The rule of the game

Every kilogram of predator needs a fixed amount of prey.

Every kilogram of meat-eating mammal needs 111 kilograms of prey to sustain it, say two ecologists. The rule holds from weasels to bears.

With many species of carnivore endangered, the discovery could help conservationists work out how to maintain a species’ resource base.

Chris Carbone, of the Institute of Zoology, London, and John Gittleman, of the University of Virginia, found the rule when they compared the population densities of 25 species of carnivore with the population densities of their prey¹.

The relationship between the mass of predator and prey holds regardless of the animals’ diet or habitat. It covers European badgers, which eat mainly worms, and big cats, for example. The smallest carnivore analysed was the 140-gram least weasel, the biggest the 310-kilogram polar bear.

“To see this rule emerging across a wide range of carnivores was a real surprise to me, and I think it’ll surprise a lot of ecologists,” says Carbone. He expects similar rules to hold for other predatory groups, such as reptiles.

“Most models [of carnivore populations] have assumed that resources aren’t limiting, but that the rate of their acquisition is,” says ecologist Pablo Marquet of the Catholic University of Chile, Santiago. “This shows that what really matters is the resources you have.”

The result means that global biological patterns may only become apparent by looking at detailed local information, such as diet and population number, Marquet adds. “Some researchers ignore local knowledge in the hope that it will average out and you’ll keep the big picture,” he says.

Rule tool

The rule will be “a very useful ballpark figure” for conservationists, says Carbone. They could use it to predict the population densities of species not included in the analysis.

Carbone intends to apply it to the conservation of the Sumatran tiger. These tigers live around oil-palm plantations, and eat mainly wild pigs. Estimating the weight of the pig population will give an idea of how many tigers an area can sustain.

And where a carnivore-to-prey ratio falls much below 1/111, it could be a clue that something else, such as inbreeding, is keeping its numbers down.

The new rule is one of several general patterns to emerge recently from studies of how organisms’ biology varies with their size. Last month, for example, researchers announced that the ratio of above- to below-ground tissue is constant across a wide range of plants².

References

1. Carbone, C. & Gittleman, J.L. A common rule for the scaling of carnivore density. Science, 295, 2273 – 2276, (2002).
2. Enquist, B.J. & Niklas, K.J. Global allocation rules for patterns of biomass partitioning in seed plants. Science, 295, 1517 – 1520, (2002).