The Ecology Book

58

the animal. It is estimated that

nearly three-quarters of flowering

plants (some 170,000 species) are

pollinated by 200,000 animal

species. Typically, a pollinating

insect is attracted to a flower by its

colours or scent to drink nectar or

collect pollen, and pollen attaches

to part of the insect’s body to be

carried to the next flower, where

it is deposited. The flower and its

pollinator have evolved to make

this mechanism work effectively.

Some plants have also evolved

a service-resource relationship in

which birds and mammals disperse

their seeds, spores, or fruit. Seeds

may become attached to the fur of

a mammal browsing the plant’s

leaves; when the mammal wanders

away, it disperses the seed. The

vile odor of stinkhorn fungi attracts

flies, which lick the fungi’s slime

MUTUALISMS

and thence disperse their spores.

When a bird swallows a fruit, it

carries the seeds with it

as it flies away; the indigestible

seeds may be excreted in faeces

far from where they were eaten.

In all these situations, the plants

provide a resource (food) and the

mammals, flies, and birds provide

a service (transport).

However, not all mutualistic

relationships involve plants. In

Africa, birds named oxpeckers

and herbivorous mammals such

as impalas and zebras practise

another kind of service-resource

mutualism. The oxpeckers pick

ticks from the mammals’ fur,

removing irritation and a source

of disease, while at the same time

having a meal. Oxpeckers also

make loud calls when they sense

danger, alerting the mammal host

as well as other oxpeckers.

In the insect world, some ants

and aphids carry out a different

form of service-resource mutualism.

While the aphids feed on plants,

the ants protect the aphids.

Subsequently, the ants consume

the honeydew that the aphids

release, using a “milking” process

on their smaller partners, by

stroking them with their antennae.

acacia would have no means of

defending itself. And if the acacia

trees were removed, the ants would

have no home.

Benefits for all

There are two fundamental types

of mutualism—service-resource

and service-service relationships.

They are defined by the nature

of the relationship between the

partner organisms, whether it is

the provision of a service or the

supply of a resource—both are

usually key to survival. Service-

resource relationships are common

in nature, with the fertilization, or

pollination, of flowers by butterflies,

moths, bees, flies, wasps, beetles,

bats, or birds the most widespread

example. The resource (pollen) is

provided by the flower, and the

service (pollination) is provided by

Life cycle of the

fig wasp

The fig wasp and the fig share a complex

service-resource mutualism, in which the wasp provides

the service of pollination and the fig plants provide

a secure environment for the wasp eggs to develop.

1. Adult female wasp


2. Wasp enters fig,
   lays eggs, pollinates
   flowers, and dies


3. Inside mature fig,
   male wasps fertilize
   new females and dig
   escape tunnels for them

4. New-generation

adult female wasps

pick up pollen and

emerge from fig

There is

mutual aid

in many species.

Pierre-Joseph

van Beneden

Belgian zoologist

US_056-059_Mutualisms.indd 58 12/11/18 6:24 PM