Predator Prey Relationship: Where Are All the Lions?

Researchers in east and southern Africa parks were wondering why they didn't see more lions. They recently found that prey animals reproduce less when living in more crowded conditions, and that lions' numbers are consequently affected by having less prey. This pattern remained consistent across a range of ecosystems, which, according to a news release, suggests that this could be a new law of nature. 

"I went to high school in Zimbabwe and spent vacations in the national parks there," Ian Hatton, lead author and Ph.D. student at McGill University, said in the release. "When I began my Ph.D. in biology at McGill, I wanted to go back and compare whole communities of African animals across protected ecosystems to see how the numbers of carnivores are related to their herbivore prey at the scale of whole landscapes. So I gathered all the animal census data I could for parks in east and southern Africa."

When Hatton and his colleagues calculated the number of herbivores and carnivores found throughout these parks, they found an unexpected, consistent relationship between predators and their prey.

"Until now, the assumption has been that when there is a lot more prey, you'd expect correspondingly more predators," Hatton said in this release. "But as we looked at the numbers, we discovered instead, that in the lushest ecosystems, no matter where they are in the world, the ratio of predators to their prey is greatly reduced. This is because with greater crowding, prey species have fewer offspring for every individual. In effect, the prey's rates of reproduction are limited, which limits the abundance of predators."

The researchers extended their study, examining predator-prey relationships in the Indian Ocean, the Canadian Arctic and tropical rainforests. In total, they analyzed plant and animal data from more than 1000 studies conducted over the past 50 years.

Overall, the researchers found that rather than the numbers of predators increasing to match the available prey, predator populations are limited by the rate at which prey reproduce. According to their study, the researchers also found that growth patterns throughout whole ecosystems, where prey seemed to naturally reproduce less, were found to have similar individual growth patterns. 

"Physiologists have long known that the speed of growth declines with size," Jonathan Davies, co-author from McGill's Department of Biology, said in the release. "The cells in an elephant grow more than 100 times more slowly than those of a mouse."

Michel Loreau, co-author and adjunct professor in McGill's Biology Department, added in the release that, "The discovery of ecosystem-level scaling laws is particularly exciting. Their most intriguing aspect is that they recur across levels of organization, from individuals to ecosystems, and yet ecosystem-level scaling laws cannot be explained by their individual-level counterparts. It seems that some basic processes reemerge across levels of organization, but we do not yet fully understand which ones and why."

Their research was recently published in the journal Science.

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