1. Lecture 16 Population Dynamics Ozgur Unal
NIS - BIOLOGY
Lecture 16
Population Dynamics
Ozgur Unal

2. Population Characteristics
Population: the members of a single species that share the same geographic location at teh same time.
How can we describe populations?
What are some observations you can make about populations of insects over the course of a year?
Populations of species are described by density, spatial distribution and growth rate.
These characteristics are used to classify all populations of organisms including bacteria, animals and plants.

3. Population Density
Population density is the number of organisms per unit area.
In order to find the average population density, simply divide the total number of organisms by total area.
Figure 4.1 and 4.2!

4. Spatial Distribution
Population density might not be uniform everywhere.
Dispersion is the pattern of spacing of a population within an area.
Mainly three types of dispersion: uniform, clumped and random.

5. Spatial Distribution Figure 4.2
Which type of population distribution allows you to predict more accurately how many individuals reside in a given area?
What advantage do smaller fish gain by forming clumped groups?

6. Population Ranges
No population (not even the human population) occupies all habitats in the biosphere.
Some species have very limited population range or distribution.
Examples: Iiwi, peregrine falcon etc.
What are the reasons why some species are not able to expand their population range?

7. Population Limiting Factors
All species have limiting factors, which keep them from continuing to increase indefinitely.
Example: Food supply..
If the food supply increases, a larger population might result. If it decreases a smaller population might result.
Population limiting factors can be grouped into two: density-independent factors and density-dependent factors.

8. Population Limiting Factors
Density-independent factors:
Any factor in an environment that does not depend on the number of members in a population per unit area is a density-independent factor.
These factors are ususally abiotic and include natural phenomena such as weather events.
Example: Ponderosa pine trees

9. Density-Independent Factors
Example: Humpback chub fish in Colorado River.

10. Density-Dependent Factors
Any factor in an environment that depends on the number of members in a population per unit area is a density-dependent factor.
These factors are often biotic factors such as predation, disease, parasite and competition.
Predation:

11. Density-Dependent Factors
Disease: This is a density-dependent factor.
When population density is high, diseases are transmitted easily from one individual to another because contact between individuals is more frequent.
Competition:
Competition between organisms
increases when density increases.
Competition can occur within
a species or between two different
species that use the same resource.

12. Density-Dependent Factors
Parasites:
The presence of parasites is a density-dependent factor that can negatively affect population growth at higher densities.

13. Lecture 17 Population Growth Ozgur Unal
NIS - BIOLOGY
Lecture 17
Population Growth
Ozgur Unal

14. Population Growth Rate
Check this out!
How is the growth rate of these bacteria?
Do they grow fast or slow by time?
How would you plot the number of bacteria vs time?

15. Population Growth Rate
Population growth rate (PGR) explains how fast a given population grows.
What factors do you think affect the size of a population?
The natality of a population is the birthrate, or the number of individuals born in a given time period.
The mortality of a population is the number of deaths that occur in a given time period.
Emigration is the term to describe the number of individuals moving away from a population.
Immigration is the number of individuals moving into a population.

16. Population Growth Models
There are two mathematical models used to describe population growth: Exponential growth model and logistic growth model.
Exponential Growth Model:
Remember the bacteria growth. The growth rate was small at first, but then it increases rapidly.
The initial growth rate is small and called lag phase.
Exponential growth occurs after the lag phase.
Similarly, a mice population in an area with large food supply grows exponentially.

17. Exponential Growth Model
The graph of exponential growth is J-shaped.
The rate of growth is proportional to the size of the population.
All populations grow
exponentially until some
limiting factor slows the
population’s growth.

18. Logistic Growth Model
Logistic growth occurs when the population’s growth slows or stops after exponential growth, at the population’s carrying capacity.
Many populations grow like
in this model, rather than
exponential growth.
S-shaped curve is typical
for logistic growth.
A population stops
increasing when natality
is less than mortality, or when emigration
exceeds immigration.

19. Logistic Growth Model
Carrying capacity is the maximum number of individuals in a species that an environment can support for the long term.
Carrying capacity is limited
by the energy, water oxygen,
and nutrients.

20. Reproductive Patterns
The logistic model shows the number of individuals increasing until the carrying capacity is reached.
However, there are additional factors that must be considered for real populations.
Number of births per reproduction cycle, the age that reproduction begins, life span of the organism..
r-strategists: The rate strategy is an adaptation for living in an environment where fluctuation in biotic and abiotic factors occur (such as availability of food or changing temperature).
Example: Fruit fly or locusts
r-strategists usually have short life span and produce many offsprings.

21. Reproductive Patterns
Populations of r-strategists usually are controlled by density-independent factors and they usually do not maintain a population near the carrying capacity.
k-strategists: The carrying capacity strategy is an adaptation for living in environments that do not fluctuate much.
k-strategists are generally larger organisms that have a long life span and produce few offspring.
Their population reaches equilibrium at the carrying capacity.
Populations of k-strategists usually are controlled by dnesity-dependent factors.