The Ecology Book

67

Oystercatchers, despite their name,

are reliant on cockles and mussels as

their primary food source. Without

these shellfish, they are forced to

forage farther inland.

See also: Evolution by natural selection 24–31 ■ Predator–prey equations

44–49 ■ Competitive exclusion principle 52–53 ■ Mutualisms 56–59

ECOLOGICAL PROCESSES

American ecologists Howard

Richardson and Nicolaas Verbeek

studied Northwestern Crows

feeding on clams in the intertidal

zone of British Columbia. The

crows put lots of effort into digging

clams out of the mud, opening the

shells, and feeding on the animal

inside. The ecologists noticed that

smaller clams went unopened and

concluded that the crows had to

make an energy trade-off between

handling time and edible food. The

time and energy needed to open up

small clams was better spent

digging for another, larger clam.

A similar study with oystercatchers

and mussels found that the largest

mussels were left—they had

thicker, barnacle-clad shells, so

opening them was more difficult.

The oystercatchers benefited more

by looking for thin-shelled mussels,

despite their smaller size.

Animals also have to make

choices about where and when to

feed. The longer a starling spends

in one patch of suitable grassland,

for example, the harder it will

become to find prey, so it has to

decide when to abandon that patch

and move to another—an example

of what is known as the “marginal

value theorem.” Foraging animals

also need to consider a range of

other factors such as the presence

of predators, the number of animals

competing for the same food, and

the impact of human activity. ■

Echolocating bats

Technological advances have

greatly helped research into

the hunting strategies of

animals. Insectivorous bats

(also known as microbats)

use echolocation in the

dark to locate and pursue

flying insect prey, such as

moths and midges. A team

of Japanese scientists set out

to study the bats’ feeding

behavior using microphone

array measurements and

mathematical modeling

analysis. The researchers

recorded the echolocation

calls and flight paths of the

bats and discovered that they

often directed their sonar not

just at their immediate prey

but at the next target they

were lining up as well.

The team also found

evidence that the bats chose

flight paths that would allow

them to plan two steps ahead,

rather like skilled chess

players. Not only were the

animals maximizing their

energy input by targeting

multiple prey items, but they

were also minimizing their

energy output by reducing

the distance they flew in

pursuit of insects. This

behavior fits in well with

optimal foraging theory.

The expected behavior of

animals with respect to

available resources can be

used to predict ... the biotic

structure ... of communities.

Ronald Pulliam

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