Population Ecology

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Population Ecology
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By the end of this class you should understand

• The language of ecology and how to describe a
  population
• The different types of growth and survivorship
  curves
• How to identify limiting factors of a population

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Ecology

• 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
  populations
• Population ecology often closely studies the size
  of a population over time
• More sex and violence!

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Key Population Terms

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

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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

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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
  happens

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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

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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
• Generation time varies wildly by species
• Bacteria 20 minutes
• Humans 20 years
• All species will have the same curve though

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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

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Infinite Resources

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

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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
  average
• Environments can be created or damaged and
  carrying capacities can be changed!

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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

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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)

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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

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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

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Windfall Population Pattern
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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

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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

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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?

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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

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Survivorship Curves
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More on Monday!

• See you in lab!