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June 12, 2007

The kapok connection: Study explains rainforest similarities

ANN ARBOR, Mich.—Celebrated in Buddhist temples and cultivated for its wood and cottony fibers, the kapok tree now is upsetting an idea that biologists have clung to for decades: the notion that African and South American rainforests are similar because the continents were connected 96 million years ago.

Research by University of Michigan evolutionary ecologist Christopher Dick and coworkers shows that kapok—and perhaps other rainforest trees—actually colonized Africa after the continents split, as a result of seeds traveling across the ocean.

The findings were published online June 7 in the journal Molecular Ecology.

"Our study shows how oceanic dispersal links the world's rainforests, and it's one of the first to catch oceanic dispersal in action at the species level for a rainforest tree," said Dick, an assistant professor of ecology and evolutionary biology and an assistant curator at the U-M Herbarium. "Although single seeds are very unlikely to survive an oceanic voyage and then successfully establish, such improbable events become probable when summed over 10 to 15 million years."

Dick studied the rainforest form of Ceiba pentandra, a species of kapok that grows taller than a 16-story building, with its head poking above the forest canopy. Its flowers produce more than 50 gallons of nectar per tree in a season, attracting bats that travel as far as 12 miles between trees and transfer pollen in the process. When the seed pods ripen, they break open to reveal fluffy fibers that are used to stuff pillows and mattresses. The seeds, which are about the size of an unshelled sunflower seed, are buoyant and able to float down rivers along which the colossal trees grow.

In the just-published research, Dick and collaborators at the Smithsonian Tropical Research Institute and the Instituto Nacional de Pesquisas da Amazônia in Manaus, Brazil investigated which of several possible scenarios best explains the current distribution of C. pentandra.

One possibility is that the species was widespread on both continents before they drifted apart—the so-called Gondwana vicariance hypothesis. If that were true, the South American and African populations would have been separated for at least 96 million years. Using "molecular clock" information, Dick was able to reject that scenario.

The process he used is something like comparing successive manuscripts written by the same author. If you know how many changes the author made each day and how many days she worked, you can determine how different the second draft must be from the first. Similarly, if you know how quickly or slowly changes accumulate in DNA, you can calculate how genetically different two populations should be, based on how long they have been separated.

Dick's analysis showed that for the Gondwana hypothesis to be true, the two populations' DNA should have diverged by 50 percent. But the actual divergence was zero. In other words, the African population is too young to have been created when the continents split.

Another scenario, the Boreotropical hypothesis, suggests that during a warm phase in Earth's history, tropical plants spread into high latitudes and crossed a land bridge between Europe and North America that disappeared some 35 million years ago. For this to be true, the expected difference between the South American and African populations is 20%, not zero. Again, the African population is too young to confirm the hypothesis.

"That leaves dispersal by wind or ocean currents or by humans as possible explanations," said Dick, who is also a research associate at the Smithsonian Tropical Research Institute. "It's been suggested that Ceiba was introduced into Africa by humans during the slave trade, but my research shows that it dispersed from tropical America to Africa much earlier than that and was already being cultivated for kapok fibers in the 10th century, when Arab traders transported it to southeast Asia." Indeed, 10th century Buddhist temples are decorated with sculptures of kapok trees.

After ruling out the other possibilities, Dick concluded that extreme long distance travel by wind or ocean currents best explains how Ceiba pentandra spread from South America to Africa. He plans to continue investigating the role of oceanic dispersal to see if the same is true of other species and entire plant communities.

Though his studies focus on dispersal that occurred in the past, his conclusions are relevant to present-day conservation issues.

"Ceiba has become locally extinct in parts of the Peruvian Amazon, due to overexploitation for plywood," Dick said. "It may be saved from widespread extinction by continuing to invade new land areas through oceanic dispersal."

Dick's coauthors on the paper are Eldredge Bermingham of the Smithsonian Tropical Research Institute and Maristerra Lemes and Rogerio Gribel of the Instituto Nacional de Pesquisas da Amazônia. The National Science Foundation, the International Plant Genetics Resource Institute and the Smithsonian Tropical Research Institute provided funding.