Population Growth

How is population growth naturally regulated?

Why?

The current world population is estimated to be over 7 billion. At present the number of births
annually exceeds the number of deaths, which means that the population is increasing, and is
estimated to reach 9 billion by 2040. In 1750 the world population was estimated at less than 800
million. How are growing populations such as ours controlled and supported, and can they continue
to grow indefinitely?

Model 1 – Population Growth

Bi

(I)
rth

on
s(
B)

ati
igr
m
Im

Total Population

Em
) igr
(D ati
hs on
e at (E
D )

1. Refer to Model 1.
a. What is the term used for populations moving into an area?
immigration
b. What is the term used for populations leaving an area?
immigration
c. Name two factors that cause an increase in the population size.
Births, immigration
d. Name two factors that cause a decrease in population size.

Population Growth 1
 Deaths, emmigration

2. Using Model 1 and the letters B, D, E, and I, write mathematical expressions to show the
types of population described below. Note: The use of > and < may be needed.

a. A stable population (total number of organisms is not changing).

B+I=D+E

b. A declining population (total number of organisms is decreasing).

B+I<D+E

c. A growing population (total number of organisms is increasing).

B+I>D+E

Model 2 – Survivorship Curves

1000

Type I:
Number of Survivors (log scale)

Human

100 Type II:

Rodents

10 Type III:
Fish, insects,
amphibians

0
Newborn Pre-reproductive Reproductive Post-reproductive age
age age

3. Refer to Model 2.
a. What does the x-axis on the graph in Model 2 represent? age

2 POGIL™ Activities for High School Biology

 b. Which type of organism shows a steady decline in its population at all life stages?
Rodents, type ii

c. Which type of organism loses most of the individuals in its population at an early life

stage? Fish, type iii

d. What survivor type are humans? Type i

Read This!

Through the process of evolution, all species have developed strategies to compensate for their
survivorship type. Insects lay eggs by the hundreds. Mammals keep their young close by and
protected until they reach adulthood. Factors such as these allow populations of species to survive
and thrive despite their survivorship curve.

4. How do you think populations with Type II or III survivorship compensate for high pre-
reproductive mortality? Have more offspriung

7. Consider the evolutionary strategies that each survivorship type has developed for producing
and rearing their young. Propose an explanation for why type I survivors have the highest
relative number of individuals/1000 births that survive until they reach post-reproductive
age?

They only focus on 1 "baby" and they would take really good care of it

8. Under what circumstances might human populations not show Type I survivorship?

Population Growth 3
 famine, war, drought, environmental stresses

Model 3 – Growth Curves

Diagram A—Exponential Growth Curve Diagram B—Logistic Growth Curve

Carrying
capacity (K)
Population Numbers

Population Numbers

Exponential
growth

Pressure begins due to

environmental resistance

Lag phase Lag phase

Time Time

9. Refer to Model 3.
a. During what phase of the growth curves in each diagram is the population just beginning
to colonize an area? Lag phase

b. Which type of population growth appears to continue unchecked? exponential

10. The growth curves in Model 3 are often referred to using the letters of the alphabet they
resemble. The logistic growth curve is sometimes referred to as an S-curve. What letter
would you use to describe the exponential growth curve?

4 POGIL™ Activities for High School Biology

 11. What causes the population to slow down during logistic growth? Limiting factors

12. The maximum population an environment can sustain is affected by environmental factors
that cause the population to level out or become stable. What term is used to describe this
level of logistic growth? Carrying ca[acoty
13. Propose some reasons why population growth is so rapid immediately after the lag phase in
both diagrams of Model 3?
Reproduction increases because of availability of resources and no competition

14. Exponential growth (diagram A) refers to the phenomena of populations that double in size
every generation. If you start with a single bacterium capable of dividing every 20 minutes,
how many bacteria would you have after just four hours?
4096

15. In most natural populations rapid exponential growth is unsustainable. As populations

increase, environmental resistance causes the growth rate to slow down, until carrying
capacity is reached. With your group, brainstorm several factors that could be considered as
environmental resistance.
Producers, space, illness, consumers, water

16. Diagram B shows that the population size fluctuates around the carrying capacity.
Considering what you know about interactions in the environment, discuss with your group
some of the factors that could cause these fluctuations. In your answer you should relate
these factors to the information from Model 1.

Population Growth 5
 Immigration and emigration would fluctuate as the species in an environment reaches
carrying capacity.

Extension Questions
Read This!

Invasive species are species that are introduced into an environment but are not naturally found in
that environment. One example of an invasive species is the American gray squirrel, introduced into
Britain at the end of the 18th century. Until 1876 the only native squirrel in Britain was the European
red squirrel, which was found in deciduous and coniferous forests. By 1940 the gray squirrel had
displaced the red squirrel across most of the British Isles, and by 1984 the red squirrel was only
found in isolated coniferous woodland areas. After its initial introduction, the gray squirrel
population increased rapidly; however, in recent years population sizes within specific environments
have become stable.

17. Explain why the newly-introduced gray squirrel initially showed rapid population growth
and why the native red squirrel showed a population decline. Use ecological terms from the
previous models in your answer.
The gray squirrel was dominant over the native one and had no competition.

18. Why has the population size of the gray squirrel become stable in recent years?

6 POGIL™ Activities for High School Biology

 The initial spike in growth from no competition is gone, and now there are limiting factors in
place stopping it from growing.

Population Growth 7