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202 Chapter 12

100

80

60

40

20

(^00100200300400)
Vegetation biomass (kg / ha)
Consumption rate (kg /3 months)
500 600
Fig. 12.5Food intake
per individual red
kangaroo per day at
varying levels of food
availability. (After Short
1987.)
0 100 200 300 400
Plant biomass (kg / ha)
Per capita rate of increase /3 months 500 600
0

• 0.2


• 0.4

Fig. 12.6Rate of
increase of red
kangaroos on a 3-
monthly basis in
relation to food
availability. (After
Bayliss 1987.)

12.5.3The numerical
response of
kangaroos to plant
abundance

They are exactly analogous. The only difference lies in the difficulty of measuring a

predator’s food intake. An ability to measure intake by way of radioactive tracers has

greatly simplified that problem. A good example is Green’s (1978) use of radio-sodium

to estimate how much meat a dingo eats in a day.

The functional response gives the effect of the animal upon a consumable resource.

In contrast, the numerical responsegives the effect of the resource on the change

in animal numbers. If the resource is used in a pre-emptive rather than a consump-

tive way (e.g. nesting holes used by parrots), then it may be adequate to represent

the numerical response by consumer density of the animals against the level of the

resource (e.g. nesting holes per hectare). If the animals’ use of the resource is con-

sumptive, however, then the relationship between the animals and the resource is

best portrayed as the instantaneous rate of population increase against the level of

the resource.

Figure 12.6 shows the numerical response relationship between rate of increase of

red kangaroos and the biomass of pasture. Bayliss (1987) estimated rates of increase

from successive aerial surveys, and pasture biomass from ground surveys. As with

the functional response, the numerical response has an asymptote: there is an upper

limit to how fast a population can increase and no extra ration of a resource will