Population size and Population size and densitydensityPopulation sizePopulation size

Determined by:Determined by: actual countactual count SamplingSampling

Population DensityPopulation DensityExpressed as:Expressed as:Number of Number of

Individuals per unit Individuals per unit area.area.

How many populations are shown How many populations are shown here? If this was a kmhere? If this was a km22 of land how of land how dense would the zebra population dense would the zebra population

be?be?

Population DynamicsPopulation Dynamics

Dynamics means Dynamics means changing over time.changing over time.

Change is due to:Change is due to: Birth rate.Birth rate. Death (mortality) rate.Death (mortality) rate. Life expectancy.Life expectancy.

Survivorship curvesSurvivorship curves Type I Type I – Few offspring, – Few offspring,

low youth mortality, low youth mortality, long life spanlong life span

Type II Type II – moderate – moderate number of offspring number of offspring with moderate with moderate mortality at of different mortality at of different age categories age categories

Type III Type III – many – many offspring , high youth offspring , high youth mortality, a few adults mortality, a few adults surviving a long time.surviving a long time.

Can you think of another example Can you think of another example of an organism that fits each one of an organism that fits each one

of the above?of the above?

Age structuresAge structures

• Populations can have different age structures

• For example sea turtles have many young and few long lived adults

• In humans age structures have a large impact on society• How many

children to care for, how many working adults

Take a moment to compare the age structures in these two nations. In 20 years how would each change?

Population GrowthPopulation GrowthPopulation growth rate is size change over time.Population growth rate is size change over time.

Growth can be negative or positive.Growth can be negative or positive.Depends on any of four factors: Depends on any of four factors:

Birth rate & death rateBirth rate & death rate Emigration (leaving ) & immigration (arriving) Emigration (leaving ) & immigration (arriving)

What would happen if the death rate was higher than the birth rate?What would happen if the death rate was higher than the birth rate?

Exponential Growth CurveExponential Growth Curve

Populations grow more Populations grow more rapidly as they increase rapidly as they increase in size.in size.

““J” curve.J” curve.Demonstrates species Demonstrates species

biotic potential.biotic potential.Unlimited resources.Unlimited resources.Only occurs in rare Only occurs in rare

cases.cases.

Can this growth curve occur for Can this growth curve occur for long periods of time? Why?long periods of time? Why?

Logistic Growth CurveLogistic Growth Curve

Same as exponential Same as exponential for short time.for short time.

Growth slows and Growth slows and finally levels out.finally levels out.

Known as “S” curve.Known as “S” curve.Carrying capacity has Carrying capacity has

been reached.been reached.Carrying capacity Carrying capacity is the is the

number of individuals number of individuals the environment can the environment can supportsupport What resource most likely What resource most likely

causes this curve to flatten out?causes this curve to flatten out?

Factors that Limit Population GrowthFactors that Limit Population GrowthCauses growth to be Causes growth to be

something less than biotic something less than biotic potential. potential. (if all offspring survive to (if all offspring survive to adulthood)adulthood)

Referred to as Referred to as limiting factorslimiting factors Density dependent: limited Density dependent: limited

resources control growth resources control growth (the larger the population the more (the larger the population the more

will die)will die) Limits on resources.Limits on resources. DiseaseDisease

Density independent: events Density independent: events that reduce population sizethat reduce population size (regardless of the size of the (regardless of the size of the

population)population) WeatherWeather FireFire

Why so many rabbits in Why so many rabbits in Australia? They are an Australia? They are an introduces species with introduces species with no natural predators.no natural predators.

Factors that Limit Population GrowthFactors that Limit Population Growth

• Predator/ prey or producer/ herbivore relationships control each other’s growth • E.g. more hares for the lynx to eat then the

more lynx will survive, the more lynx preying on the hares the more hares die… and the cycle continues

Dangers of Small Dangers of Small PopulationsPopulations

Lack of genetic variation.Lack of genetic variation.Inbreeding.Inbreeding.Lower birth rates.Lower birth rates.Susceptible to extinction.Susceptible to extinction.

Summary review1. What is the difference

between population size and density?

2. What factors influence population size?

3. What are the 2 growth models (graphs)?

4. What are limiting factors?5. Compare/ contrast density

dependent and independent factors.

6. ( extra challenge question) Cheetahs have experienced a genetic bottleneck and are endanger of extinction. Explain what this statement means.