Biology 12

498 MHR • Unit 5 Population Dynamics

a particular habitat. For example, they might be

specialists at finding, hunting, or eating a particular

type of food, or at physiologically coping with a

particular temperature and moisture regime.

In a stable environment, individuals that

produce a few large offspring that are cared for to

ensure their survival are favoured. They are more

likely to pass their genes along to subsequent

generations. An organism like this is said to have

an equilibrial life history. This type of life history

is also called K-selected, since it is seen in

populations that are near Kin size. In these

environments, K-selectionis at work.

As always, it is important to realize that the

distinction between these two general types of life

history is not absolute. Real environments are

highly complex. So, too, are the selective forces

that act within those environments. Most

organisms therefore, experience both r- and

K-selection to a greater or lesser extent, and many

display features of both types of life histories. It is

probably useful to think of purely opportunistic

(r-selected) and purely equilibrial (K-selected) life

histories as forming opposite ends of a theoretical

continuum. Most populations lie somewhere

between these two extremes. In the Thinking Lab

on page 497, you will illustrate your understanding

of the differences between an opportunistic life

history and an equilibrial life history.

Looking Ahead

Human life history features are closer to what

ecologists would consider the equilibrial or

K-selected end of the scale. This suggests that

for most of its evolutionary history, the human

population has been at or near its carrying capacity.

Several lines of evidence from archeologists,

anthropologists, and historians support this

viewpoint.

However, as will be discussed in the next

chapter, about 8000 years ago humans managed to

change their environment’s carrying capacity. The

human population then entered into a period of

rapid growth that is still underway. Much of the

biosphere has been affected by this growth, and the

ecology of many species has been altered as a

result. The future survival of an array of species,

including Homo sapiens, may depend on whether

the growth of the human population and its impact

on Earth can be controlled.

SECTION REVIEW

1. Identify each of the elements in the (K-N)/N
   portion of the logistic growth equation (that is, what
   does each of the letters represent?). How does this
   portion of the equation affect the overall rate of
   growth of a population?


2. Distinguish between opportunistic and
   equilibrial life histories.


3. Distinguish between r-and K-selection.


4. What factors typically limit the population size
   of species with very high birth rates?


5. Draw a survivorship curve for a population in which
   most of the mortality occurs among the youngest
   members of the species. Provide examples of two
   species that demonstrate such a survivorship pattern.


6. What stage of an organism’s life cycle typically
   has the highest fecundity rate? Explain your answer.


7. Do humans typically have only one reproductive
   event or several over their lifetimes? Explain your
   answer.


8. What is the survivorship pattern for Canada’s
   human population?
   9. Compare the typical age structures seen in
   populations with Type I, Type II, and Type III
   survivorship patterns. Draw a diagram comparing
   the three types of survivorship patterns.


9. How might generation time affect r, the
   population growth rate?


10. Design a study that would allow you to determine
    whether a particular species had a more
    opportunistic or equilibrial life history.


11. Think of a habitat near your school or home.
    List 10 species that this habitat contains. Do these
    species have equilibrial or opportunistic life histories?
    Design a study that would allow you to find out.


12. Compare two species of plants, the dandelion
    (Taraxacum officinale) and the red maple (Acer
    rubrum) and two species of mammal, such a mouse
    and a black bear (Ursus americanus), with respect to
    whether they are more r- or K- selected. Specifically,
    compare them with respect to their fecundity,
    the shape of their survivorship curves, life span,
    population density, and whether the environment
    each species typically inhabits is more or less stable
    or unpredictable.

C

I

I

K/U

K/U

MC

K/U

K/U

C

K/U

K/U

K/U

K/U