LESSON
1.
POPULATION DYNAMICS
A population is a group of interbreeding individuals
of the same species that occupy a specific area (Sutton and
Harmon, 1973).
Population dynamics is the study of changes in
population size and density. The principles of population
dynamics are fundamental to understanding how ecosystems
function (Cunningham and Saigo, 1990).
When resources are plentiful, most organisms reproduce
very fast. Population size doubles at regular intervals. We
can call this population explosion. Some species will grow
exponentially until they exceed the environment’s carrying
capacity, exhausting all available resources.
When this
happens, population “crash” or a sudden population decline
could occur.
In other species, population growth is
regulated so it reaches an equilibrium before exceeding the
carrying capacity of the environment.
Many factors can influence population size.
Among
these are natality (birth rate), mortality (death rate),
fertility rate, immigration, emigration, climate, weather,
competition, predation and disease. The sum total of gains
and losses due to these influences determines the net rate
of growth.
Stress and crowding due to competition for
limited resources can be important population limiting
factors
both
within
a
species
and
between
species
(Cunningham and Saigo, 1990)
Population density is a population size in relation to
some unit of space (Odum, 1971) or number of organisms in
one unit area (Sutton and Harmon, 1973).
The formula for getting the population density is as
follows:
Pd =
number of individuals or population biomass
unit area or volume
Imagine yourself in a forest and you would like to
know the population density of all dipterocarps. The best
thing to do is count all the dipterocarp trees and divide
the number by the area of the forest.
For example 200
trees per hectare.
Population growth is the increase or decrease in the
total number of organisms in a population due to the
interplay
between
biotic
potential
and
environmental
resistance.
Biotic potential refers to the ability of
organisms to reproduce themselves under optional conditions
whereas environmental resistance refers to the biotic and
abiotic factors that keep the organisms from reaching and
continuing their biotic potential.
This process can be
viewed as a cybernetic system, with negative feedback
tending to keep the population in some sort of balance.
The feedback process is based on the fact that as
population density increases, environmental resistance
increases, which in turn decreases subsequent population
density (Sutton and Harmon, 1973).
A common population growth pattern is the S-shaped
curve. This is typical of an organism in a new environment.
This curve begins with a slow rate of growth (the lag
phase), then experiences very rapid exponential growth (the
logarithmic phase), and then it levels off (the equilibrium
level).
If individuals of a population are introduced into an
area suitable for their growth and reproduction, an
increase in their numbers as time passes can be reasonably
predicted, and several characteristics descriptive of that
growth can be determined. Population growth can be plotted
on a graph as a curve relating the numbers of individuals
to the passage of time. The best curves result from
laboratory populations that are easy to grow but have a
very short life cycle. Figure 1 illustrates the growth of
yeast cells.
700
600
500
400
300
200
100
0
0
Figure 1.
2
4
6
8
10
12
14
16
18
S-shaped growth curve of yeast cells in a
laboratory setting (Sutton & Harmon, 1973).
This growth curve takes the form of an S that is why
it is called an S-shaped curve.
This very typical growth
pattern for an organism in a new but favorable environment.
The curve has three phases:
lag phase: a slow initial phase of growth in which the
organisms are acclimating to a new environment (1-4
hours on Figure 1)
logarithmic phase: a period of rapid exponential
growth following the lag phase (called logarithmic
because it appears as a straight line on a logarithmic
graph) (4-10 hours on Figure 1)
equilibrium level: a phase of gradual leveling off at
some set point (10-18 hours on Figure 1)
The curve tells us the total number of organisms in
the population at any given time on the graph. It can also
tell us something about the rate of population growth. The
steeper the curve, the faster the population is growing.
Thus, the rate of growth is greater between the sixth hour
and the eighth hour than it is between the second hour and
the fourth.
After the fourteenth hour, the population
levels off at about 650 individuals.
The growth rate at
this point is zero (Sutton & Harmon, 1973).
At the equilibrium level, the population has reached
the maximum density that the environment can support. This
is usually referred to as the carrying capacity of the
environment.
The S-type of growth pattern is not universal to all
populations. A J-shaped curve shows a more dynamic growth
and never levels off long enough to define a carrying
capacity.
J-shaped growth curves are typical of many insect
populations which produce only one generation of offspring
per year. Plankton algae species that quickly replace each
in lake and ocean waters and bacteria in various solutions
also show a J-shaped pattern. The early part of the growth
curve in Figure 2 shows some similarities to the previous
S-shaped curve.
Both the S and J curves have a similar
phase of slow initial growth.
Both proceed to a phase of
rapid increase. But the final phase of the S-shaped curve
is characterized by a gradual leveling off, while the Jshaped curve is marked by a sharp decline once a maximum
value is reached (Figure 2) (Sutton & Harmon, 1973).
One explanation for the J-shaped curves is that many
species have very specific environmental requirements.
When the environment changes unfavorably, perhaps because
of the population’s own growth (accumulation of toxic waste
products of exhaustion of food resources), the population
cannot continue to survive. A few individuals may persist
(perhaps in some dormant state like an egg or spore), but
the majority of the members making up the population die.
When conditions again become favorable, any survivors will
start to grow.
Organisms having a J-shaped curve usually
have tremendous biotic potentials. From a systems point of
view, the J-shaped curve represents an imperfect regulatory
system in which adequate corrective (negative) feedback is
absent, weak, or very, very slow in responding
(Sutton &
Harmon, 1973).
11000
D. divergens
10000
9000
8000
7000
6000
D. sociale
5000
4000
3000
2000
1000
0
A
Figure 2.
M
J
J
A
S
O
N
D
J
F
M
J-shaped growth curves of two species of
golden-brown algae, Dinobryon divergens
and D. socials (Sutton & Harmon, 1973).
Activity 1.
Answer all the questions to determine whether or not
you have mastered the first lesson.
1.
Define population: ____________________________________
_______________________________________________________
_______________________________________________________
2.
What is population dynamics? __________________________
_______________________________________________________
_______________________________________________________
3.
Match the following:
Match Column A with Column B.
A
____
____
____
____
____
____
1.
2.
3.
4.
5.
6.
B
a. ability of organisms to reproduce
themselves
under
optimal conditions
b. number
of
organisms
per
unit
environmental resistance
area/volume
c. sudden decline in
population
d. population
size
doubles at regular
intervals
e. increase
or
decrease
in
the
number
of
population
f. biotic and abiotic
factors that could
affect
population
growth
g. number
of
birth
per unit of time
population explosion
population crash
population density
population growth
biotic potential
B
No. of organism
No. of organism
A
Time
a.
Time
Identify and describe graph A & B. Give example
organism having growth curve A & growth curve B.
of