Population Size

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SWBAT construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.

Big Idea

Students explore how environmental factors put limits on population sizes.


10 minutes

To engage students in lesson I show students the following video titled Penguins:  Popularity, Peril, and Poop by Dyan deNapoli.

Penguins are odd birds. For one, they cannot fly (but they are amazing swimmers), and, contrary to popular belief, the majority of penguin populations live in warmer regions. But these beloved birds are in danger, with populations declining up to 90%. Dyan deNapoli explains the reasons behind the decline -- and why penguins are like the proverbial canary in the coal mine of our oceans. (MS-LS2-4. Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations./Stability and Change - Small changes in one part of a system might cause large changes in another part.) 

As student watch video they answer the following questions in their interactive notebooks:

1.  How many of the 18 penguin species are listed as Near-threatened, Threatened, or Endangered?

a.  8

b.  10

c.  12

d.  15

2.  Which four penguin species regularly live and breed in Antarctica?

a.  Emperor, King, Chinstrap, Gentoo

b.  Emperor, King, Chinstrap, Adélie

c.  Emperor, Chinstrap, Gentoo, Adélie

d.  Emperor, Chinstrap, Gentoo, Macaroni

3.  What percentage of a penguin's life is spent at sea?

a.  50

b.  75

c.  85

d.  90

4.  Penguins are negatively impacted by humans in many ways but what are the two main threats to penguins today?

a.  Global warming and overfishing 

b.  Global warming and oil spills

c.  Overfishing and introduced predators

d.  Overfishing and oil spills

5.  Over the last 50-100 years, the population of most penguin species has declined close to _____%.

a.  50

b.  75

c.  80

d. 90


20 minutes

In this section of lesson students explore the affects of abiotic and biotic factors on population size by completing a module from The Concord Consortium.

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Population Explosion 

Many factors influence the success and survival rate of a population of living things. Explore several factors that can determine the survival of a population of sheep in this NetLogo model. Start with a model of unlimited grass available to the sheep and watch what happens to the sheep population! Next try to keep the population under control by removing sheep periodically. Change the birthrate, grass regrowth rate, and the amount of energy rabbits get from the grass to keep a stable population.(SP2 - Developing and Using Models/SP4 - Analyzing and Interpreting Data)



Teacher Note: Students complete module in pairs using student laptops.  I will add to narrative once students have completed module for insight/modifications.   


10 minutes

In this section of the lesson students read an article titled Population Size from cK-12. 

The article discusses:

  • Biotic and abiotic factors relationship to population size
  • Introduces the concept / vocabulary of Carrying Capacity 
  • Introduces the concept / vocabulary of Limiting Factors

Key Points

  • Biotic factors that a population needs include food availability. Abiotic factors may include space, water, and climate.
  • The carrying capacity of an environment is reached when the number of births equal the number of deaths.
  • A limiting factor determines the carrying capacity for a species.

Prior to reading the text, students complete a Pre Read Activity

Pre Read Objective:

To use prior knowledge of root words to learn new vocabulary words by using the etymology of the scientific vocabulary. 

Once student have read the article, they answer the following text-dependent questions (RST.6-8.1-Cite specific textual evidence to support analysis of science and technical texts): 

  1. Why don't populations continue to grow and grow?
  2. What happens if a population exceeds its carrying capacity?
  3. What happens if a factor that has limited a population's size becomes more available?


15 minutes

In this section of the lesson, students complete a Carrying Capacity activity.  

Learning Objective:

To be able to describe the carrying capacity of an ecosystem (SP4- Analyzing and Interpreting Data)

Teacher Instructions:

1.  Hand out the page with the pictures on it (page 7) and allow students time to interpret it.

2.  Based on the information conveyed in the pictures, have the students develop a definition for carrying capacity.  

3.  Based on their definition of carrying capacity, have the students determine the factors on which carrying capacity depends.

4.  Provide each student with the rest of the handout (pages 2-4) Read through it together to have the students determine if their definition for carrying capacity and the factors on which it depends is appropriate.

5.  Go over what the graph of a population at its carrying capacity looks like.  Emphasize that a population fluctuates around it's carrying capacity.  

6.  Ensure that all students realize that the carrying capacity is the maximum number for a certain population and that populations do no need to be at their carrying capacities to be stable. 

7.  Have the students answer the questions in the handout.
8.  Correct the questions (answer key on pages 5 and 6).


5 minutes

In this section of lesson students answer the following question. Students are required to analyze data in order to choose the correct answer and they must use evidence from the lesson to support their answer. (WHST.6-8.1-Write arguments to support claims with clear reasons and relevant evidence./SP7 - Engaging in Argument from Evience)

Use the graph below to answer the next question.  Use evidence from todays lesson to support your answer.  


Which of the following situations MOST likely produced this population of deer on WallaWalla Island?

  1. Deer predators were removed from the island before the deer arrived.
  2. The deer overgrazed the grasses immediately upon their arrival to the island.
  3. After a period of time the deer adjusted to the available food on the island.
  4. The deer contracted a new disease just after their arrival on the island.