Scientists describe population growth using models like exponential and logistic growth. Exponential growth assumes unlimited resources leading to rapid growth, while logistic growth incorporates limiting factors that slow growth as the population approaches the carrying capacity. Population size is regulated by biotic factors like competition for resources, disease, predation, and physiological stress at high densities, as well as abiotic factors such as climate, weather, and natural disasters. No population perfectly fits either the exponential or logistic growth models due to the complex interplay of these limiting factors.

1. POPULATION GROWTH
THEORIES AND FACTORS
By
Prachee Rajput
Zoologist

2. NEW AIM: How do scientists describe population
growth?
Population Ecology
- The study of changes in population size and the factors that
regulate it.
Population
- group of individuals of a single species that occupy the
same general area
1. Rely on same resources
2. Influenced by the same biotic and abiotic factors
3. High likelihood of interbreeding

3. AIM: How do scientists describe population
growth?
Let’s look at population growth of bacteria starting from a single bacterium
- binary fission every 20 minutes under ideal conditions
Q: How many will there be after 36 hours?

4. AIM: How do scientists describe population
growth?
Let’s look at population growth of bacteria starting from a single bacterium
- binary fission every 20 minutes under ideal conditions
Graph it:
Q: How many will there be after 36 hours?
A: 2108
(enough to cover the planet one foot deep)
2n
Where n = # of divisions

5. AIM: How do scientists describe population
growth?
1. Exponential Growth Model
(J-shaped curve)
What does the rate of growth depend on in this model?
The number of individuals ONLY
So why is Earth not covered in bacteria?

6. AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
Ex. Fur seals on St. Paul Island

7. AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
Ex. Fur seals on St. Paul Island
1. Uncontrolled hunting until 1925
2. Population jumped until 1935
- Almost exponential
3. Population stabilized
- Some hunting
- limited breeding grounds

8. AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
3. Logistic Growth Model
(S - shaped curve)
Q: How does the logistic model
differ from the exponential model?

9. AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
3. Logistic Growth Model
(S - shaped curve)
Q: How does the logistic model
differ from the exponential model?
A: It accounts for limiting factors
in the environment

10. AIM: How do scientists describe population
growth?
2. Population limiting factors
- environmental factors that restrict population growth
3. Logistic Growth Model
(S - shaped curve)
Carrying capacity (K)
- max. population the environment can
support without degrading the habitat
Determined by:
The species itself and the
environment (resources
available, predation, etc…)

11. AIM: How do scientists describe population
growth?
- environmental factors that restrict population growth
When is growth rate the lowest?
When is it the highest?
NO POPULATION FITS EITHER
MODEL PERFECTLY
1. When population is small or large
Limited number of individuals
and then limited resources
1. Population at intermediate level
relative to carrying capacity.
2. Plenty of resources
2. Population limiting factors
3. Logistic Growth Model

12. AIM: How do scientists describe population
growth?
Population limiting factors
- environmental factors that restrict population growth
Human Population
We have a major problem here. What is
our future? The number one problem on
this planet for humans and many, many
other species is our own overpopulation.
What is the solution?

13. AIM: How do scientists describe population
growth?
Population ecology
Exponential growth model
Logistic Growth Model
- S-shaped
- NO limiting factors
Review
- Growth rate determined by population
size and type of organism
- J-shaped
- Population limiting factors
NO POPULATION FITS EITHER
MODEL PERFECTLY

14. AIM: How do scientists describe population
growth?
Let’s look at some specific population limiting factors and examples.

15. NEW AIM: What are some of the factors that
limit population growth?
Population limiting factors
1. Biotic factors
a. Competition among members (intraspecific)
Song Sparrow Population Experiment
(small island in British Columbia)
Fig. 35.4A
- add food
- Clutch size increased
- food supply, water, space, energy
(oil), mates, etc…

16. AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
a. Competition among members (intraspecific)
- food supply, water, space, energy
(oil), mates, etc…
Competition for space…

17. AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
a. Competition among members (intraspecific)
i. food supply
ii. space (territory)
A. Dispersion patterns
1. clumped
2. uniform
3. random

18. AIM: What are some of the factors that limit
population growth?
Dispersion patterns

19. AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
b. Health
- increased spread of disease with increased pop. density
- accumulation of waste

20. AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
c. Predation
Fig. 35.5
“boom-and-bust” cycles
As the hare population increases, so
does the Lynx population, which will
cause the hare population to decrease
and in turn the lynx population
decreases, etc…

21. AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
d. Physiological factors
White-footed mouse experiment
1. Enclose in a small field
2. Reproduce quickly to 30 or 40
3. Reproduction declines and pop. stabilizes
around 40
4. Add more food and water
- no change
RESULT: high population induces stress syndrome (hormonal changes)
- sexual maturity delayed, reproductive organs shrink, depressed immune system

22. AIM: What are some of the factors that limit
population growth?
Population limiting factors
2. Abiotic factors
a. Climate and weather
Ex. Aphids
- insects that feed on phloem sap of plants
1. Exponential growth in spring
2. Rapid die out in hot, dry summer
3. A few individuals survive and reproduce
when conditions improve
What happens to mosquitoes and grasshoppers during winter?
- die out completely, leaving only fertilized eggs
Fig. 35.4B

23. AIM: What are some of the factors that limit
population growth?
Population limiting factors
2. Abiotic factors
b. Fire, flood, etc…

24. AIM: What are some of the factors that limit
population growth?
Population limiting factors
1. Biotic factors
a. Competition among members
- food supply
- space
b. Health
c. Predation
d. Physiological factors
2. Abiotic factors
a. Climate and weather
b. Natural disasters
Almost all populations are regulated by a mixture of these and other factors
(25-year study of the song sparrow)
Fig. 35.4C