The Ecology Book

141

they attack and eat prey, they affect

the number and behavior of that

prey—since prey move away when

predators are present. The impact

of a predator can extend down to the

next feeding level (trophic level),

affecting the population of the prey’s

own food source. In essence, by

controlling the population density

and behavior of their prey, predators

indirectly benefit and increase the

abundance of their prey’s prey.

Indirect interaction that occurs

across feeding levels is described

by ecologists as a trophic cascade.

By definition, trophic cascades must

cross at least three feeding levels.

Four- and five-level trophic cascades

are also known, although these are

less common.

Controlling factors

In 1960, the American ecologist

Nelson Hairston and his colleagues

Frederick Smith and Lawrence

Slobodkin published a key paper

entitled “Community Structure,

Population Control, and Competition,”

which examined the factors that

control populations of animals on

ECOSYSTEMS

See also: Predator–prey equations 44–49 ■ The food chain 132–133 ■ The ecosystem 134–137 ■ Energy flow through

ecosystems 138–139 ■ Evolutionarily stable state 154–155 ■ Biodiversity and ecosystem function 156–157

Ocher starfish prey on sea creatures

such as mussels and limpets. In a

famous experiment, Robert Paine took

them out of their rock pools to observe

the effect on the rest of the food web.

different trophic levels. They

concluded that populations of

producers, carnivores, and

decomposers are limited by their

respective resources. Competition

occurs between species on each of

these three trophic levels. They also

found that herbivore populations

are seldom limited by the supply of

plants, but are limited by predators,

so they are unlikely to compete

with other herbivores for common

resources. The paper highlighted

the important role of top-down

forces (predation) in ecosystems,

and bottom-up forces (food supply).

American ecologist Robert

Paine was the first to use the term

“trophic cascade” when, in 1980,

he described changes in food

webs that were brought about

by the experimental removal

of predatory starfish from the

intertidal zone in Washington

State. The concept of trophic

cascades is now generally accepted,

although debate continues as to

how widespread they are.

Top-down cascades

This type of cascade is clearly

demonstrated when a food chain

is interrupted by the removal of

a top predator. The ecosystem

may continue to function despite

the shift in species composition;

alternatively, the removal of one

species may lead to the ❯❯

Predators move into the

area and numbers increase

Predators eat

herbivore prey

Increased rainfall

encourages vegetation

Overgrazing brings

habitat change and loss

of species richness

Herbivore population

increases Prey population increases

If predators are

removed ...

Herbivore prey eat plants

Bottom-up cascade Top-down cascade

US_140-143_Trophic_cascades.indd 141 12/11/18 6:24 PM