1. Biology Chapter 4, Section 1: Population Dynamics
Population Biology
Biology Chapter 4, Section 1: Population Dynamics

2. Principles of Population Growth
A population is a group of organisms, all of the same species, that live in a specific area.
A healthy population will grow and die at a relatively steady rate unless it runs out of water, food, or space, or is attacked in some way by disease or predators.
Scientists study changes in populations in a variety of ways.
One method involves introducing organisms into a controlled environment with abundant resources; then watching how the organisms react.
Ex. Bacterial cells are placed in a dish of sterile, nutrient-rich solution and population growth is observed over a period of time.
Through this study, scientists have been able to identify some trends in the growth of bacterial cells, which helps fight diseases.

3. How fast do populations grow?
Think about a job
You are paid a certain amount per hour that you work.
Let’s say you are paid $5 per hour to help your brother cut yards. You work 3 hours on Monday, 5 hours on Tuesday, and 7 hours on Wednesday. What would think look like if we plotted it on a graph?

4. How fast do Populations grow?
Populations of organisms do not experience this linear growth.
A population graph starts out slowly and then begins to increase in numbers.

5. Is growth unlimited?
A J-shaped growth curve illustrates exponential population growth.
Exponential growth: as a population gets larger, it also grows at a faster rate.
Results in unchecked growth.

6. QUESTION
THINK: Can a population of organisms grow indefinitely? Why or why not?

7. OR What can limit growth? Population CANNOT grow indefinitely!!!
Eventually, limiting factors such as…..cause population growth to slow.
Food
Disease
Predators
Lack of space
With this information, what would our population graph look like now? OR

8. Carry Capacity
Carrying capacity: the number of organisms of one species that an environment can support indefinitely.
When a population is developing in an environment with resources, there are more births than deaths and the population increases until the carrying capacity is reached or passed.
When a population overshoots the carrying capacity, then the limiting factors may come into effect.
Deaths begin to exceed births and the population falls below the carrying capacity.
The number of organisms in a population is sometimes more than the environment can support and sometimes less than the environment can support.

9. Reproduction Patterns
In nature, animal and plant populations change in size.
For example, mosquitoes are more numerous at certain times of the year than others.
THINK: Why don’t populations reach carrying capacity and remain stable?
To answer this question, scientists must study the factor that determines population growth- an organism’ s reproductive pattern, also called its life-history pattern.
THINK: What is the difference in the life-history pattern of a mosquito and an elephant?

10. Rapid life-history patterns
Common among organisms from changeable and unpredictable environments.
Organisms who exhibit a r-strategy show:
Small body size
Mature rapidly
Reproduce early
Have a short life span
Populations increase rapidly, then decline when environmental conditions such as temperature suddenly change and become unsuitable for life.
The small population that survives will reproduce again when conditions are favorable once again.

11. slow life-history patterns
Larger species usually exhibit a slow life-history pattern.
Animals who exhibit s-strategy include:
Elephants
Bears
Whales
Humans
Plants
Slow life-history organisms reproduce and mature more slowly, and are long-lived.
They maintain population sizes at or near carrying capacity.

12. Density factors and population growth
Ecologists have identified two kinds of limiting factors that are related to dispersal:
Density-dependent
Density-independent
Population density describes the number of individuals in a given area.
Density-dependent: includes disease, competition, predators, parasites, and food.
These factors have an increasing effect as the population increases
Density-independent: These include volcanic eruptions, temperature, storms, floods, drought, chemical pesticides, and major habitat destruction.
These factors harm populations not matter their density!

13. Organism interactions limit pop. size
Population sizes are limited not only by abiotic factors, but also are controlled by various interactions among organisms that share the community.

14. Predation affects pop. size
Populations of predators and their prey are known to experience cycles or changes in their numbers over periods of time.
Under controlled conditions, predator-prey relationships often show a predictable cycle of populations increases and decreases over time.

15. Competition within a population
THINK: What happens when organisms within the same population compete for resources?

16. The effects of crowding and stress
When populations of certain organisms become crowded, individuals may exhibit symptoms of stress.
As populations increase in size in environments that cannot support increased numbers, individual animals can exhibit a variety of stress symptoms.
These effects are negative symptoms to a population and keep populations below the carrying capacity.
This includes:
Aggression
Decrease in parental care
Decreased fertility
Decreased resistance to disease

17. Biology Chapter 4, section 2
Human population
Biology Chapter 4, section 2

18. World Population Every ten years a census is taken in the US.
One of the most important pieces of data from this census is the rate at which the country’s population is growing or declining.
These figures are the basis for demography: the study of human population size, density and distribution, movement, and its birth and death rates.

19. Human population growth
THINK: What factors affect growth of human population?
Human population growth is different because humans can consciously change their environment.
Over time, humans have:
Eradicated the smallpox disease
Developed methods for producing more food
Infant mortality rate has decreased and technological developments have improved the delivery of clean water.

20. Calculating Growth rate
There are numbers of factors that determine population growth rate.
These factors include:
Births
Deaths
Immigration
emigration
Birthrate: number of live births per 1000 population in a given year.
Death rate: number of deaths per 1000 population in a given year.
Immigration is the movement into a population.
Emigration is the movement out of a population.
Equation needed: (Birthrate + Immigration rate) – (Death rate + Emigration rate) = Population Growth Rate (PGR)

21. Calculating growth rate
What if immigration and emigration rates are not available or are inaccurate?
Use the equation: Birthrate – Death rate = Population Growth Rate (PGR)
FACTS TO REMEMBER!!!
If the birthrate of the population equals the death rate, then the population growth rate is zero.
This does not mean the population isn’t changing; rather that new individuals entering the population at the same rate they are leaving.
If the PGR is above zero.
More new individuals are entering the population than are leaving, so the population is growing.
The PGR CAN be less than zero!
There are more individuals leaving than are entering the population.

22. Doubling Time
Doubling time: the time needed for a population to double in size.
This time varies per population because it depends on the current population and the growth rate of the population.
A slow or negative growth rate means it will take a long time for the population to double.
A rapid growth rate means the population will double in a shorter time.
Think about the populations of different countries:
Slow doubling time means that the country is developed.
Rapid doubling time means that the country is developing.

23. Age structure
Age structure: refers to the proportion of the population that are different age levels.
Based on information from population counts, an age structure graph can be calculated.
An age structure graph can tell you approximately how many males and females there are in a population, and how many people there are at each age level.
Rapidly growing countries have age structures with a wide base because a large percentage of the population is made up of children and teenagers.
If the percentage of people in each category is fairly equal, the population is stable.

24. Ecology and growth
Needs of populations differs greatly around the world.
What do populations need???
Resources could include: uncontaminated drinking water and agriculture, adequate sewage facilities, and the ability to provide food for a growing population.
Sometimes a population grows more rapidly than the available resources can handle.
Resources needed for life become scarce or contaminated, the amount of waste becomes too great to dispose of, conditions become difficult and lead to stress on the population.