The Ecology Book

55

See also: Ecological niches 50–51 ■ Modern view of diversity 90–91 ■ Animal behavior 116–117 ■ The ecosystem 134 –137

■ Niche construction 188–189

ECOLOGICAL PROCESSES

street lights could be switched off to

evaluate the impact of light pollution

on their behavior change.

Scottish barnacles

In 1961, American ecologist Joseph

Connell published the results of

his research on barnacles on the

Scottish coast. Since free-swimming

barnacle larvae can settle anywhere,

Connell had tested why the lower

part of the intertidal zone was

colonized by Balanus balanoides

barnacles and the upper part by

Chthamalus stellatus. He wanted to

know if this was due to competition,

predation, or environmental factors.

Connell manipulated the local

environment, and monitored it for

over a year. In one area, he removed

the Chthamalus barnacles. They

were not replaced by Balanus,

which suggested that Balanus could

not tolerate the desiccation that

occurred in the upper zone at low

tide. Connell then removed the

Balanus population from the lower

zone, and found that Chthamalus

barnacles did replace them. Both

species could live in the lower zone,

but only one could survive higher

up. This suggested that Chthamalus

was better able to deal with the

harsh conditions of the upper zone,

but was outcompeted by Balanus

lower down. The “fundamental

niche” of Chthamalus (where the

species would normally be able to

survive) encompassed both zones,

but its “realized niche” (the actual

area it inhabits) was more restricted.

Diversity experiments

In the early 1970s, Connell and

American ecologist Daniel Janzen

published an explanation of the

degree of tree diversity in tropical

forests: the Janzen–Connell

hypothesis. Connell mapped

trees in two rain forests in North

Queensland, Australia, and found

that seedlings tended to be less

Joseph Connell’s barnacle experiment

Highly desiccated

area during low tide

Fundamental

niches

Realized

niches

High tide

Chthamalus

Balanus

Low tide

Ocean

This experiment showed

that Balanus could live only

in the lower intertidal zone,

while Chthamalus could live

in both the upper and lower

zones, but was outcompeted

by Balanus in the lower zone.

successful when their nearest

neighbor was of the same species.

Each species is targeted by specific

herbivores and pathogens, which

will also eat or attack smaller,

weaker individuals of the species

nearby. This prevents “clumping”

of one tree species.

In 1978, Connell proposed

the intermediate disturbance

hypothesis (IDH). This states

that both high and low levels of

disturbance reduce species

diversity in an ecosystem, so the

greatest range of species can be

expected between those extremes.

Several studies support IDH. One,

carried out in waters off Western

Australia, examined the effects of

wave disturbance on diversity.

Species diversity was found to be

low both at exposed offshore sites

and at sheltered sites. ■

[Connell’s] studies ... have

improved our understanding

of the mechanisms

that shape population

and community

dynamics, diversity,

a nd demo g raphy.

Stephen Schroeter

Marine scientist

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