One would think that the more prey animals–zebras, antelopes, rabbits, etc.–there are, the more predators there would be, since a larger food supply should support a larger population of animals that eat them. But it doesn’t work that way. According to a new study, predator populations decrease when the prey increases. In fact, this seems to be the case with all animal populations, including herbivores whose “prey” is particular kinds of plants. And the ratios can be described with a specific mathematical formula, suggesting that this discovery constitutes a new law of nature.
Now the obvious layman’s question is whether the causes might be reversed, that a decline in the number of predators allows for a greater population of prey, but apparently this is considered to be a different effect having to do with the fact that predators breed more slowly in more crowded environments.
From Emily Chung, Predator-Prey Study May Reveal a Surprising Law of Nature, CBC news:
A lush savannah teeming with zebras, gazelles and buffalo may look like an all-you-can eat buffet for lions.
But a new Canadian study has revealed a surprise: When prey abound, there are relatively fewer predators. And a look at ecosystems on land and sea around the world shows that this might be a fundamental law of nature.
Intuitively, you’d expect the populations of lions, leopards and hyenas to increase proportionately when there are more zebras and antelopes around for them eat, acknowledges Ian Hatton, the McGill University researcher who led the study published this week in Science.
“If you double the prey, you should double the predators,” said Hatton. “And we found that this was not the case.”
Hatton conducted the study for his PhD research under Michel Lareau, who is now the with Centre national de la recherche scientifique in France. He looked at the populations of prey species such as zebras and antelopes and predator species such as lions and hyenas in different African parks with different climates and environments – some were richer and lusher than others, so some had more animals than others. He found that as the number of prey animals increased, the relative number of predators declined at a very predictable rate. In other words, predator populations grew much more slowly than prey populations, following a distinct mathematical pattern called a power law.
And it didn’t just apply to African ecosystems. A look at data from 1,000 studies worldwide over the past 50 years showed it also applied to predators and prey in other parts of the world, in other environments, even to animals living in lakes and oceans. In fact, it even applied to herbivores, whose populations don’t keep up with the growth of plants. “You could call a herbivore a predator on plants,” Hatton said.
[Keep reading. . .]
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