The rule of the gameEvery kilogram of predator needs a
fixed amount of prey.
25
March 2002
JOHN
WHITFIELD
 |
| The polar bear was the
biggest carnivore in the study. |
| ©
GettyImages | | |
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. | 
