untitled

then determined by two things: on one hand the amount of food available and on

the other the intrinsic ability of the species to convert that extra energy into

enhanced fecundity and diminished mortality. Thus, it depends on an environmental

effect and an intrinsic effect but neither is without limit. From the viewpoint of

the animal both are constrained. There comes a point at which the animal has

all the food it can eat, any further food having no additional effect on its reproduc-

tive rate and probability of survival. Similarly, an animal’s reproductive rate is

constrained at the upper limit by its physiology. Litters can be only so big and the

interval between successive litters cannot be reduced below the gestation period. The

potential rate of increase can never be very high, irrespective of how favorable

the environmental conditions are, if the period of gestation is long (e.g. 22 months

for the African elephant, Loxodonta africana). All species, therefore, have a maximum

rate of increase, which is called their intrinsic rate of increase(Fisher 1930) and

denoted rm. It is a particularly important parameter in estimating sustainable yield

(see Section 19.3).

Populations do not attain that maximum very often. It requires a very high avail-

ability of food and a low density of animals such that there is negligible competition

for that food. These conditions are most closely approached when a population is

in the early stage of active growth subsequent to the release of a nucleus of indi-

viduals into an area from which they were formerly absent. Figure 6.2 gives intrinsic

rates of increase of several mammals, most of the data being gathered in that

way. Alternatively the rate could be estimated from the initial stages of growth

of a population recovering from overhunting. That would work for blue whales

(Balaenoptera musculus) for example, which are presently recovering from intense over-

harvesting between about 1925 and 1955 (Cherfas 1988).

Intrinsic rate of increase rmtends to vary with body size. The relationship has been

calculated (Caughley and Krebs 1983; Sinclair 1996) for herbivorous mammals as:

rm=1.5W−0.36

where Wis mean adult live weight in kilograms. Table 6.1 gives rmcalculated by

that equation for a range of body weights. In the absence of other data it provides

an approximation that can be used to make a first estimate of sustained yield (see

Chapter 19).

POPULATION GROWTH 81

800

600

400

200

0

14

12

10

8

01020300102030

t (years)

N

(thousands)

log

Ne

t (years)

Fig. 6.1Exponential
population growth of
the George River
caribou herd, as
discussed in the text.
(After Messier et al.
1988.)