152
See also: Human activity and biodiversity 92–95 ■ Island biogeography 144–149
■ Climax community 172–173 ■ Open community theory 174–175B
iodiversity is shaped
globally by new species
appearing and others
becoming extinct. Community
ecology has traditionally held that
interactions between species play
a vital role in determining this
process. If two species compete for
similar resources, for example, either
the stronger pushes the weaker
to extinction, or each is driven into
a narrower niche of specialism.
In 1976, however, American
ecologist Hal Caswell proposed
a “neutral” theory of biodiversity. It
maintained that ecologically similar
species are competitively equal, and
whether species become common
or rare is down to chance processes.The “null” model
In the early 2000s, American
ecologist Stephen P. Hubbell
developed a mathematical model
known as the “null” hypothesis,
published in The Unified Theory
of Biodiversity and Geography (20 01),
that supported Caswell’s theory.
He tested his model by studying
real communities.Neutral theories of biodiversity have
dominated community ecology in
recent years. However, an Australian
study of coral reefs, published in
2014, focusing on once-dominant
species that have been almost lost
to overfishing, did not support the
theory. According to Hubbell,
species are interchangeable, so
others should have increased to take
their place. The fact that this did not
happen in this case suggests that
the neutral theory is flawed. The
question of what maintains diversity
remains an open one. ■IN CONTEXT
KEY FIGURES
Hal Caswell (1949 –),
Stephen P. Hubbell (1942–)BEFORE
1920 Frederic Clements
describes how plant species
are associated with each other
in communities.1926 Henry Gleason proposes
that ecological communities
are organized more randomly.1967 Richard Root introduces
the concept of the ecological
guild—a group of species
exploiting resources in
similar ways.AFTER
2018 A review headed
by Dutch ecologist Marten
Scheffer suggests that,
although species that use
the same resources may
be competitively equivalent,
they may also differ according
to their response to stress-
inducing factors, such as
drought or disease.Caswell made a bold attempt
to create a neutral theory of
community organization.
Stephen P. HubbellPOPULATIONS ARE
SUBJECTED TO
UNPREDICTABLE FORCES
THE NEUTRAL THEORY OF BIODIVERSITY
US_152-153_Neutral_theory_of_biodiversity.indd 152 12/11/18 6:25 PM