Biology 12

Chapter 14 Population Ecology • MHR 479

Often, as is true of the fur seal population shown
in Figure 14.14B, a population grows substantially
above the carrying capacity of the environment.
This can happen if there is a delay between the
time at which a population reaches high density
and the time at which there is a response to this
increased density (in other words, before the
negative effects of increased density cause a
decrease in the birth rate). For example, imagine a
population so dense that its food is beginning to
run out. At this point, members of the population
might be expected to have fewer offspring. Some
individuals, however, may have energy (fat)
reserves that they may be able to use so they can
continue (at least for a brief period) to produce the
same number of offspring.

Figure 14.14Why does the population size fluctuate
around the carrying capacity of the environment, rather
than remaining stable at K?

The use of these reserves to maintain a high
birth rate may cause the population to overshoot its
carrying capacity. As is predicted by the logistic
growth equation, the population will then decrease
in size (dN/dtwill be < 0 , since [N−K]/K< 0 )
until it falls below K, at which point it may then
start to grow again. Thus, real populations often
fluctuate (sometimes a considerable amount)

around the carrying capacity rather than remaining

perfectly stable at K. Of course, if a population

overshoots its carrying capacity by so much that

its resources are seriously depleted, it may never

recover (see Figure 14.15).

Figure 14.15Four male and 21 female caribou were

introduced to St. Paul Island, Alaska in 1911. There, the

caribou fed on lichens, which grow slowly and are not

replaced quickly after being eaten. Although many caribou

migrate seasonally (giving their food supply time to recover),

this herd was confined to an island that had few human

hunters and no natural predators. The population grew

exponentially to about 2000 individuals. Overgrazing almost

completely depleted the food supply, and the population

crashed to about eight animals in 1950.

It is important to realize that the Kdrawn on

any graph represents an estimate of the carrying

capacity of the environment at a given moment

in time. The carrying capacity may change if

conditions are altered (for example, if a drought or

flood occurs, or if global temperatures increase). In

fact, a population may actually affect the carrying

capacity of its own environment as well as be

affected by it. As a population grows, increasing

numbers of individuals produce larger amounts

of metabolic waste and perhaps other types of

garbage (see Figure 14.16 on the next page). The

accumulation of this waste may degrade the habitat

so that its carrying capacity is lowered.

What exactly determines the carrying capacity

of the environment for a given species? What about

populations that grow exponentially — why does

this period of rapid growth rarely last long in

nature? Put another way, what regulates the size

of various populations? The answers to these

questions are provided in the next section.

1910 1920 1930

500

1000

1500

2000

1940 1950

decline as a result

of sudden resource

depletion

exponential

growth

Number of caribou

0

(^0515)

200

400

600

10

800

1000

Paramecium/mL

Time (days)

(^019151935)

2

4

6

1925

8

10

1945

Breeding male fur seals

(thousands)

Time (years)

A

B