The Ecology Book

105

See also: Evolution by natural selection 24–31 ■ Mutualisms 56–59

■ The ecosystem 134–137 ■ Energy flow through ecosystems 138–139

THE VARIETY OF LIFE

a much greater amount of minerals.

When the fungal hyphae attach to

the plant roots, they extend the root

system, causing extra nutrients to

seep into the plant.

Albert Frank realized that this

partnership worked both ways. It

was a winning combination for both

plant and fungus. In exchange for

passing on a share of its minerals,

the fungus receives sugar from the

plant—made by photosynthesis in

the leaves and transported to the

roots via the plant’s sap. This boosts

the nutrient supply that the fungus

derives from dead organic matter.

Ancient networks

Fossils of plants dating from 400

million years ago—when vegetation

was first spreading across dry

land—show traces of fungal threads.

This suggests that the mycorrhizal

partnership was key to the evolution

of terrestrial life. Today, the majority

of plant species continue to rely on

fungi in this way. Trees supported

by mycorrhizae are more resistant

to drought and disease, and can

even communicate alarm signals by

releasing chemicals in response to

attack by herbivores. This fungal

network connecting trees has been

dubbed “the wood-wide web.” ■

Mycorrhizae as

pollution indicators

Mycorrhizal fungi are not only

good for the health of plants—

they can also act as indicators

of the health of the entire

environment. Laboratory

experiments with these fungi

have shown that some grow

badly in the presence of toxins,

which means that they can be

used to detect pollutants in

the air or soil. For instance,

some fungi fail to grow when

exposed to heavy metals such

as lead or cadmium, and

because different kinds of

fungi react differently to

environmental change, certain

species can be used to identify

specific kinds of pollution.

Mycorrhizae are also useful

indicators of the health of their

native habitat. Many form

cauliflower-like growths

on tree roots, but these are

smaller in polluted soil. The

trees themselves may also

respond to pollution with

weaker shoot growth, but

the mycorrhizal response is

more acute and serves as a

valuable early-warning sign

of a habitat in decline.

Weak growth in the russet

brittlegill, a mycorrhizal fungus

of European and North American

spruce forests, can be an early

indicator of habitat air pollution.

Beneficial exchange between

mycorrhizae and plant roots

Supplies sugar from

photosynthesis

Connects plants in an

extensive network

Increases uptake of

water and nutrients

Allows plants to share

nutrients with others

Boosts protection

against soil diseases

The mutualistic

relationship between

mycorrhizae and plants

is highly evolved. As

many as 90 percent of

all plant species rely on

fungi for nutrients and

protection. In return,

plants supply the fungi

with a vital food source.

Mycorrhiza Plant

[the fungus] performs

a “wet nurse” function

and performs the

entire nourishment

of the tree from

the soil.

Albert Frank

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