Population Ecology

Population Characteristics

Population Density: – The number of organisms

per unit area

Spatial Distribution:– Dispersion: The pattern of

spacing a population within an area

– 3 main types of dispersion• Clumped• Uniform/Even• Random

– The primary cause of dispersion is resource availability

• Aside from the examples listed, give me an example of each of the types of dispersion–Clumped–Uniform–Random

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Population Limiting FactorsPopulation growth rate

– How fast a given population grows– Factors that influence this are:

• Emigration: the number of individuals moving out of a population

• Immigration: the number of individuals into a population

• Natality: Birth rate• Mortality: Death rate

• Give me a factor that influences population growth and explain what that factor is.

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Population Limiting Factors• Density-independent factors

– Factors that limit population size, regardless of population density.

– These are usually abiotic (non-living) factors

– They include natural phenomena, such as weather events

• Drought, flooding, extreme heat or cold, tornadoes, hurricanes, fires, etc.

• Give some examples of density-independent factors.

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Population Limiting Factors• Density-dependent factors

– Any factor in the environment that depends on the number of members in a population per unit area

– Usually biotic factors– These include

• Predation• Disease• Parasites• Competition

• What are some differences between density-dependent and density-independent factors?

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Understanding Exponentials• Think about this:

– An employer offers you two equal jobs for one hour each day for fourteen days.

– The first pays $10 an hour.– The second pays only 1 cent a day, but the

rate doubles each day.– Which job will you accept?

Understanding ExponentialsPopulation Ecology: Population Growth Rate

Job 2 lags for a long time before

exponential growth kicks in!

Now, how much would your employer owe you if you stayed at this job for another 2 weeks?

What would happen if this type of growth took place

within a population?

Population Limiting Factors• Population growth models

– Exponential growth model• Also called geometric growth or J-shaped growth.

– Limits to exponential growth• Population Density (the number of individuals per unit of

land area or water volume) increases as well• Competition follows as nutrients and resources are used

up

– The limit to population size that a particular environment can support is called carrying capacity

• What are some limits to population growth?

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What population do you think this is?

So, what do you think is going to happen to the human population?

• We will probably reach our carrying capacity.• Our growth rate will start to look like most

organisms, which is the Logistic Growth Model

Population Limiting Factors• Population growth models

– Logistic Growth Model• Often called the S-shaped growth curve• Occurs when a population’s growth slows or

stops following exponential growth.• Growth stops at the population’s carrying

capacity• Populations stop increasing when:

– Birth rate is less than death rate(Birth rate < Death rate)

– Emigration exceeds Immigration (Emigration > Immigration)

• What causes the logistic growth model to form?

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Population Limiting Factors• Population growth models

– Logistic Growth ModelThe S-curve is not as pretty as the image looks1. Carrying capacity can be raised or lowered.

How?Example 1: Artificial fertilizers have raised kExample 2: Decreased habitat can lower k

2. Populations don’t reach k as smoothly as in the logistic graph.• Boom-and-Bust Cycles• Predator-Prey Cycles

• Species that follow this pattern usually1. Have a Boom-and-Bust Life

Cycle2. Have short life spans3. Produce many offspring4. Smaller organisms

• i.e., fruit flies, mice, locusts

5. Don’t maintain a population near carrying capacity

6. Controlled by density-independent factors

• They are called r-strategists (for rate of increase)

Population Limiting Factors

• What animals are r-strategists?

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• Species that follow this pattern usually1. Have long life spans2. Produce few offspring

that have a better chance of living to a reproductive age

3. Are larger organisms• i.e., elephants

4. Maintain a population at or near k

5. Controlled by density-dependent factors

• They are called k-strategists (for karrying kapacity)

Population Limiting Factors

• What type of animals are k-strategists?

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