5 Population Potential
1. Identify factors that affect population size.
2. Compare real population growth curves to models
3. Explain what factors affect carrying capacity of an environment.
4. Predict changes in population size due to various abiotic or biotic factors.
Readings: Chapter 13: 343-355; Lester Brown interview “World in the Balance” linked in Week 5
Reminders: Midterm Friday Oct 7 from 8:00 to 8:50 in your normal lecture room
A population is made up of the Individuals of one species living in a given geographic area at the
same time. Populations have certain attributes/characteristics of interest to population ecologists:
- the density or the overall size of a population.
- the dispersion or spacing (spatial distribution) of individuals in a population - clumped,
uniform, or random.
- the numbers of individuals in different age classes, or sexes.
The spatial distribution of individuals in a population
Under what conditions would individuals in a population manifest a clumped distribution?
- uniform distribution?
The size of a population changes over time due to births, deaths and movements of individuals.
These processes are influenced by the interactions of individuals with their environment and with one
Plot a figure using time as the X axis and Total No as the Y axis; what shape is the line you drew?
1 2 3 4
Time No. of No. born Total No.
individuals in No.
0 1 0 1 50
1 1 1 2
2 2 2 4 30
3 4 4 8
4 8 10
6 1 2 3 4 5 6
Time The growth rate of this population (no. of new individuals added as the percentage of the current no.;
column 3 divided by column 2) is constant (in this case, 100%). However, the actual number of
individuals added onto the population at each time interval increases steadily - exponential growth
A population experiences an exponential
growth when the resources are not limited
(e.g., when the population size is small).
Twelve pairs of rabbits were released in
Victoria, Australia in 1859. The population
increased to over 120,000 in 6 years.
Yet, the present day population sizes of rabbits in Australia remain more or less stable.
For this population of yeast cells growing in a culture in a test tube draw a simple plot below of this
data and predict the population size at 120 and 135 minutes.
Time in Number of
QUESTION: What has begun to happen beyond 60 minutes?
Natural populations are regulated by density dependent factors, whose strengths and impacts on
the population are proportional to the population size (population density).
Shortage of essential resources for survival, growth and reproduction
Risks of infectious diseases
Accumulation of (toxic) wastes excreted by individuals
Do you see any commonality among these factors? Due to the effects of these factors, the birth rate begins to decline while the death rate begins to
increase. Logistic growth models take into account the effects of density dependent factors on
In the logistic growth model on the left, the number
of individuals added to the population begins to
decline as the population reaches a certain level
(where the arrow pointed).
As the population size reaches the carrying
capacity of the environment, the population ceases
to grow and stays more or less stable.
Carrying capacity – the maximum population size
that can be supported in an environment
indefinitely (without degrading the environment).
Figure 13.6 in your textbook
Besides density-dependent factors,
density-independent factors also
affect the size of populations.
Examples of density independent
factors include severe weather, fire,
storms, drought, and other
QUESTION: Which segments of this figure could represent the following scenarios ?
1) extreme climatic event?
2) rapid growth due to ample resources and
lack of predators?
3) competition for resources and a gradual
reduction of reproductive rate