496 MHR • Unit 5 Population Dynamics
Because individuals of different ages have
different probabilities of surviving, every population
has a different age structure— a different
distribution of individuals through the age classes.
The age structure of a population can be represented
by an age pyramid, which shows the proportion of
individuals in the population in each age class.
Populations like the one shown in Figure 14.31A
have by far the largest number of individuals in the
youngest age class. In other populations, individuals
are spread more evenly throughout all age classes
(see Figure 14.31B). You will look more closely at
the effect of age structure on population growth in
Chapter 15.Figure 14.31In both of these age pyramids, the bottom
level represents the youngest members of the population,
which are not yet of reproductive age. The other levels
represent older age classes containing sexually mature
adults that are reproducing or past reproductive age. Note
that in (A) three quarters of the population is very young,
while in (B) less than half of the population is in the
youngest age class.Life History and
Environmental Stability
Recall that life history traits are genetically
controlled and are the result of natural selection
acting to increase the evolutionary fitness of the
individuals in a population. How might an
organism maximize the number of its offspring that
survive to reproductive age? Theoretically, the best
way to do this would be to start reproducing at a
young age, live for a long time, reproduce many
times over that long life, produce a large number of
offspring each time, and give each of them enough
parental care that they survive to reproductive age.
Unfortunately, this is not possible in real life.
The amount of nutrients, energy, and time that
can be gathered or used by each individual is
finite, so not all of these things can be maximized
simultaneously — trade-offs must be made. Many
studies have shown the existence of such trade-offs
in nature. For example, female beetles that produce
fewer offspring during one reproductive event have
a higher chance of surviving to reproduce again.
Given that trade-offs must occur, in which aspect
of its life history should an individual invest the
most? What combination of survival and
reproduction will maximize its fitness? The optimal
combination of life history features is determined
by the environment in which the organism lives.
One factor that ecologists have recognized as
particularly important in selecting for specific life
history features is the stability or predictability of
the environment.
In many habitats (such as the shorelines of
certain oceans and mountain highland areas),
conditions are relatively variable and
unpredictable. In such situations, an individual
may have only a small chance of surviving from
one breeding season to the next, regardless of how
large and healthy it is. Here, an organism that
expends all its available energy on producing as
many young as possible while conditions are good
will probably leave behind more offspring than one
that produces fewer young and invests some of its
energy in survival. Chances are high that the latter
individual will lose this energy investment, since
conditions are likely to become unsuitable for
reproduction before it can try again.
Environments of this type tend to be
unpredictable because of abiotic factors, such as
variable weather conditions. Populations in these
environments are regulated by density-independent
factors and can grow rapidly and reach a large size76
48:52
low production
of young76:24
high production
12 of young8
4
48
12
28
8
4
B
A
Age class
(years of age)Number of
individuals in
studied cohort
alive at
beginning of
age class
0 – 1
1 – 2
2 – 3
3 – 4
4 – 5
5 – 6
6 – 7
7 – 8530
134
56
39
23
12
5
2
Table 14.2
Life table for a grey squirrel (Sciurus carolinensis) populationProportion of
cohort alive
at beginning
of age classFecundity
(average
number of
offspring born
to females in
that age class)
1.000
0.253
0.106
0.074
0.043
0.022
0.009
0.003