August 10, 1998
A cheetah watches over its cubs as they feed.
By Tim Stephens
The prospects for cheetah cubs in East Africa's Serengeti plains are grim: Only about 5 percent make it to adulthood, while most fall prey to lions and hyenas. Surprisingly, a careful analysis of cheetah population dynamics now suggests that in the long run, cheetah populations are affected far less by the deaths of cubs than they are by adult deaths, whether from natural causes such as disease or human activities such as hunting and poaching.
The new study by UCSC researchers provides a fresh perspective in an ongoing debate over the factors that limit populations of the endangered cheetah. Cheetahs were once widespread in Asia and Africa, but their numbers have been decimated by hunting, poaching, loss of habitat, diseases, and declines in their prey. The new findings may have important implications for conservation of the remaining cheetahs, the world's fastest land animals.
The study began as an argument between two UCSC graduate students on a fishing trip in 1995. Kevin Crooks, a graduate student in biology and lead author of the study, and coauthor M. A. Sanjayan, who earned his Ph.D. in biology in 1997, said they eventually realized there was no way to understand the impact of cub mortality without doing a demographic study.
To carry out such a study they turned to Daniel Doak, Pepper-Giberson Associate Professor of Environmental Studies and an expert on computer-based techniques for analyzing population dynamics. By creating a model cheetah population on the computer, the researchers were able to simulate the effects of different parameters such as cub mortality rates on the growth of the population.
To construct a realistic population model, however, they needed data from field studies of wild cheetahs. For this they relied on published results from 10 years of field studies conducted in the Serengeti by Timothy Caro, a conservation biologist at UC Davis, and other researchers.
The results of the computer simulations indicated that cub survival was far less important than adult survival in influencing population growth in cheetahs. Crooks, Sanjayan, and Doak published their findings in the August issue of the scientific journal Conservation Biology.
"When you look at the population as a whole, it is adult survival that drives population growth," Crooks said.
From a conservation perspective, this means that wildlife managers may want to focus their efforts on ensuring the survival of adult cheetahs rather than trying to limit predation of cubs by lions and hyenas, Sanjayan said. Preventing poaching, halting habitat loss, and working with ranchers to discourage them from shooting cheetahs on their land are likely to be the most effective approaches to protecting cheetah populations, he added.
The results of the study can be explained in terms of cheetah biology, Crooks noted. A key observation made by Caro's group was that when a female cheetah loses her cubs, she rapidly enters estrus (the receptive state in which the female is capable of conceiving) and breeds again.
"Although cub predation is high, cheetahs are able to reproduce again rapidly, which suggests they have experienced these [high] predation rates throughout their recent evolution," Crooks said.
The researchers incorporated this and other relevant features of cheetah biology into their population model. Elaborate equations were used to account for factors such as how long it takes a female to have a new litter after losing her cubs.
The model divided the population into six age classes: newborn cubs, young cubs, weaned cubs, adolescents, young adults, and adults. Values for survival rates and reproductive rates in each class were estimated from field data. Running the model on the computer generated a population growth rate for any given set of inputs.
This type of demographic modeling is not new, but Doak added a new twist that makes the technique especially robust, even when the raw data are imperfect or incomplete. The researchers ran the computer simulation over and over again, varying the inputs and plotting the results of hundreds of simulations. This enabled them to assess how sensitive the population growth rate was to variations in specific parameters.
"This is a powerful approach for studying animals with complicated life histories, such as cheetahs," Sanjayan said.
The results showed that the loss of a litter is less devastating to the population as a whole than it might appear to be, he said. In contrast, the death of an adult cheetah can have a big impact on the reproductive potential of the population.
Sanjayan is currently working with a conservation group in the United States (Round River Conservation Studies, Salt Lake City) and the Cheetah Conservation Fund on efforts to protect cheetahs in Namibia, where the largest remaining population of free-ranging cheetahs occurs.
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