The forests of Barro Colorado Island are teeming with life. Weevils crawl along branches in search of leaves. Birds land on piper plants, eating the fruit just like a corncob.
These piper plants are just one of a number of species currently being analyzed as a part of a multi-institutional project to understand the chemistry behind plant-animal interactions.
Edwin Perez is a Panamanian biology field technician with the Smithsonian Tropical Research Institute. He just finished collecting fruit from Hirtella triandra, and other fruiting plants for the day. He is chatting with Utah State University researcher Elsa Jos about which species are currently fruiting.
One of the primary researchers on this project is Susan Whitehead, associate professor of biology at Virginia Tech.
“We want to try to understand how the animals that disperse seeds, how they might have influenced plant chemical evolution,” Whitehead explains. “We know plants produce hundreds, sometimes thousands, of different specialized metabolites that seem to mainly function in biotic interactions, but we don't really know why plants do that. Why do they have to produce thousands of different things, and there's such huge diversity. And so we think that the seed dispersal interactions might be a really important part of that story but haven't been very well-studied.”
Plants protect themselves from potential predators like insects, herbivores, and pathogens through toxic chemicals within the plant. At the same time, they need to attract seed dispersers like birds, bats, and monkeys in order to move their seeds, so over time they have evolved mechanisms to both attract and deter other organisms.
These researchers are trying to understand the relationship between the evolution of these mechanisms and ecosystem interactions.
Answering complex questions about relationships like this requires looking at a number of angles. In this case, USU Associate Professor Noelle Beckman and her team are looking at both the plants themselves as well as the animals involved in the dispersal of their seeds.
“The first part of the project is quantifying, describing the diversity of chemicals that are in fruit and in leaves over a wide range of species that vary in how they're dispersed,” said Beckman.
The team first identified plant species that were dispersed by different means — some that are primarily dispersed by birds, mammals, bats, and by wind or other non-animal methods. Next, they collect the fruits, seeds, and leaves of those plants in order to identify secondary metabolites and those that are specifically associated with differing methods of seed dispersal.
“So we want to understand how those different types of dispersal, right, might shape the chemistry of the plant,” said Whitehead.
In the previous episode, we learned that secondary metabolites are chemical compounds found in organisms, such as plants, that assist in things like defense against predation. This group of researchers is hoping to understand the relationship between those metabolites in plants and their seed dispersers.
The next step is to use mathematical modeling that takes into account the dispersal mechanism, as well as the chemistry of the leaves and fruits.
“Using different types of models to think about the ecology and evolution of why this chemistry in these various plant species may have arisen due to the different interactions they have with animals, or that they don't have,” Beckman said.
Ray Dybzinski is an associate professor in Loyola University Chicago’s School of Environmental Sustainability whose interests lie in understanding systems from a mathematical perspective.
“My role in this project is to do some, what I call deterministic modeling of the system. So trying to understand the system conceptually, translate that into math, and try and determine what novel insights we can get by doing that,” said Dybzinski.
The goal of Dybzinski’s work is to use modeling to determine which parts of the system are important by looking at the evolutionary relationships between plant chemistry, mode of dispersal, and predation pressures.
“The beauty and challenge of that is trying to articulate all of the aspects of the system that really matter,” Dybzinski said. “And also leaving out the aspects of the system that don't really matter for the question they're asking.”
The final step in their project is to focus on the food preferences of the seed dispersers themselves. They collected large numbers of fruits from those plants in order to extract specific secondary metabolites. Then they will feed these to captive Seba’s short-tailed fruit bats and palm tanagers.
“We take fruits that we've collected from the forest, we extract all the different chemicals that are in the fruit, and then we try to look at the broad bioactivity of those extracts,” explained Whitehead. “And so we are doing experiments with birds and with bats, where we offer them different food choices, and we ask them, okay, how do the compounds we extracted from this fruit influence your preference for this diet that we're feeding you.”
Whitehead is the lead on this part of the project and expressed that she expects the food containing chemical compounds extracted from the fruits will not actively deter the bats and birds.
“We can measure their preferences for that diet versus that same diet where we added some of the compounds that we extracted from the fruit. So we keep everything controlled, everything the same, other than just that chemical composition of what, what their food choices are,” said Whitehead.
The food preference study is set to begin in the next few months, completing the years-long process of exploring the roles plant chemistry plays in greater ecosystem relationships.
