Nature equips plants with a variety of tools to disperse their seeds. They can simply drop them or release them to the wind or a nearby stream. Plants with fleshy fruits have the advantage of enticing hungry animals to partake of their delicious bounty, carrying viable seeds in their digestive tracts, thereby depositing them far and wide.
However, these seed dispersers are not the only organisms eating the fruit. Insects and plant pathogens are also drawn to the fruit, but rather than aiding propagation, these predators consume and kill the seeds.
Utah State University ecologists, along with colleagues at Virginia Tech and Loyola University, hypothesize these varied interactions are driving the evolution of the high diversity of chemical compounds found in plants.
“One small plant tissue sample may contain thousands of plant compounds,” says Noelle Beckman, assistant professor in Utah State’s Department of Biology and the USU Ecology Center. “A complex combination of processes explains and affects this biodiversity.”
Plants produce an enormous diversity of secondary metabolites — that is, compounds not essential for their growth and reproduction, she says. “We’re exploring the evolutionary mechanisms that maintain this diversity and how these secondary metabolites may mediate interactions between the plants and animals.”
Beckman is the lead principal investigator for the multi-institution effort, which is supported by National Research Foundation grants totaling more than $1.5 million. Much of her lab’s research depends on data collected from Panama’s Barro Colorado Island, site of the century-old Smithsonian Tropical Research Institute. Situated on the man-made Gatun Lake in the middle of the Panama Canal, the site is one of the most intensely studied topical forest systems in the world.
Beckman and her lab members Gerald “Jerry” Schneider, a postdoctoral fellow, and Elsa Jos, a doctoral student, have spent time on the Central American island, observing birds and mammals feeding from fruit-bearing plants.
“Barro Colorado is an incredible place not only for its nature, but as a place for international research collaboration,” Schneider says. “Our current project involves investigating what’s chemically distinctive about fruits and seeds that rely on a particular mode of seed dispersal, as well as whether these distinctive secondary metabolites might also be found in the plants’ leaf tissue.”
At Utah State, Beckman and her team carefully extract chemicals from the collected leaf and fruit samples from a range of species.
“The goal of the project is to collect fruit and leaves from 50 species that capture the scope of variation in seed dispersal mode in a diverse tropical forest,” she says. “Some are dispersed by birds, some by mammals and some by the wind.”
In addition to this large-scale collection effort, Beckman’s team is investigating the fruit chemistry of 21 species of Psychotria, a hyperdiverse genus found throughout the tropics. In the same family as the coffee plant, Psychotria includes species that are small trees and shrubs with seeds dispersed by small birds, such as manakins and thrushes.
Beckman says the team’s research is aimed at determining evolutionary inferences about the selected forces that seed dispersers may exert on fruit chemical traits.
“We expect to find greater diversity of secondary metabolites in the plant’s fruits than in the leaves,” she says. “We think this is linked to diverse evolutionary pressures from plant pathogens and insect seed predators that promote fruit defense, as well as from seed dispersers that promote the attractiveness of the fruit for consumption.”
“Our efforts represent the most integrative study to date of the chemical ecology of seed dispersal and fruit defense,” Beckman says.
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