The Ecology Book

77

See also: Evolution by natural selection 24–31 ■ Predator–prey equations 44–49 ■ Ecological niches 50–51

■ Competitive exclusion principle 52–53 ■ Mutualisms 56–59 ■ Optimal foraging theory 66–67

ECOLOGICAL PROCESSES

were in the tank, the tadpoles were

less active, swam to other parts of

the tank, and metamorphosed into

adults when they were smaller. The

predator had changed the toads’

morphology and their behavior just

by being there.

In 1991, Werner investigated

what happened when more than

one prey species was involved. In

the absence of a predator, bullfrog

and green frog tadpoles grew at

virtually identical rates. However,

when predatory dragonfly larvae

were introduced to the tank, both

prey species became less active

and chose different places in which

to swim. The bullfrog tadpoles grew

more quickly than they had in a

predator-free tank, but the green

frog tadpoles decreased their

feeding activity and grew more

slowly. Werner concluded that for

prey species there was a trade-off

between the need to grow as fast

as possible and the risk of predation.

Growing more quickly requires

more feeding activity, and this in

turn increases the chances of being

eaten by a predator. As the larvae’s

presence altered the behavior of

the prey species differently, the

bullfrog’s new behavior gave it

a competitive advantage over the

green frog by making it bigger.

Terrestrial animals

Early studies of nonconsumptive

effects (NCEs) were concerned with

aquatic organisms under laboratory

conditions, but more work has now

been done in the wild with land-

dwelling animals. German field

research published in 2018

focused on lynx and their roe deer

prey. When lynx were present,

researchers found that the roe deer

avoided areas they knew to be

high-risk, both during the day and

on summer nights when nocturnal

predation is more common. The

deer treated some grazing areas as

out of bounds, presumably due to

fear of being attacked by lynx.

Wherever there are predators,

they exert NCEs. They also affect

some sessile (nonmoving) species,

as well as mobile prey. This can

happen when certain dominant

competitors are displaced by

predators and, in their new

habitats, outcompete sessile

animals for food. Small fish that

are displaced, for example, could

outcompete sponges for food. ■

... species react [to predators]

by reducing activity and

altering space use.

Earl Werner

A green darner dragonfly laying

its eggs in a pond. The larvae that

hatch out are predators and have been

shown to influence the behavior of their

tadpole prey.

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