Biology 12

482 MHR • Unit 5 Population Dynamics

small enough to be far below its carrying capacity,

density-dependent factors have no effect and

population growth is rapid. Eventually, however,

the population reaches a density at which these

factors start to have an effect; after this point the

population growth slows, and it eventually stops

when the carrying capacity is reached.

Later in this chapter, Investigation 14-A will give

you an opportunity to examine the effect of both

density-dependent and density-independent

regulating factors on a species of micro-organisms.

The remainder of this section describes some specific

types of density-dependent factors in more detail.

Competition

In contrast to density-independent factors, density-

dependent factors are typically biotic — they

involve living things. As has been described, these

living things are often members of the population

itself. For example, the carrying capacity of a

population’s environment may depend chiefly

on the availability of food. When the population

reaches the inflection point in its logistic growth

curve, there is no longer an abundance of food for

each member of the population. Members must

now compete with each other for the limited food

supply, which becomes even more limited as the

population size increases. The result is that the

birth rate decreases or the death rate increases, or

both (see Figure 14.19), and the population growth

slows more and more as the density increases.

This type of competition among the members

of a population is referred to as intraspecific

competition. You have encountered its effects

already, since it leads to evolutionary change as a

result of natural selection. A higher proportion of

successful competitors survive longer and have

greater reproductive success; they are said to have

higher fitness. This allows them to pass on more

copies of their alleles to subsequent generations.

The result is a change in allele frequencies within

a population — which results in evolution.

100

1000

10 000

10 100

Average number of seedsper reproducing individual

Seeds planted per m^2

8

9

10

11

12

10 20 30 40 50 60 70 80 90

Average clutch size

Number of breeding pairs

20

40

60

80

100

20 60 100

Survivors (%)

Density (beetles/0.5 g flour)

A B

C

0

0

0

Figure 14.19(A) A varying number of plantain seeds (a

common Ontario weed) were planted in experimental plots.

The average number of seeds produced by an adult plant

decreased as the density of the plants in the plots

increased. (B) Many songbird populations are limited by the

amount of food available in their habitat. In some species,

the average number of eggs laid by females declines as

the density of breeding pairs increases. (C) In some cases,

increased population density reduces the ability of

individuals to survive rather than (or in addition to) reducing

reproduction. This is true for laboratory populations of flour

beetles. This graph shows the percentage of beetles that

survive from egg to adult, in relation to the population

density of the beetle.