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Population Ecology


Population Ecology – PowerPoint PPT presentation

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Title: Population Ecology

Population Ecology
By the end of this class you should understand
  • The language of ecology and how to describe a
  • The different types of growth and survivorship
  • How to identify limiting factors of a population

  • Ecology is the study of how organisms behave and
    interact with each other
  • Population Ecology is the study of a single
    population of animals (all the same species)
  • Community Ecology is the study of interactions of
  • Population ecology often closely studies the size
    of a population over time
  • More sex and violence!

Key Population Terms
  • Population Size
  • Number of individuals in a given population
  • Population density
  • How closely packed these individuals are on
  • Population distribution
  • Whether the individuals are evenly spread out or
    clumped in some way

Population Size
  • The number of individuals in a population depends
    on how you define population
  • May be all animals in a particular region
  • May be the entire species
  • May be more specific, such as all female or all
    with a particular gene
  • Usually a population includes all organisms that
    regularly interbreed
  • Geographically isolated groups are usually
    different populations

Population Size Change
  • A populations numbers can change due to any
    number of factors
  • Death of organisms
  • New organisms being born
  • Arrival (immigration) and/or leaving (emigration)
    of organisms (especially animals)
  • If a populations numbers do not change over time
    it is never because none of these are happening
  • This is a stable population and almost never

Why No Stable Population?
  • Remember the struggle for existence!
  • Even if a population is isolated, by natural
    selection only the horniest of organisms have
    reproduced over the years, so they will always be
    trying to make babies
  • Even if there are only limited resources, the
    impulse to make babies cannot be denied
  • Also, there are always predators and disease
  • A population will only be stable if the birth and
    death rates perfectly cancel out
  • They might on one given year but not constantly

No Predators Or Disease!
  • When there is no emigration and no death, a
    population will experience exponential growth
  • This means a population will double in size every
  • Generation time varies wildly by species
  • Bacteria 20 minutes
  • Humans 20 years
  • All species will have the same curve though

Exponential Growth
  • Exponential growth cannot be sustained without
    unlimited resources
  • A resource is anything organisms need to survive
  • Some resources are in much larger supply than
    others and it varies by environment
  • In the ocean sunlight is a limited resource and
    water is a plentiful resource
  • In the desert sunlight is a plentiful resource
    and water is a limited resource

Infinite Resources
  • It is obviously not possible to have infinite
  • Eventually space and air would become limited
  • That means whichever resource runs out first will
    become a limiting factor
  • Normally predators, parasites and natural
    disasters are also limiting factors by causing

Limiting Factors
  • The limiting factors of a population create a
    certain theoretical maximum of a population size
    that would be stable
  • This limit is called the carrying capacity
  • The carrying capacity is dictated by predators,
    limited resources, etc. and is not fixed but an
  • Environments can be created or damaged and
    carrying capacities can be changed!

Environmental Change
  • During the time of the bible, the Levant (in the
    middle east) was a very fertile region that was
    the birthplace of civilization
  • After centuries of overfarming the plains and
    clear-cutting the forests, it is now a desert
  • NOT due to climate change!
  • Elephants also instigate major changes in local
    environments, even without climate change

Lets make a list!
  • What are some limiting factors on populations?
  • Note some of these are density-dependent (more
    problematic as organisms are more crowded)
  • Others are density-independent (no change in
    danger as population density increases)

Carrying Capacity
  • When a population hits carrying capacity it can
    hit it in one of two ways
  • It can level out at the carrying capacity
    (logistic growth)
  • It can overshoot the capacity and then crash
    (windfall pattern)
  • If there are many strong limiting factors then
    you usually see logistic growth

Windfall Pattern
  • True story an empty island off the coast of
    Alaska had a substantial buildup of moss when
    some humans left a few reindeer on this island
  • No predators or disease lots of babies!
  • The population underwent exponential growth until
    the moss was all gone, then crashed during a
    harsh winter
  • Eventually all the reindeer died

Windfall Population Pattern
Why Is This Relevant?
  • All populations have carrying capacities
  • Including the human population!
  • A pre-industrial society has limiting factors
    like disease and lack of food
  • Industrial revolutions are awesome for sanitation
    and farming
  • A post-industrial society has a low birth rate
    because babies are expensive

Human Population Growth
  • The world didnt hit 1 billion people until
    around 1800
  • Hit 2 billion around 1927
  • 4 billion by 1974
  • 7 billion back in 2012
  • There are several projections

Reproductive Strategies
  • Different organisms have different strategies for
    survival of the species
  • Some make tons of babies and hope they survive
  • Some put a lot of energy into keeping each
    offspring alive
  • What are some organisms that are on each extreme?

Survivorship Curve
  • Closely related to the reproductive strategy is
    the survivorship curve
  • Describes the percentage of a group of organisms
    that are alive after a given percent of their
    maximum lifespan
  • Organisms that produce many babies have type III
    survivorship while organisms that produce few
    babies but protect them have type I

Survivorship Curves
More on Monday!
  • See you in lab!
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