f(N 1 ) =where β(N 1 ) depicts the probability of foraging on the poorer prey, which is a func-
tion of the density of preferred prey, calculated according to the optimal diet choice
rule (Section 5.2.1).
This equation predicts that there would be a sharp drop in consumption of the
more profitable prey at the point at which the forager expands its diet to include
less profitable prey (Fig. 5.3). This drop implies that mortality risk for the more
profitable species would decline accordingly. This process has been observed with
beavers in large enclosures with either a single type of prey or a mix of prey species
(Fryxell and Doucet 1993). When presented with a single species of prey, beavers
had a Type II functional response, smoothly climbing with prey density, but at a decel-
erating rate (Fig. 5.4). When presented with a mix of prey, however, beavers
expanded their diet as preferred prey declined in abundance, in accordance with an
optimal diet model (Fryxell 1999). This led to a pronounced decline in predation
risk to preferred prey as the diet expanded (Fig. 5.4).aN 1
1 +ah 1 N 1 +β(N 1 )ah 2 N 264 Chapter 55 4 3 2 1 00 20406080100
Prey 1 densityRate of consumption of preferred prey3210
0102030
Aspen saplings / 0.15 haSaplings cut /daySingle
speciesMultispeciesFig. 5.3Predicted effect
of optimal diet choice
on the rate of intake of
the preferred prey.Fig. 5.4Functional response of
beavers presented with a preferred
species of prey (trembling aspen
saplings) or a mix of prey species
(trembling aspen, red maple, and
speckled alder saplings) in two
separate trials. The rate of
consumption of preferred prey
(aspens) dropped precipitously as
the animals expanded their diet to
include less profitable prey (maples
and alders) in the multispecies
trial, causing the pronounced
deviation between the two curves
at low densities of aspen saplings.
(After Fryxell 1999.)WECC05 18/08/2005 14:42 Page 64