Biology 12

Chapter 14 Population Ecology • MHR 493

the population live past the age when sexual
maturity is reached, and the percentage of
survivors from a cohort does not start to decline
rapidly until a relatively old age. The Type II
survivorship curve lies in between these two. In
populations with this pattern, the percentage of
individuals dying is constant over all ages.
It is important to realize that not all populations
fit one of these three survivorship patterns. There
is, in fact, a continuum of survivorship curves and
the survivorship pattern for a particular population
may fall somewhere between Types I and II, or
between Types II and III. As is often the case,
ecologists start by simplifying the complexities
of survival patterns in order to try to understand
them. Nonetheless, many populations do fit one
of these three survivorship curves fairly closely.
Populations that share a common survivorship
pattern also tend to be similar with respect to
other life history features, as described below.

Fecundity

As mentioned, populations vary with respect to
patterns of reproduction as well as survivorship.
The average number of young produced by a
female over her lifetime can be referred to as the
fecundityof the population. In some populations,
females produce their lifetime quota of young all at
once and then die. This is typical of many species
of insects and annual plants (those in which
individuals live for only one growing season, such
as sunflowers and petunias), as well as some
vertebrates, including several species of salmon.
Populations in which females only reproduce once
and then die have non-overlapping generations.
In other populations, females typically survive
their first reproductive event and go on to reproduce
several more times. Many types of vertebrates,
including humans, songbirds, and elephants, are
like this. So are perennial plants (those that survive
for more than one year), including many species of
shrubs and trees. The fact that females reproduce
more than once leads to populations with
overlapping generations. In such cases, it is
common for a female and her previous offspring to
all be reproducing during the same time interval.
Populations also vary with respect to the number
of offspring females typically produce each time
they give birth. The females of some species (for
example, humans, elephants, and grizzly bears)
commonly produce only one offspring at each
reproductive event. In contrast, some females have
many young each time they reproduce. A single

marigold plant may produce hundreds of seeds. In

some populations of Chinook salmon (Oncorhynchus

tshawytscha), each female may lay over 15 000 eggs.

Since the energy a female can accumulate and

use for reproduction is limited, the size of

individual offspring tends to be inversely related

to the number produced. Newborn humans and

elephants are much larger in relation to the size of

their mothers than are newly hatched salmon or

spiders. For the same reason, the number of

offspring also tends to be inversely related to the

amount of care parents give their young. A mother

that produces hundreds of offspring cannot feed or

protect each one for long, if at all. A single young,

however, often receives (and requires) a substantial

amount of care, in some cases staying with one or

both parents for years.

Age Effects

As the survivorship curves show, in any given

population an individual’s probability of dying

varies with its age. In populations with a Type I

survivorship pattern, a very young individual has

a low probability of dying, compared with an older

one. As you might expect, the fecundity of an

individual also varies with its age. Often very

young and very old individuals have a zero

probability of producing offspring. In many

populations, individuals have the highest fecundity

in the early to middle period of their lives (see

Figure 14.30). Of course, in populations in which

females reproduce only once, individuals do not

survive after reproducing; therefore, there are no

older, non-reproducing members.

Figure 14.30The fecundity (measured in terms of the

number of eggs laid) of female snow geese (Chen

caerulescens) increases until the birds are about nine

years old and then gradually declines.

2 6 10

2.0

2.5

3.0

4

3.5

4.0

4.5

5.0

8 12 14 16

Fecundity (total number

of eggs laid)

Age (years)

0