untitled

is denoted V. Field data suggest that it is rare to observe higher browse availability

than 100 g /m^2 , which is equivalent to K=100 Mg / km^2. Maximum moose density is

thought to be 2 moose / km^2 (Messier 1994), and woody plant rmaxis estimated as

3.33 Mg / km^2 / year (Turchin 2003). We see that the growth term for the edible plant

biomass is maximized at low biomass, not at intermediate biomass, as it would be

for a logistic growth function. The rationale for low edible biomass is that moose

have access only to regrowing tissues, such as twigs and leaves, so that the rest of

the plant functions as an ungrazeable reserve. Regrowth capacity should not be inversely

affected by herbivory so long as it does not jeopardize plant survival. The indigestible

component is the same kind of refuge demonstrated in Robertson’s (1987) study of

food plants fed upon by kangaroos in semi-arid Australian grasslands.

The maximum rate of plant consumption by moose was set at 2 Mg /individual/

year, based on maximum values quoted in the feeding studies literature (Crête and

Bédard 1975). Fitting various curves to Vivås and Sæther’s (1987) studies of moose

foraging in Norway suggests a foraging efficiency of b=40 Mg / km^2. Moose can

just meet their metabolic requirements at a level of intake of half the maximum, pro-

viding an estimate of d=1 Mg / individual /km^2. Given a maximum exponential rate

of increase of 0.2 for moose (Fryxell et al. 1988b) and values for all the other para-

meters, one can solve for eusing the following relationship:

yielding e=0.467.

Rates of wolf consumption of moose are modeled as a Type II functional response,

based on Messier’s (1994) review of several moose–wolf studies throughout North

America. Each of these studies provides one or more estimates of consumption rate

by wolves at a given moose density. By combining all of the recorded data together

in a single graph (Fig. 12.10), Messier was able to illustrate one of the most difficult

kinds of ecological relationships, functional responses of large organisms under

free-living conditions. Such patterns are essential to our understanding of con-

sumer–resource interactions, yet are prohibitively costly to gather in a single study.

Use of aggregate data is a very useful way to solve this problem.

e

r

aK

bK

d

=

+

−

0

208 Chapter 12

4

3

2

1

0

0 0.5 1.0 1.5 2.0 2.5

Moose density (moose/km^2 )

Consumption rate by wolves

(kills/100 days)

r = 0.73 P = 0.01

y =0.46 + 3.36xx

Fig. 12.10The
consumption rate by
wolves of moose in
relation to moose
density. (After Messier
1994.)