The Ecology Book

109

A spider traps a damselfly,

demonstrating that the principle

of food size can be modified by the

comparative aggression and strength

of the predator and its prey.

A lone polar bear surveys the

sea for prey from a piece of floating

ice in the Arctic. The shrinking

area of sea ice in the region

threatens this species’ survival.

in nature, so if the population

of a given species grows too large

it will be regulated, most often by a

lack of food. However, relationships

between organisms and their

environment change from place

to place and through time.

Chain of dependence

In Animal Ecology, Elton outlined

the key principles of the study of

animal communities: food chains

and webs, food size, and ecological

niches. Each food chain and web,

he asserted, is dependent on

producers: plants and algae that

support plant-eating consumers

(herbivores). These herbivores in

turn support one or more levels of

meat-eating consumers (carnivores).

Large carnivores generally eat

smaller animals, but because small

animals reproduce more quickly,

their numbers are able to support

the larger predators.

Competition for food is very tight

near the top of a food web. Although

apex (top) predators, such as big

cats and large birds of prey, have no

natural predators, this often means

that they have to defend territories

THE VARIETY OF LIFE

See also: Keystone species 60–65 ■ The food chain 132–133 ■ The ecosystem

134–137 ■ Energy flow through ecosystems 138–139 ■ Trophic cascades 140–143

against rivals of their own species

to ensure there is enough food for

themselves and their offspring.

Food size

One of Elton’s most important

points was the notion that food

chains exist primarily because

of the principle of food size. He

explained that every carnivorous

animal eats prey between upper

and lower limits. Predators are

physically unable to catch and

consume other animals above a

certain size because they are not

large enough, strong enough, or

skillful enough. That is not to say

that predators cannot kill and eat

larger animals than themselves; a

weasel can easily kill a larger rabbit

because it is more aggressive.

However, an adult lioness, one

of the world’s top predators, is not

capable of killing a healthy adult

African elephant. Likewise,

a dragonfly larva on the bottom

of a pond may be able to prey

on a small tadpole, but it would

not be able to eat an adult frog. ❯❯

Forecasting the effects

of climate change

Ecologists examine changes

to animal populations and

distribution and apply climate

change models to forecast how

these will change further in

the future, over the course of

5, 10, 50, or more years. In the

Arctic, for example, where

average temperatures are

rising more rapidly than

anywhere else, the sea ice is

contracting. As a result, polar

bears have to travel farther in

search of ice where they can

catch seals, rest, and mate.

The farther they swim, the

more energy they burn. As

the sea ice declines, the polar

bears starve. Scientists

monitor their numbers and

movements and compare this

data with changes in sea ice.

The polar bear plays a

vital part in the ecology of the

Arctic. As an apex predator

and keystone species, it must

have access to seals, which

are its almost exclusive diet.

The number of seals regulates

the density of polar bears,

while polar bear predation in

turn regulates the density and

reproductive success of seals.

Every animal is closely

linked with a number of

other animals living

around it—and these are

largely food relations.

Charles Elton

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