Ecosystem Stability and Species Equilibrium Models

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Ecosystem Stability and Species Equilibrium Models
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Ecosystem Stability

• Ecosystems are relatively stable over time
• a rain forest today will look similar to the rain
  forest in its place a hundred years from now
• Ecosystems have a stable number of species over
  time, although species turnover will change the
  specific species in an ecosystem over time

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Ecosystem Stability

• What are the three types of stability?
• Persistence ability of an ecosystem to resist
  disturbance
• Constancy ability of a population to maintain
  adequate population size
• Resilience ability of an ecosystem or population
  to bounce back after a disturbance

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What are the characteristics of stable ecosystems?

• complex food webs
• generalist species
• high species diversity
• complex species interactions
• high primary productivity
• low to moderate climate variation
• However, no cut and dry rules (at times, simple
  can be stable too

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What determines how many species will exist in an
area?

• Species Equilibrium Model
• or, the
• Theory of Island Biogeography

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The Theory of Island Biogeography

• The theory was originally established for
  islands, but can be adapted to any habitat
  island
• The number of species on an island is determined
  by the balance of extinction and immigration
• After some time, the immigration rate and
  extinction rate will be equal equilibriujm
  number of species on island

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The Theory of Island Biogeography

• The theory was originally established for
  islands, but can be adapted to any habitat
  island
• The theory looks at the question from a
  historical perspective How did the island get
  from no species to the stable number that it has
  now?

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The Theory of Island Biogeography

• The number of species on an island is determined
  by the balance of extinction and immigration

Immigration rates start out high, because each
species to hit the island is likely to be a new
species, and will successfully immigrate Over
time the number decreases because there are no
new species left to colonize, and immigration is
difficult
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The Theory of Island Biogeography

• The number of species on an island is determined
  by the balance of extinction and immigration

Extinction rates start out low because there is
little competition for abundant resources Over
time, that number increases as species become
extinct due to competition, predation, or random
fluctuations in the environment
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The Theory of Island Biogeography

• The number of species on an island is determined
  by the balance of extinction and immigration
• After some time, the immigration rate and
  extinction rate will be equal equilibrium
  number of species on island

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Affect of Island Area on Species Equilibrium

• Larger islands have a larger catching area which
  results in higher rates of immigration
• Larger islands have lower rates of extinction
  because of a more diverse habitat more niches to
  fill, more resources

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Species-area relationships

• The island biogeography model predicts, and most
  people agree, that there is a positive
  relationship between the number of species (and
  species diversity) and the area of a piece of
  land
• This is called the Species-area relationship

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Affect of Island Distance from Mainland on
Species Equilibrium

• Nearer islands have higher rates of immigration
  because there is a greater chance that organisms
  will immigrate to the island

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So, how does this apply to environmental science?

• As humans destroy habitat, we are leaving habitat
  fragments behind
• This theory can help us answer
• Should we preserve this fragment?
• Would it be better to give up this small fragment
  in exchange for saving a larger one?
• Is this fragment close enough to untouched land
  to be able to be a viable piece of land for
  organisms to colonize?
• Etc...

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Species Diversity

• Species diversity is a measure of the number of
  species there are in an ecosystem plus the
  distribution of individuals in all species
• The most diverse ecosystems are those that have
  the greatest number of species, and the most
  even distribution.
• The species-area relationship suggests that the
  greatest diversity should be in the largest
  habitat areas

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Calculation of Species Diversity

• One diversity index is the Shannon-Weiner
  Diversity Index
• It is calculated as
• H -
• pi the number of individuals in one species
  divided by the total number of individuals in all
  species

• pi log pi

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Calculating Diversity Example
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The good and the bad

• Good
• can compare different ecosystems in different
  habitats
• reflects number of species and eveness
• Bad
• means nothing by itself
• doesnt reflect biomass or individual size
• no units

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Species Richness

• Species richness is often calculated as the
  number of species in an environment
• In the previous example, it would be 5