Fleshing Out the Predator-Prey Balance

David Christianson sleeps next to lions. For an expert in large mammal conservation and ecology, that’s just the nature of the beast.

Christianson, an assistant professor in the School of Natural Resources and the Environment at the University of Arizona, focuses on large mammal behavior and the predator-prey relationships of Africa’s most iconic carnivores and herbivores, as well as elk and wolves in Montana and the endangered Sonoran pronghorn in the southwestern United States.

By studying the behavior of these animals, Christianson hopes to identify why certain species or populations teeter on the brink of extinction and what forces determine whether they live to flee or fight another day, regardless of where they fall on the food chain.  

“Populations ebb and flow, sometimes disappearing.” Christianson said. “The role of carnivores as ‘killers’ is one force we often look at to try to understand these dynamics, but predation alone can be a poor explanation for the trends we see in prey populations. That’s partly because predators can play a larger role in the ecosystem beyond simply killing and eating prey.”

One of Christianson’s projects involves better understanding how predation risk from large carnivores drives the behavior of large herbivores in Zambia in southern Africa. As co-investigator on the project, he collaborates with Montana State University and the Zambian Carnivore Programme, a non-profit trust dedicated to conserving the region’s large meat eaters, to study the ways populations persist. In particular, they examine behavior in dominant herbivores such as wildebeest, antelope, and Cape buffalo in relation to lions, cheetahs, African wild dogs, leopards, and hyenas.

Christianson also has assisted conservation and resource managers with reintroducing lions to Liuwa Plain National Park in western Zambia, where proximity to the Angolan civil war and poaching had reduced the lion population to a single lioness by the late 1990s. A documentary film features this “Last Lioness,” Lady Liuwa, who slept near Christianson’s tent when he last visited there.

During the Liuwa Plain project, one 250-pound lioness bolted toward the park boundary. Christianson helped track and dart her, but she plunged into a swamp before the tranquilizer completely kicked in.   

“She was in the water with her head barely above the surface,” Christianson said. “Every time she almost fell under sedation, I had to tug her tail to keep her awake. Eventually, once she was completely sedated, I held her head until we could pull her out of the swamp and into the helicopter sling.”

Christianson sinks his teeth into this kind of heart-pounding work out of a passion for these charismatic carnivores and their herbivorous counterparts.

“I was drawn to the field due to the intelligence of these large mammals, and their need for vast, wild places,” he said. “They can have such an impact on their ecosystems, but they can also be very vulnerable.”

Through direct observation, radio collars, and analysis of what herbivores eat and where, Christianson’s work examines how animals respond to the presence of a predator and how that behavioral response itself can affect population dynamics and reproduction.

“For example, there may be higher predation rates in herbivores that do not react strongly to the presence of carnivores,” he said. “But those that do react strongly, by avoiding risky habitats or behaviors, may face other consequences, including increased disease, depressed nutrition, and low reproduction. These less obvious consequences of living with predators can be just as real and important as being killed and eaten.”  

Christianson’s research in the Greater Yellowstone region, which covers 20 million acres in Montana, Wyoming, and Idaho, has shown just that. Elk herds that moved out of open areas, increased vigilance, and decreased foraging when wolves were immediately nearby had higher rates of starvation and lower rates of reproduction over the course of a single winter. Subsequently, the number of elk in the region has fallen by 50 percent, far more than can be explained by wolf predation alone.

“It was a very exciting discovery, but like most good research, it leads to more questions than answers,” Christianson said. “What other outcomes can arise between predators and prey? What happens when there is more than one top predator in the ecosystem, or more than one dominant herbivore? That’s what we hope our work in Zambia will begin to address.”

Despite the success of his work in the northwest, Christianson was drawn to the Southwest to discover more about large mammal behavior.

“I liked that the University of Arizona had so many opportunities for what I can do with my current research and what I can initiate as new research,” Christianson said. “There is such diversity in Southern Arizona.”

He recently received a four-year, $482,000 grant from the U.S. Fish and Wildlife Service to examine how humans affect Sonoran pronghorn populations in Arizona. The reddish-brown and white pronghorn—the speediest land mammal in North America—once roamed as widely as buffalo; now only about 160 free-ranging animals live in the U.S., with an additional 240 living in Sonora, Mexico, according to the U.S. Fish and Wildlife Service.

Despite their low numbers, Christianson maintains a cautiously optimistic outlook for pronghorn and other struggling mammals.

“Significant strides are being made in conservation today,” he said. “There is still a lot of work to do, and we hope this type of research will arm decision makers with the knowledge to successfully maintain these populations well into the future.”