In a talk packed with elephants, African buffalo, and Yellowstone wolves, Sean B. Carroll, AB ’79, explored four key lessons of conservation biology in his William C. Ferguson lecture on March 19. 

A renowned biologist, award-winning author, Emmy- and Peabody-winning film producer, Carroll has spent much of his career studying and explaining the population dynamics of the Serengeti, the vast, grassy plains of Tanzania and Kenya that teem with large mammals. 

The talk included data and personal photos from that biodiversity hotspot, but the lessons could apply to any ecosystem with grazers and predators, whether it’s a lake, a rainforest, or a single rose bush in a backyard. “Any place is a Serengeti,” said Carroll, a Distinguished University Professor and Balo-Simon Chair of Biology at the University of Maryland, an investigator of the HHMI, and a professor emeritus at the University of Wisconsin–Madison.

Carroll acknowledged that ecosystems all over the world are under enormous stress. But the lessons of the Serengeti offer some room for optimism. If we can understand the rules that regulate populations at a large scale, he said, we can protect those populations and intervene when needed. “There’s hope for wildlife and wild places across the globe,” he said. 

Before diving into the rules, Carroll paid tribute to his time at WashU and those who shaped his career in research, education, and public outreach, including his academic adviser Simon Silver; James Jones, a professor of Romance languages and literature; and Owen Sexton and David Kirk, both professors of biology.

Here are the four “Rules of the Serengeti,” the lessons of Africa that can apply just about anywhere.

Keystone species: Some animals are “more equal” than others

Like a keystone that supports an entire Roman Arch, some animals support an entire ecosystem. “Certain species have a disproportionate impact on the effect on the abundance and diversity of other creatures,” Carroll said. The wildebeest that graze in a 600-mile-long circuit make the Serengeti what it is. “It’s like a million lawn mowers,” Carroll said. “They shape the landscape for so many other things by suppressing fire, allowing trees to grow, creating habitat for things like butterflies.” 

Trophic cascades: Nature’s domino effect

In the Serengeti, species are connected in ways that surprise even experts. Carroll explained that after domestic cattle were vaccinated for rinderpest — a virus deadly to wild grazers — wildebeest and buffalo populations exploded. That boom rippled through the ecosystem: fires became less frequent, and more trees grew, creating habitat for giraffes, elephants, birds, and other species. “The eradication of rinderpest worked its way through the food web to lead to the abundance of giraffes,” Carroll said. “It’s a domino effect.”

A similar chain reaction unfolded in Yellowstone. Wolves, reintroduced in the 1990s, thinned elk herds, which allowed aspen and willow trees to recover. The returning vegetation provided new homes and food for beavers, industrious animals that reshaped the landscape with new dams and ponds.

Density helps explain destiny

Population growth isn’t limitless. After rinderpest’s eradication, wildebeest numbers surged but eventually stabilized around one million, roughly the number the ecosystem could support. “It’s like a thermostat,” Carroll said. Density-dependent regulation helps explain why populations expand or contract in response to food availability, predators, and other pressures.

Nature is resilient

Carroll highlighted Gorongosa National Park in Mozambique, once a thriving refuge for lions, elephants, and giraffes, which was devastated during a brutal civil war from 1977 to 1992. Decades later, with protection and a few reintroductions, wildlife returned in impressive numbers. African wild dogs were brought in intentionally, but most species came back on their own. “If you just keep the pressure off these animals, nature will do the rest,” Carroll said.