Dec. 24, 2003 While visiting the herbarium at the Natural History Museum in London and wandering through rooms piled high with plant materials from Cook’s and Darwin’s voyages, University of Arkansas professor Cindy Sagers contemplated the role of insects in the diversification of plant species. Her musings led to a collaborative grant that will allow her to study plants at high altitudes in Ecuador.
Sagers, Nancy Garwood of the Museum of Natural History in London, and Paola Barriga of Universidad Pontifica Catholica in Quito, Ecuador, have received a Women’s International Scientific Collaboration award from the National Science Foundation and the American Association for the Advancement of Science to study a select group of Cecropia plants in Ecuador. These particular species of Cecropia stand out because they have no ants associated with them—almost all of these tropical plant species have an ant species that lives on them and uses them as food. Of the seven ant-free species, six are found at high altitudes in Ecuador.
Cecropia trees sport huge, lobed leaves that form in spirals on slender stems and frequently grow in disturbed landscapes—often along roadsides. Male and female cigar-shaped flowers grow on separate trees. Sagers and her colleagues will spend the summer in Ecuador collecting leaf samples from the high-altitude and lower-altitude plants for genetic analysis. They hope to gather specimens from about 23 species in all. This will allow them to trace the evolutionary history and relatedness of these plants. Sagers and her colleagues also will examine nitrogen stable isotopes in the plants and the carbon stable isotopes in any ants found on the plants to see what the ants are eating. Because plants and animals have distinct isotope signals, the contribution of each item to the ant diet can be assessed, and this may lead to better understanding of the relationship between plant and insect.
Mutual relationships, where two species live together and benefit one another, prove to be rare in nature. The ant-plant mutualism, where ants defend the plant against more destructive bugs while still taking food from the plant, stands as a classic example of this model.
But the lack of ants on the high-altitude plants may mean that at some point the plant had to shed the ants to continue its survival. A shift from abundant to harsh conditions, such as cold temperatures and low nitrogen levels, might change a mutual relationship to one of parasitism, where survival becomes a matter of exclusion rather than cooperation.
"Losing ants may have allowed Cecropia to move uphill," said Sagers. "This research will help us determine what conditions in nature allow mutual associations to exist."
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