1. Population Growth Ch. 4 sec. 3

2. Factors That Determine Population Growth
Growth rate: change in a population’s size over time.
A population’s growth rate is determined by births, deaths, immigration and emigration.
Why are there so many bacteria and so few elephants?

3. 1. Birth and Death Rates Mortality: rate of death
Natality: rate of birth
Survivorship Curves: A graph that shows how the likelihood of death varies with age.

4. 2. Age Structure
Age structure of a population is related to birth rates, death rates and population growth.
Young populations (pre and reproductive age) = more births than deaths.
Population increase
Elderly population = more deaths
Population decrease
Sex Ratio also influences population size.
Imbalance of males and females = changed population size

5. Immigration and Emigration
Immigration is the arrival of individuals from outside a given area.
Emigration is the departure of individuals from a given area.
Migration: temporary movements into and out of an area as a part of a seasonal routine.
Fish, mammals, insects and birds all migrate

6. Calculating Population Growth
(birth rate + immigration rate) - (death rate + emigration rate) =
net change in population size
Population changes are often expressed as percentages, which we can calculate using the following formula:
GROWTH RATE x 100%

7. How Populations Grow
Exponential Growth: when a population increases by a fixed percentage each year.
Logistic Growth: describes how a population’s increased growth is slowed and stopped by limiting factors.

8. Exponential Growth Faster and faster growth!
Only occurs in nature when populations have plenty of food, water, space, and no competition.
Ex. Mold on bread

9. Logistic Growth Growth that slows and stops due to limiting factors.
Limiting factors: Any environmental characteristic that can slow population growth and determine carrying capacity
Water
Space
Food
Predators
Disease

10. What Limits Population Growth?
1. Carrying capacity
2. Limiting factors
3. Competition

11. Carrying Capacity Maximum population an ecosystem can support.
A population may grow past the carrying capacity, but cannot stay at that size.
Example: The state of Pennsylvania can only support a certain number of white tailed deer. There is a limited amount of food, water, and shelter for the deer that live here. If the population grows so much that deer do not have enough of what they need, that population has gone over the carrying capacity and it will shrink.

12. Limiting Factors
Whatever resource is lacking or missing for a population
Will ultimately cause a decline in the population.
Water
Space
Food
Predators
Disease

13. Competition
Members within a population use the same resources and therefore will compete with each other.
Part of natural selection: those species that can access resources and make best use of those resources will survive.
Examples: Wolves: territory, food
Trees: sunlight

14. Biotic Potential
Potential that each living thing has to produce offspring in ideal conditions.
Not every organism is created equally – some can reproduce quickly and easily – others take quite a long time!

15. Review Questions
What is the difference between exponential growth and logistic growth? Which is more common over long term in nature?
You are a population ecologist studying white-tailed deer populations in your state. Populations have been growing exponentially for some time, and food is becoming a limiting factor. Many deer are dying of starvation, and others are in bad health. What do you recommend to state officials? Should people intervene and try to limit deer populations through relocation or hunting? Or should they do nothing and wait for the population to regulate itself? Explain your reasoning.