colorado Island and at the Sherman Canopy Crane near Panama’s Atlantic coast were advised to be on the lookout for increases in butterfly and moth larvae populations. However, lepidopteran numbers remained stable there. Whatever it was that caused the the Pacific Coast outbreak did not occur or have the same effect where rainfall levels were more constant.

"This outbreak was unusual because it involved a dozen insect species, and followed a drought associated with one of the most severe El Niño events experienced here. We don’t know how common outbreaks like this are in the tropics because people have begun to associate events like this with global climate change fairly recently," says Van Bael.

What caused the outbreak? Most likely the drought kept the caterpillars’ natural enemies out of play, but the exact cause will remain a mystery. The team monitored lepidopteran populations again, after a milder El Nino event in 2003, but saw no large-scale outbreaks.

Drought stress of host trees is thought to be a factor in temperate area outbreaks of spruce bud worm and gypsy moths. But such outbreaks often last for up to 10 years. This short-lived (5-6 week) event in Panama was brought under control quickly by hungry birds, parasitoid flies and wasps, and aggressive caterpillar diseases.

Climatologists predict that ENSO events will become more common as global temperatures rise. And because herbivores hit some tree species harder than others, repeated herbivore outbreaks could change the species composition of the forest. In a recent volume of the Journal of Tropical Ecology, ecologists in Borneo reported a similar outbreak in response to the ENSO event in 1998.

Van Bael cautions: "It’s becoming increasingly clear that we should pay attention to these harbingers of climate change. A better understanding of the deep natural history of these systems may help us to predict outbreaks involving crop pests or human disease vectors."

Beth King | EurekAlert!
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