untitled

A population’s rate of increase is determined by its size, by how many animals are

born, and by how many die during a year. Hence, birth rate is an important com-

ponent of population dynamics that can be measured in a number of ways. Of these

the most useful is fecundity rate.

We measure fecundity rate as the number of female live births per female per unit

of time (usually 1 year). That figure is often broken down into age classes to give a

fecundity schedule as in Table 6.2, and each value is denoted mx, female births per

female in the age interval xto x+1.

The number of animals that die over a year is another important determinant of rate

of increase, and again it can be measured in a number of ways. We measure it as the

mortality rate, the number of animals that die during a unit of time (usually 1 year)

divided by the number alive at the beginning of the time unit. As with fecundity, the

rate is often given for each interval of age.

The pattern of mortality with age is summarized as a life table, which has a

number of columns as in Table 6.3. The first is the age interval labeled by the age

at the beginning of the interval and denoted x. The second is survivorship lx, the

probability at birth of surviving to age x. The third is mortality dx, the probability at

birth of dying in the age intervalx, x+1. The fourth, the most useful, is mortality

rate qx, the probability of an animal age xdying before the age of x+1. The fifth,

82 Chapter 6

2

1

0

–1

–2

–3

101 102 103 104 105 106 107

Weight (g)

log

re

m

Multimammate rat

Short-tailed vole

Norway rat

Orkney

vole

White-tailed deer

Roe deer Wapiti

Man (not used) African elephant

Fig. 6.2Intrinsic rate of rm = 18 W– 0.36
increase of mammals
plotted against body
weight. (After Caughley
and Krebs 1983.)

Weight (kg) rm

1 1.50

10 0.65

100 0.29

1000 0.08

Table 6.1Expected
intrinsic rates of
increasermon a yearly
basis for herbivorous
mammals as estimated
from mean adult live
weight.

6.3 Fecundity rate

6.4 Mortality rate