The Ecology Book

53

The red squirrel is smaller than the

gray, and has a more restricted diet

and habitat. Reds may also die from the

squirrel parapoxvirus, which is carried

by the grays but does not affect them.

See also: Evolution by natural selection 24–31 ■ Ecological niches 50–51 ■ Animal ecology 106–113 ■ The ecosystem

134 –137 ■ The ecological guild 176–177 ■ Niche construction 188–189 ■ Invasive species 270–273

ECOLOGICAL PROCESSES

nature, he proposed, there were too

many variables to draw conclusions

about how ecological mechanisms

work. He argued that little progress

had been made since Darwin’s era

in understanding how species

compete for survival, whereas the

experimental method had produced

great advances in areas such as

genetics. In fact, the competitive

exclusion principle—although a

useful theoretical model—is rarely

seen in nature, simply because, in a

bid to survive, a weaker competitor

tends to quickly move on or adapt.

Avoiding competition

Most creatures can make the

changes necessary for survival. A

variety of birds can live in a garden

during any one year because they

all operate in different “niches.”

They have contrasting beak shapes

and sizes that allow them to eat

different types of food—the robin

preferring insects, the finch eating

seeds. Their choice of habitat and

feeding times might also vary; this

is known as resource partitioning.

In 1957, Robert MacArthur

noted this phenomenon in North

American warblers. The five

species he observed, each with

distinctive, colorful markings,

flitted in and out of coniferous

trees, feeding on bugs and other

insects. They could coexist in one

habitat because they did not try to

feed in the same part of the tree but

at different heights and depths of

the foliage. In this way they avoid

competing with each other.

An invasive competitor

Problems often arise if an exotic

species is suddenly introduced

to an ecosystem. Britain’s red

and gray squirrels provide a clear

example. When the grey arrived

from America in the 1870s, both

squirrel species competed for the

same food and habitat, which put

the native red squirrel populations

under pressure. The gray had

the edge because it can adapt

its diet; it is able, for instance, to

eat green acorns, while the red can

only digest mature acorns. Within

the same area of forest, gray

squirrels can decimate the food

supply before red squirrels even

have a nibble. Grays can also live

more densely and in varied

habitats, so have survived more

easily when woodland has been

destroyed. As a result, the red

squirrel has come close to

extinction in England. ■

Types of competition

The Competitive Exclusion

Principle covers two main types

of competition. Intraspecific

competition is between

individuals of the same species

and ensures the survival of

the fittest, so that only the

healthiest individuals—or those

best adapted to a particular

environment—will breed. The

second type is interspecific:

competition between two

different species that rely on

the same resources. The most

important of these will be the

“limiting resource,” the one that

both require in order to breed.

Ecologists make a further two

distinctions. Interference is

when two organisms fight

directly with each other over a

limited resource, such as a mate

or a preferred food. Exploitation

is indirect competiton, such as

stripping out a resource so there

is none left for the competitor;

this can be seen in plants, when

a species’ uptake of nutrients or

water is more efficient than that

of its neighbors.

Let us make for this purpose

an artificial microcosm... let

us fill a test-tube with nutritive

medium and introduce to it

several species of protozoa

consuming the same food

or devouring each other.

Georgy Gause

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