Lesson: Calculating total population size and population density

David Kujawski Bird Middle East Walpole, MA
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Lesson Objective

YWBAT: Calculate population density

Lesson Plan

DO NOW: Have a clear, rectangular container in the front of the room filled with jelly beans or another type of candy.  Have students try to estimate how many jelly beans are in the top layer of the container only.  NOTE: Just have the students estimate the jelly beans on the top layer, otherwise this becomes a volume calculation and not an area one.  We don’t want to confuse them with the difference between area and volume!  Field responses and ask students to explain how they came up with that answer.  Write predictions on the board.

State: Ecologists, women and men who study ecology, sometimes need to know how many organisms of a certain species live in a certain area.  You may remember from math that area is LENGTH x WIDTH, so if you had a length of 5 meters and a width of 2 meters, you would have an area of 10 meters squared.  We represent meters squared like this: m2

Activity 1:

State: We will now do an activity to practice calculating area (Student worksheet 1)

Handout activity and circulate the room to insure that all students can correctly calculate area.  You should verify that they’re using correct units, as well.

Review activity

State: Once ecologists have determined the area that they will be studying, they then determine how many organisms of a certain species are in several smaller areas.  You may remember the term ‘sampling’ from our lesson on methods used to estimate population sizes.  A common sample size that ecologists use is 1 meter x 1 meter, commonly referred to as a meter squared (m2).  Simply put, ecologists will walk around their sample area and randomly drop their m2.  They will then count how many organisms of the species that they’re studying that are found in the sample.  Upon taking several samples, they will get an average amount of species.  The final step is to multiply the average amount of organisms of that species by the TOTAL sample area to estimate the total population of that species in the area.

Example: Perform on board or Smart board.

Let’s say the total area has the following dimensions: 10 meters by 50 meters and there is an average of 5 Dandelions in 1 m2.  Approximately how many Dandelions would be in the entire area?

Well…10 x 50 = 500 m2 so that’s the area of the study area.  Now we need to multiply 500m2 by 5 Dandelions.

Answer: 2,500 Dandelions in the 500m2 area.

Activity 2: Student worksheet 2 (Estimating total population size)

Students complete and review with class.  Circulate to insure that all students can complete the task.

State: You may remember the formula for calculating density from our chemistry unit.  It was Density = Mass divided by Volume.  Mass is how much matter something has and volume is the amount of space that something takes up. 

Ask: What would a picture of atoms/molecules that are really dense look like?  Answer: They would be really close together.

Ask: What would a picture of atoms/molecules that are not very dense look like?  Answer: They would be spread out, not very close together.

State: The same idea applies to population density; dense populations would mean that the organisms of the same species live in close proximity to each other, less dense populations do not live in close proximity to each other. 

We can use a similar, yet somewhat different, formula for calculating population density.

Population density = the number of organisms in an area / Total area

Example on board:  What would the population density of oak trees be if the number of organisms is 10 and the total area is 5 m2.

Population density = 10 oaks trees (number of organisms in a species) / 5m2 (total study area)

Answer: Population density = 2 oak trees/m2 (This is said that there are 2 oak trees PER meter squared) In other words, you can expect to find, on average, 2 oak trees in every meter squared in the 5 meter squared area.

Activity 3: Students should complete worksheet with their partner or on their own.  Review as a class to identify misunderstanding and re-teach as necessary.

Revisit DO NOW (Formative assessment)

Ask: Based on what we know now, turn to your partner and discuss a way that we can now estimate the population density of the top layer of the jelly beans?

Have students collect the information that they need (area and number of jelly beans in sample area) and have them estimate the number of jelly beans.  You can then count to see how accurate their estimates were!  If you school allows eating then you can share jellybeans with the students.

Homework: complete (Student worksheet 4)

Reflection: be sure to check in with students who struggle in math on this lesson.  In addition, make sure that students know the different between area and volume--they commonly mix these up!

Lesson Resources

Student worksheet 1 area calculations    Classwork
Student worksheet 2 (Estimating total population size)   Classwork
Student worksheet 3 Calculating population density    Classwork
Student worksheet 4 homework    Homework


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