STUDYING POPULATIONS

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?

DETERMINING POPULATION SIZE – WARM-UP

Direct observationCount all of the members of a populationWorks well for large animals in open areasPlants in a small area

HOW SCIENTISTS ESTIMATE POPULATION

Observing signs of organismsNests# of animal tracksSounds (frog chirps)

INDIRECT OBSERVATION

Samplingcounting the number of organisms in a small area and then multiplying to estimate the amount in a larger areaAssumes that the number of organisms is equally spread out through the area

Scientist collect organisms of a single species

Mark each individualLet them goRecapture 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

Mark and Recapture

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: _______________

LET’S TRY IT!

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?

POST QUESTIONS

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.

OUTPUT!

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?

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?