The Ecology Book

171

The process of primary succession begins in barren

environments such as bare rock. Hardy species, usually lichens,

appear first and then give way to a stable climax community of

more complex and diverse life forms over hundreds of years.

See also: Field experiments 54–55 ■ The ecosystem 134–137 ■ Climax community 172–173 ■ Open community theory

174 –175 ■ Biomes 206–209 ■ Romanticism, conservation, and ecology 298

ORGANISMS IN A CHANGING ENVIRONMENT

an 1860 address to members of the

Middlesex Agricultural Society,

Massachusetts, Henry David

Thoreau had stated: “Though I do

not believe that a plant will spring

up where no seed has been, I have

great faith in a seed.”

Growth of an ecoystem

French geographer Adolphe Dureau

de la Malle is regarded as the first

person to use the term “succession”

with reference to ecology when he

witnessed the progression of plant

communities after all the trees

were removed from a forest. Cowles

provided a more formal articulation

of his ecological succession theory,

in The Ecological Relations of the

Vegetation on the Sand Dunes of

Lake Michigan, published in 1899.

In this seminal paper, he proposed

the idea of primary succession—

the gradual growth of an ecosystem

originally largely devoid of plant life.

The stages of primary succession

include pioneer plants (often lichens

and mosses), followed by grassy

plants, small shrubs, and trees.

Life after disturbance

Secondary succession occurs after a

disturbance that destroys plant life,

such as a flood or a fire. The plant

life reestablishes itself and develops

into an ecosystem similar to the one

that existed before the disturbance.

The stages of secondary succession

are similar to those of primary

succession, although the ecosystem

may start at different points in the

process, depending on the level of

damage caused by the trigger.

A common example of secondary

succession occurs after a wildfire in

oak and hickory forests. Nutrients

from burned plants and animals

provide the right conditions for

growth of annual plants. Pioneer

grasses soon follow. After several

years, due at least in part to the

environmental and soil changes

resulting from the pioneer species,

shrubs and oak, pine, and hickory

trees will begin to grow. As the

trees grow higher, shading out more

of the underbrush, the grasses are

replaced by plants able to survive

with low sunlight, and, after around

150 years, the forest once more

resembles the prefire community. ■

Bare rock Lichens

Small annual

plants and

lichens

Grasses and

perennials

Hundreds of years

Grasses,

shrubs,

and shade-

intolerant trees

Shade-

tolerant trees

Primary succession

Pioneer species Intermediate

species

Climax

community

Soil

I ... found indisputable

evidence (a) that forests

succeeded prairie, and

(b) that prairie had

succeeded forest.

Henry Allan Gleason

American ecologist

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