untitled

increasing food availability, often termed a Type II response (Holling 1959). The

rate of energy gain f(N 1 ) that an animal would experience as a consequence of the

Type II functional response can be calculated as follows:

f(N 1 ) =

where e 1 is the energy content of each item of the more profitable prey type 1, ais

the area searched per unit time by the consumer, h 1 is the time required to consume

each item of prey type 1, and N 1 is the population density of prey type 1. The energy

gain function f(N 1 ) grows with increasing abundance of prey type 1, but there are

diminishing returns to this relationship (Fig. 5.1). Indeed, there is an upper limit

e 1 /h 1 to the rate of energy gain, even when food is superabundant. This upper limit

is set by the limited capacity of the animal to handle prey.

If a forager specialized by feeding only on prey type 1, it would realize a rate of

energy return equivalent to f(N 1 ). How would this compare with the energy gain if

the forager generalized, by feeding on both prey types 1 and 2? If both prey types

are mixed indiscriminately over the landscape traveled by our hypothetical forager,

then the energy gain by a generalist would be calculated as follows:

g(N 1 , N 2 ) =

This equation raises the following question: when does it pay to be a specialist

and when to be a generalist? The answer is to specialize when f(N 1 )>g(N 1 ,N 2 ),

but act like a generalist when f(N 1 )<g(N 1 ,N 2 ). Both strategies are equivalent when

f(N 1 ) =g(N 1 ,N 2 ). This special case occurs when f(N 1 ) =e 2 /h 2. So, a forager that changed

from being a specialist to a generalist whenever f(N 1 ) fell below e 2 /h 2 would do better

than one that acted all the time as a specialist or as a generalist (Fig. 5.1). Such a

foraging strategy is termed the “optimal” strategy, meaning that it yields the highest

energetic returns over time. It is straightforward to extend this logic to any number

of resource types. First, we rank prey in terms of profitability. Then we add prey to

e 1 aN 1 +e 2 aN 2

1 +ah 1 N 1 +ah 2 N 2

e 1 aN 1

1 +ah 1 N 1

THE ECOLOGY OF BEHAVIOR 61

10

8

6

4

2

(^0) 0 20406080100
Prey 1 density
Energy gain
Expected gain from prey 1
Profitability of prey 2
Fig. 5.1Comparison
of the expected rate of
energy gain by a forager
specializing on the most
profitable prey (prey 1,
solid curve) relative to
the profitability of
alternative prey 2
(broken line). An
optimal forager would
expand its diet whenever
the abundance of prey 1
drops below the density
at which these two
curves intersect (slightly
more than 20 per unit
area in this hypothetical
example).
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