The Ecology Book

47

A cheetah pursues a Thomson’s

gazelle. The predator–prey equations

are able to model the way populations

of both species will change in response

to the activities of the other.

See also: Evolution by natural selection 24–31 ■ The selfish gene 38–39 ■ Ecological niches 50–51 ■ Competitive exclusion

principle 52–53 ■ Mutualisms 56–59 ■ Keystone species 60–65 ■ Optimal foraging theory 66–67

ECOLOGICAL PROCESSES

war years, but D’Ancona could not

explain why less fishing did not

produce more fish of all kinds in the

nets. Using the same equations as

Lotka, Volterra eventually explained

the fluctuations in both the predator

and the prey species.

Population principles

At the time Lotka and Volterra

made their calculations, the science

of population dynamics was still

in its infancy, having barely moved

on since the population studies of

British economist Thomas Malthus

in the late 18th century. According

to Malthus’s theory, a population

grows or declines rapidly as long

as the environmental factors for

survival are constant, and the rate

at which that population changes

increases as the population grows.

From this theory, Malthus predicted

a catastrophic future for humanity.

The number of humans was

growing much more quickly than

the amount of food that could be

produced by the world’s farmlands.

Eventually, Malthus argued, a point

would be reached when the human

population would succumb to

global famine and decline.

Malthus’s bleak vision did not

happen, thanks to technological

advances in agriculture and the

development of artificial fertilizers,

but his population model became

applicable to species populations

within ecosystems. Every habitat,

and the niche occupied by a species

within its community of organisms,

has a carrying capacity—the

maximum population that can

be supported by the resources

available, such as water, space,

food, and light. Any rise in

population above this level is

likely to be reduced by naturally

occurring factors. As a result, wild

populations should in theory be

more or less static, fluctuating only

around the carrying capacity,

assuming the random impacts of

catastrophic events are ignored.

However, this relative

equilibrium did not always match

up with observations—as in ❯❯

The food species

cannot, therefore, be

exterminated by the

predatory species, under

the conditions to which

our equations refer.

Alfred J. Lotka

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