Studying%20Populations

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Studying Populations
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Determining population size Warm-UP

• How do you think scientists can determine the
  size of a population of organisms in an
  ecosystem?
• What are some factors that might affect how
  accurate the population count is?
• Can a scientist ever really know the exact number
  of organisms in a population? Why or why not?

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How scientists estimate population

• Direct observation
• Count all of the members of a population
• Works well for large animals in open areas
• Plants in a small area

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Indirect Observation

• Observing signs of organisms
• Nests
• of animal tracks
• Sounds (frog chirps)

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• Sampling
• counting the number of organisms in a small area
  and then multiplying to estimate the amount in a
  larger area
• Assumes that the number of organisms is equally
  spread out through the area

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Mark and Recapture

• Scientist collect organisms of a single species
• Mark each individual
• Let them go
• Recapture them at a later date and count the
  total with the original mark
• Use a math formula to determine the total
  population size
• Works well with animals that move around, or that
  live in water

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Lets Try it!

• Purpose Given an area, field or other space, use
  random sampling to estimate the population size
  of dandelions (Taraxacum officinale).
• Procedure Using a meter stick (or tape measure),
  and some kind of marker (stick, rocks), mark a
  square area 2 meters by 2 meters.
• Count the dandelions in the area, be sure to
  count even the ones that have not blossomed.
  Record in your notebook
• of dandelions in 2m2 ____________
•  
• Repeat in at least one other spot. Record in
  notebook
• Calculate the populations density of dandelions
  per m2 (divide by 2).
• Record in notebook of dandelions per m2
  _______________

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Post questions

• If the total area you sampled is 66,300 m2, how
  many dandelions would you expect there to be
  overall?
• Describe two factors that would result in errors
  with your estimate. What could be changed to
  improve the accuracy?
• What would be another organism whose population
  could be measured this way?

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OUTPUT!

• A certain bird of prey will only nest in areas
  that have large oak trees. As part of a
  reintroduction project, an environmental agency
  needs to determine if Wild River State Park has
  enough oak trees to support the birds. How would
  you determine this number? Explain why this is
  the BEST method.

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• Prairie dogs have been spotted throughout western
  Minnesota in the tall-grass prairies. These
  animals live in underground burrows, eat mostly
  plants and avoid large birds of prey by running
  underground. The tall grasses of the prairie can
  make them difficult to spot. How would you
  determine the population size of the prairie dog
  in western Minnesota?

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• Fish biologists must strike a balance between
  allowing for recreational fishing and maintaining
  a healthy fish population. How would you
  determine the number of fish in Forest Lake?
• Students are worried about the number of spiders
  living on the soccer field at school. The
  principal says that there's really not that many
  and that it's not a big deal. How can you find
  out how many spiders are living on the field?