In Oosting Lecture, John Terborgh Asks: Which Processes Have Most Effect on Species Diversity: Top-down or Bottom-up?

April 12, 2015
Contact:

Tim Lucas, 919-613-8084, tdlucas@duke.edu

By Kati Moore (MEM ’16), Nicholas School Communications Student Assistant

DURHAM, N.C. – Renowned tropical ecologist John Terborgh delivered the 2015 Henry J. Oosting Memorial Lecture in Ecology to a crowd of students, faculty, and professionals in Love Auditorium in Duke University’s Levine Science Research Center on April 10.

The lecture was presented by Duke’s Nicholas School of the Environment, the University Program in Ecology and the Department of Biology.

Terborgh, who is research professor of environmental science the Nicholas School and director of Duke’s Center for Tropical Conservation, lectured on the long-disputed question of whether top-down or bottom-up processes are more important for maintaining species diversity in an ecosystem.

He started with a brief introduction to this field of study, explaining the current theories and models of species coexistence and how research has supported or refuted these ideas. He included many examples from his own work, including a 17-year study of the Guri Reservoir in Venezuela.

When the valley was flooded to create this reservoir, hundreds of hilltops became islands, drastically limiting the habitats of the plants and animals that remained. Top-level predators, such as jaguars and harpy eagles, were lost entirely.

Loss of these predators resulted in a trophic cascade: with no predators, prey populations, which were mainly herbivores, increased dramatically, leading to overgrazing of plants.

Terborgh cited this as just one example of top-down ecosystem control. He described similar processes that have been documented around the world. These examples included the changes in Yellowstone National Park’s ecosystem after the reintroduction of wolves, forest retreat in the Serengeti after wildebeest populations suddenly increased, and dramatic decreases in native predator populations in the Everglades since the introduction of Burmese pythons.  

Top-down processes, such as predators controlling prey populations, and bottom-up processes, such as competition for resources, both play a role in a “fully fleshed-out” ecosystem, Terborgh said. In many cases, however, it is still difficult to determine which force has the stronger effect.

To illustrate this point, Terborgh closed his talk with the example of Caribbean grouper and parrotfish species:

Grouper have low local diversity and high competition, whereas parrotfish have high local diversity and low competition. Is this because grouper have more limited resources, and parrotfish have more abundant resources? Or is it because grouper don’t have any predator species controlling their populations, and parrotfish do?

“One of those two is wrong – they can’t both be right. What do you think?” Terborgh said, and left the question hanging.

Students said they found the talk interesting and engaging.

“It was great to hear about his research on topics we’ve been learning about all semester,” said Trisha Gopalakrishna, MEM ’16, who is in John Poulsen’s Tropical Ecology class. “It was exactly what we studied.”

Terborgh’s newest book, “Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature,” was published by Island Press and is available here: http://islandpress.org/trophic-cascades.

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