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Populations. Chapter 36. Populations. Is a groups of organisms that belong to the same species and live in a particular place at the same time. Population Ecology.

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Is a groups of organisms that belong to the same species and live in a particular place at the same time.

Population ecology

Population Ecology

The study of how members of a population interact with their environment, focusing on factors that influence population density and population growth.

Properties of populations

Properties of Populations

  • Populations Size

  • Population Density

  • Dispersion

    • Random

    • Uniform

    • Clumped

Population density

Population Density

  • The number of individuals of a species per unit area or volume

  • Density is determined by sampling and extrapolating

    • # red oak / km2

    • # of earthworms / m3

Dispersion patterns

Dispersion Patterns

  • clumped: most common-due to clumping of resources

  • examples:

    • fungus growing on

      a rotting log

    • schools of fish

Dispersion patterns1

Dispersion Patterns

  • uniform: evenly spaced

  • examples:

    • shore birds (gannets and king penguins)

    • plants secrete chemicals to decrease competition

Dispersion patterns2

Dispersion Patterns

  • random: unpredictable spacing, no pattern

  • example:

    • dandelions due to windblown seed distribution

Population dynamics

Population Dynamics

Birth Rate

Death Rate

Mortality rate- the number of deaths in period of time

  • The number of births occurring in a period of time

Survivorship curves

Survivorship Curves

Life Tables track survivorship in populations. Insurance companies use them for human life expectancy.

Survivorship curves1

Survivorship Curves

  • Show the probability that members of a population will survive to a certain age.

  • Type I

    • Likelihood of dying is small until late in life

    • Characterized by high parental care

  • Type II

    • The probability of dying does not change throughout life

    • Individuals are no more vulnerable at any age.

      • example: squirrels preyed upon by hawks

  • Type III

    • Likely to die when young

    • Very little or no parental care

      • example: insects, mollusks, fish, frogs

Survivorship curves2

Survivorship Curves

Measuring populations

Measuring populations

Population growth rate

Population Growth Rate


  • The amount by which a population’s size changes in a given time.

  • Is figured by taking the number of births and subtracting the number of deaths

    • (assuming immigration and emigration are equal)

Exponential growth model

Exponential Growth Model

The rate of population increase under ideal conditions

Logistic growth model

Logistic Growth Model

Due to limiting factors (resources), population growth rate will level off (possible crash)

Carrying capacity

Carrying Capacity

is the maximum population size that a particular environment can sustain (“carry”) with available resources.

Carrying Capacity = K.

Limiting factors


Any factor that limits or restrains the growth of a population


Competition for limiting resources


Competition for limiting resources:

Food or nutrients:

As resources are divided between more and more individuals, birth rate decreases.

Example: song female bird density vs. clutch size results in an indirect relationship.

Nesting sites



Biotic factors


Influences on health and survival


Search image established for most common prey

  • Crowded plants tend to be smaller with fewer flowers, fruits, seeds.

  • Increased disease transmission

  • accumulation of toxic waste products

Physiological factors


Physiological factors

  • High population densities in mice appear to induce a stress syndrome. Hormones change, delay sexual maturation

  • Cause reproductive organs to shrink

  • Depress the immune system

Abiotic factors


  • Complex interaction of both density-dependant and abiotic factors (density-independent)

  • First hard freeze kills all adult insect, eggs hatch in spring

  • Aphids show exponential growth with rapid decrease with high heat and low humidity (abiotic).

  • A few individuals remain to perpetuate the species.

Boom and bust cycles

Boom-and-Bust Cycles

“Booms” : rapid exponential growth followed by “busts”: population falls back to a minimal level.

Limiting factors1

Limiting Factors


Density Dependent

An individual’s chance of surviving or reproducing depends on the number of individuals

Resources limitations

  • Reduce the population by the same proportion

    • Weather

    • Flood

    • Fires



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