untitled

Populations are often disturbed from their equilibrium, K, by temporary changes in

limiting factors (a severe winter or drought or influx of predators might reduce the

population; a mild winter or good rains might increase it). The subsequent tendency

to return toKis largely due to the effect of density-dependent factors, and this

process is called regulation. Therefore, regulation is the process whereby a density-

dependent factor tends to return a population to its equilibrium. We say “tends to

return” because the population may be continually disturbed so that it rarely reaches

the equilibrium. Nevertheless this tendency to return to equilibrium results in the

population remaining within a certain range of population sizes. Superficially it appears

as if the population has a boundary to its size, and it fluctuates randomly within this

boundary. However, it is more constructive to picture random fluctuations in both

the density-independent (d 1 ) and density-dependent (d 2 ) mortalities as the shaded

range in Fig. 8.6a. This results in a fluctuation of the equilibrium population

POPULATION REGULATION, FLUCTUATION, AND COMPETITION WITHIN SPECIES 113

Rate

Population density

d 3

d 2

d 1

b 1

K 4 K 3 K 2 K 1

b

Fig. 8.5Model showing
that the equilibrium
point, K, can vary with
both density-dependent
and density-independent
processes. Birth rate, b,
is held constant over all
densities. In sequence,
a density-independent
mortality d 1 reduces the
input to the population
to b 1. There follows a
density-dependent
mortality d 2 or d 3. The
intercept of bor b 1 with
d 2 or d 3 determines the
equilibrium (K 1 – K 4 ).

8.3.3Regulation

Population density

d 2

d 1

b 1

b

Population density

(a) Random variation (b)

in d 2

Random

variation

in d 1

Range

of K

Range of K

d 1

d 2

b 1

b

Rate Rate

Fig. 8.6Random variation in the mortalities d 1 and d 2 (indicated by the shaded area) are the same in (a) and (b). In (a) there
is stronger density dependence at the intercept of b 1 and d 2 than in (b), and this difference results in a smaller range of
equilibria, K, in (a) than in (b).