Predators Can Only Grow as Fast as Their Prey: Why There Aren't More Lions

Why aren't there more lions? Scientists have long wondered this when looking at the proportion of predators to prey across Africa. Now, scientists have found that in crowded settings, pretty produced less than they did in uncrowded settings.

The researchers actually wanted to compare communities of African animals across protected ecosystems to see how the numbers of carnivores are related to their prey at the scale of whole landscapes. When the scientists put the data together, though, they found an unexpected pattern: in every park, there seemed to be a consistent relationship to predator and prey in an unusual way.

"Until now, the assumption has been that when there is a lot more prey, you'd expect correspondingly more predators," said Ian Hatton, one of the researchers, in a news 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."

After they found this pattern, the researchers began looking at the relationship between predators and prey in ecosystems that included the Indian Ocean, the Canadian Arctic and tropical rainforests. In all of these settings, they found a surprising consistency in the relations of predators to prey, and confirmation that rather than the numbers of predators increasing to match available prey, predator populations are limited by the rate at which prey reproduce.

In fact, the researchers found that large numbers of prey seemed to reproduce less, which is very similar to the patterns of growth in individuals.

The findings reveal a bit more about predators, and may help researchers better manage both predator and prey populations.

The findings are published in the journal Science.

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