The Ecology Book

174

AN ASSOCIATION

IS NOT AN ORGANISM

BUT A COINCIDENCE

OPEN COMMUNITY THEORY

W

hen American plant

ecologist Frederic

Clements proposed the

idea of climax communities in 1916,

he envisioned the community as a

superorganism in which all plants

and animals interact to develop the

community. A year later, American

plant ecologist Henry Gleason

dismissed the idea; he argued that

plant species have no common

purpose but merely pursue their

own individual needs. Gleason’s

hypothesis became known as the

“open community” theory. The

dispute initiated a debate that still

rages in ecological circles today.

Gleason did not deny that plant

communities could be mapped and

their interactions identified, but he

IN CONTEXT

KEY FIGURE

Henry Allan Gleason

(1882–1975)

BEFORE

1793 Alexander von Humboldt

uses the word “association” to

sum up the range of plant

types in a particular habitat.

1899 In the US, Henry Cowles

states that vegetation develops

in stages, in a process he calls

plant succession.

1916 Frederic Clements posits

the idea of a climax community

as a single organism.

AFTER

1935 Arthur Tansley coins

the term “ecosystem.”

1947 Robert H. Whittaker

begins field studies that will

refute Clements’s holistic idea

of plant communities.

1959 John Curtis boosts

Henry Gleason’s reputation

with numerical studies of

prairie plant communities.

could see none of the integration

proposed by Clements. Instead,

Gleason believed that groups of

plants were random growths

of individuals and species,

responding to local conditions.

Individual needs

Gleason maintained that the

changes that occur during plant

succession, as the composition

of a community evolves, are

not integrated stages, as in the

development of a single organism.

Rather, they are a combination of

responses from individual species

as they seek to meet their own

needs within a locality. “Every

species of plant,” Gleason argued,

“is a law unto itself.” Gleason also

An ecological

community is not

an organism

There is no evidence of

integrated development

between plants

They grow randomly,

influenced only by the

environmental conditions

Plants grow according to

their individual needs

US_174-175_Open_community_theory.indd 174 12/11/18 6:25 PM