6 Population growth
In this chapter we deal with the internal workings of a population that result in a
change of population size. The speed of that change is measured as rate of increase.
Any such change alerts us that the fecundity rate, the mortality rate, or the age dis-
tribution, or more than one of these, has changed. Each of those parameters will be
considered in turn and the relationships between them explained.
This chapter has two quite distinct functions. The first is to arm the reader with
the theory of population dynamics. The second is to indicate which parts of that
theory are immediately applicable to wildlife management and which parts are
necessary only for a background understanding. The first function may appear to load
a manager with unnecessary mental baggage, but without such knowledge mistakes
are more than just possible, they are likely. Knowledge of atomic theory is not needed
to mix a medicine, but without that knowledge a pharmacist will, sooner or later,
make a critical mistake.If a population comprising 100 animals on (say) January 1 contained 200 animals
on the following January 1 then obviously it has doubled over 1 year. What will be
its size on the next January 1 if it continues to grow at the same rate? The answer
is not 300, as it would be if the growth increment (net number of animals added
over the year) remained constant each year, but 400 because it is the growth rate
(net number of animals added, divided by numbers present at the beginning of the
interval) that remains constant. Thus the growth of a population is analogous to the
growth of a sum of money deposited at interest with a bank. In both cases the growth
increment each year is determined by the rate of growth and by the amount of money
or the number of animals that are there to start with. Both grow according to the
rules of compound interest and all calculations must therefore be governed by that
branch of arithmetic.
Populations decrease as well as increase. The population of 100 animals on
January 1 might have declined to 50 by the following January 1, in which case we
say that the population has halved. If its decline continues at the same rate it will
be down to 25 on the next January 1. Halving and doubling are the same process
operating with equal force, the only difference being that the process is running in
opposite directions. The terms by which we measure the magnitude of the process
should reflect that equivalence. It is poorly achieved by simply giving the multiplier
of the growth, 2 for a doubling and 0.5 for a halving, and it becomes even more con-
fusing when these are given as percentages. We need a metric that gives exactly the
same figure for a halving as for a doubling, but with the sign reversed. That would
make it obvious that a decrease is simply a negative rate of increase.78