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Population Growth

Increase in population = population growth

Population Growth Rate = the change in the number of individuals in a population over time

Population Dispersion

The way in which individuals of a population are spread in an area

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Clumped vs. Uniform Dispersion

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Live in close proximity for mating, protection, and/or food access

Due to territoriality and intraspecific competition

Random Dispersion

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Spread randomly within an area

Survivorship Curves

Survivorship Curve

Shows the number of surviving members over time from a measured set of births.

–Type I—low level of infant mortality and will survive until old age (mammals and humans)

–Type II—survivorship rate is equal at all stages of life (birds, mammals, reptiles)

–Type III—very high birth rate, very high infant mortality (inverts, amphibians, plants)

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A population is a group of individuals belonging to a single species that occupies a certain area.

What are some ecological populations we could find outside? Discuss with your neighbor.

Population StudiesCHECK YOUR NEIGHBOR

Population Studies

Population size over time depends on four variables—birth rate, death rate, the rate of immigration into the population, and the rate of emigration out of the population.

Population Studies

Exponential growth occurs when a population grows at a rate that is proportional to its size.

Population Studies

Logistic growth occurs when population growth slows as it reaches the habitat’s carrying capacity, the maximum number of individuals or maximum population density the habitat can support.

Population Studies

Organisms have different life history strategies.

r-selected organisms produce a lot of offspring and invest little in each offspring. For example, they provide little parental care. Few of the offspring reach adulthood.

K-selected organisms produce a few offspring and provide a great deal of investment in each. For example, they provide a lot of parental care. Most of their offspring reach adulthood.

These are two extremes along a continuum.

Population Studies

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Population Growth

Biotic Potential• The rate at which a population will grow if all

individuals in a population survive and reproduce at maximum capacity

• This is the ideal environment• Unlimited resources

• No hazards

• Does not exist in real world

Population Growth

Four factors can affect the size of a population:• Immigration

• Births

• Emigration

• Deaths

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Growth Curves

Graphs showing the number of individual in a population over time

2 types• Exponential (J shaped curve)

• Logistic curve (S shaped curve)

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Exponential Growth

Exponential Growth• Due to an increase in resources

• J shaped curve

• 2 phases• Lag phase – little or no increase in population

growth

• Exponential phase – rapid population growth

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Logistic Growth

Logistic Growth• S shaped curve

• Represents the Carrying Capacity

• Carrying Capacity = the max number of individuals the ecosystem is capable of supporting• Just enough resources

• Sudden drop in population causes a population crash.

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Limits to Growth

Density Dependent Factors Density Independent Factors

Limiting factors are factors that cause population growth to decrease.

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Limits to Growth Density Dependent Factors

• Factors that affect populations depending on population density

• Competition• Food

• Water

• Space

• Shelter

• Predation

• Availability of mates

• Availability of parasites and disease

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Limits to Growth

Density Independent Factors• Factors that affect populations regardless of

population size

• Generally abiotic factors

• Ex: weather changes

• Ex: natural disasters

• Ex: human activities

Human Population Growth

Some human populations have undergone a demographic transition, a shift from high birth and death rates to low birth and death rates.

Usually, death rate decreases first, due to medical and public health advances. Later, birth rate also declines.

During the period between the fall in death rate and the fall in birth rate, the combination of low death rate and high birth rate causes the population to grow very rapidly.

Human Population Growth

A pyramid-shaped age structure diagram, such as that of Kenya, is a sign of a rapidly growing population. Most of the population is young and therefore at or approaching reproductive age.

The United States has an age structure that is much more even—its population is growing slowly.

In Italy, much of the population is older, and there are relatively few children. Italy’s population is stable or even declining.

Human Population Growth

Human Population Growth

AIDS has affected world population more than any other disease since the bubonic plague of the Middle Ages.

AIDS strikes primarily adults of reproductive age.

Some of the countries struck hardest by AIDS, including Botswana and South Africa, are expected to experience population declines as a result of the disease.

Human Population Growth

Human population is currently about 6.5 billion.

Although world population continues to grow rapidly, the rate of growth has slowed, so that growth is no longer exponential.

Scientists now believe that if present trends continue, the global human population will peak at around 10 billion soon after 2050.

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Human Population

From 6000 BC to 1800 AD the number of people increased slowly

Mid 1800’s human population began to grow exponentially• Why?

• Agricultural revolution

• Industrial revolution

• Advances in medicine

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Human Population

In 1650 – 500 million people In 1930 (about 300 years later) – 2 billion

people Today (about 70 years later) – over 6

billion people By 2050 – between 7.8 and 12.5 billion

• This is based on population growth in China and India

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Human Population

Humans have been able to push the carrying capacity of the Earth past what it really should be• Ability to cultivate crops

• Ability to domesticate animals

• Ability to cure diseases

Eventually we will run out of space and possibly resources