Predators always increase with prey

Scientists said this week they have uncovered what seems to be an unusual law of nature that keeps big predator numbers low across vast spaces of the Earth and its oceans.

Even when there are plenty of prey around for larger creatures like lions to eat, the number of lions in an area does not increase, said the findings in the journal Science.

The same pattern holds just as true for big animals as it does for tiny sea creatures like zooplankton, which eat phytoplankton.

"Where prey are abundant, there are not proportionally more predators," said the study, which analyzed data going back 50 years on plants and animals across 2,260 ecosystems in 1,512 distinct locations worldwide, including grasslands, lakes, forests and oceans.

Rather than predators rising in number to match the available prey, predator populations are limited by the rate at which prey reproduce.

And in crowded settings, prey reproduced less than they did in settings where there were fewer prey around, suggesting that competition for resources may be working to limit prey offspring.

"Until now, the assumption has been that when there is a lot more prey, you'd expect correspondingly more predators," said study author Ian Hatton, a doctoral student at McGill University.

"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," Hatton said.
"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."

Co-author Kevin McCann, of Guelph University's department of integrated biology, said researchers were "astonished" by what they viewed as an "amazing pattern."

The relative amounts of predator and prey biomass in diverse ecosystems could be "remarkably well-predicted by a simple mathematical function called a power scaling law," said McCann.

This "power law" shows there are always fewer top predators than expected in resource-rich ecosystems than in resource-poor ecosystems.

A better understanding of predator-prey dynamics would help conservationists monitor endangered species and could show how many large predators should be in a given area based on the available prey.

"Confirming these hypotheses would mark a major milestone in ecosystem science," said an accompanying editorial in the journal Science by Just Cebrian, professor in the department of marine sciences at the University of South Alabama.
Source: AFP