Ecologist’s studies on the effects of prey depletion on populations of large African carnivores

“The studies are related in a lot of ways,” said Scott Creel, lead author of a paper about the effects of prey depletion on endangered African wild dogs that was published this week in the journal Proceedings in the National Academy of Sciences, and co-lead author of another paper about the effects of poaching controls on African lion demographics that was featured on the cover of the Jan.11 issue of Conservation Science and Practice.

“The common denominator is that across sub-Saharan Africa, populations of animals like wildebeest and impala are declining pretty dramatically almost everywhere as a consequence of habitat loss and bushmeat poaching,” Creel said. “The densities of those animals, which are prey for species like African wild dogs and lions, are a tiny fraction of what they would have been 40 or 50 years ago.”

The two papers document studies by Creel and his students of the relationship between prey depletion and population decline of large carnivores. For several decades, Creel, a Distinguished Professor in the Department of Ecology in MSU’s College of Letters and Science, has studied the behavior, ecology and conservation of large mammals, with particular emphasis on African carnivores.

For each study, the researchers monitored groups of animals in Zambia between 2013 and 2021, including prides of lions living in the massive Kafue ecosystem. The area is located in an economically depressed area of the country that has experienced significant declines in prey animals due to poaching.

“For very understandable reasons, the people living in villages next to the protected areas turn to bushmeat poaching as a way to get food and make money,” Creel explained.

The researchers monitored the lions through both visual observation and data from collars worn by at least one animal in each group. During the first five years of the study, the lion population in the ecosystem declined by 3% per year, and Creel said there was debate among ecologists about whether that trend could be reversed. Some advocated for fencing to deter poachers, while others suggested employing other means of protecting the lions. In 2018, patrols were systematically increased to curb poaching in several well-defined areas of the ecosystem.

That’s when the researchers started to see growth of the lion populations in those areas — most notably, a 29% increase in the number of cubs produced.

“More prey means it’s easier for a female lion to raise her cubs,” Creel said. “What used to be a litter of three cubs is now four cubs, and that will change population growth rate pretty dramatically.”

Also in the patrolled areas, the survival rate of the lions rose by about 8%, partly because fewer of them were being killed by snares in the patrolled areas, but also because more prey translates into better odds of survival. However, Creel said, the most significant benefit for lions was that fewer animals they hunt for food were dying in those snares. Overall, the study showed that in protected areas, the lion population grew by about 8% annually, while in unprotected areas, the population continued to decline.

“That’s a good news story in the face of these problems. It definitely shows that even in a big ecosystem that potentially can hold a huge population, increased investment works,” he said. “With 8% annual growth, a population will double in just 10 years.”

The wild dog study told a different story, Creel said.

African wild dogs, about half the size of hyenas, are pack animals that typically live in groups of eight to 10 adults. They were listed as endangered in 1990, and fewer than 1,500 breeding adults remain in the world among a total population of about 6,500 individuals. The dogs hunt in teams, allowing them to prey upon the same species as larger carnivores, but their kills are often stolen by hyenas. And hyenas aren’t the only thing they have to worry about, because lions kill African wild dogs.

“They really are between a rock and a hard place,” Creel said. “In places with low prey density, they die at higher rates and reproduce at lower rates but they still have to avoid places with high lion density, because they’re just so dangerous.”

Creel and his team tested the physical effects of prey depletion on African wild dogs, focusing on two populations in ecosystems where prey and competitor densities varied. One or two adult members in each of 16 packs were fitted with collars that measured their energy expenditures, which provided researchers with highly detailed data about the animals’ activities.

“You can tell when they make a kill, you know how far they’ve run, you know how much energy they’ve burned,” Creel said. “It’s crystal clear.”

Combined with direct observation, the data revealed that the energy costs of hunting were higher and the benefits lower for wild dogs in prey-depleted areas, regardless of the number of competitors present — a fundamental change from past studies that showed the most important factor limiting population size was the presence of dominant competitors, Creel said.

“Wild dogs were always limited from the top down by their competitors. They’re limited now from the bottom up by their food supply,” Creel said. “The reason they do badly in those places is they have to go farther to find an opportunity to hunt, and they usually kill a smaller thing when they succeed. They burn much more energy and get less in return.

“That they switched from being limited by their competitors to being limited by food is a new thing, and unfortunately, very worrisome,” he said.

Both studies were conducted in coordination with the Zambia Department of National Parks and Wildlife and the Zambian Carnivore Programme. Matthew Becker, the co-lead author of the PNAS article, is CEO and program manager of the Zambian Carnivore Programme, an MSU alumnus and an affiliate faculty member in the Department of Ecology. Creel said the cooperation of those agencies made it possible to conduct the years of intensive field research that enabled scientists to become familiar with the lives of each individual animal and understand very well the factors that affect them.

“We’ve now shown that even in a huge, unfenced ecosystem that’s already heavily affected by humans, we can reverse the decline by increasing the investment in protection — we just need the will to do it,” Creel said. “We know what works, and it’s a pretty simple thing.”

He said he is encouraged that the Zambia Department of National Parks and Wildlife allows research and data to guide decision making, and by the potential economic benefits from tourism that could result from investing in protecting animals.

“I think the lion study is really exciting, in that if we can do it for lions, we can do it for all wildlife,” he said. “All boats are going to rise on the same tide if you increase the protection.”