Populations (HS) Teacher Guide

Unit

Populations

Subject

Biology

Grade Level

HS

Activity Name(s)

Extinction

Population Balance

Population Explosion

Being Prepared

Ideally, students would have one to one computer access where they are close to other students in order to discuss the activities. They may also work in groups of 2-3 if necessary.

Getting Started

Both of the activities in the unit are simulations so they only require a computer with internet access.

Suggested Timeline

This unit will take three days, or each activity needs about 35 minutes to complete so it is possible to do both in one block period but that could be a lot of rigor in one period.

Thinking about the Discovery Questions

The populations unit focuses on how populations grow and which factors contribute to growth and decline. This includes the concepts of evolution, stabilization of population size, and explosions of population growth.

The first activity's (Extinction) discovery questions, "Why do species sometimes disappear completely?" allows students to investigate factors that contribute to extinction.

The second activity's (Population Balance) discovery questions, "What keeps populations stable?" allows students to investigate carrying capacity and how populations strike a balance between the factors introduced in the first activity.

The third activity's (Population Explosion) discovery questions, "Can populations go out of control?" allows students to investigate further factors that contribute to population size, such as death and limited food supplies.

Misconceptions

Common misconceptions held by students include the notion that populations grow or decline constantly and do not find a “peak” population size. For example, they tend to see human population as limitless in size, rather than susceptible to the same environmental factors that affect all life on Earth. Another commonly held belief is that competition between organisms is direct and involves fighting. In these simulations we can see that competition is for resources and does not involve physical confrontation. Students may also think that animals of the same species do NOT compete for the same resources or “cooperate” to share resources. This model addresses this false understanding.

Learning Objectives

NGSS

• Performance Expectations
  • HS-LS2-1. Use mathematical and/or computational representations to support explanations of factors that affect carrying capacity of ecosystems at different scales.
• Disciplinary Core Ideas
  • Ecosystem Dynamics, Functioning, and Resilience
    • A complex set of interactions within an ecosystem can keep its numbers and types of organisms relatively constant over long periods of time under stable conditions. If a modest biological or physical disturbance to an ecosystem occurs, it may return to its more or less original status (i.e., the ecosystem is resilient), as opposed to becoming a very different ecosystem. Extreme fluctuations in conditions or the size of any population, however, can challenge the functioning of ecosystems in terms of resources and habitat availability.
  • Interdependent Relationships in Ecosystems
    • Ecosystems have carrying capacities, which are limits to the numbers of organisms and populations they can support. These limits result from such factors as the availability of living and nonliving resources and from such challenges such as predation, competition, and disease. Organisms would have the capacity to produce populations of great size were it not for the fact that environments and resources are finite. This fundamental tension affects the abundance (number of individuals) of species in any given ecosystem.
• Practices
  • Asking Questions and Defining Problems
    • Ask questions that arise from examining models or a theory to clarify relationships.
  • Developing and Using Models
    • Develop and use a model based on evidence to illustrate the relationships between systems or between components of a system.
  • Engaging in Argument from Evidence
    • Construct an oral and written argument or counter-arguments based on data and evidence.
  • Using Mathematics and Computational Thinking
    • Use a computational representation of phenomena or design solutions to describe and/or support claims and/or explanations.
• Crosscutting Concepts
  • Cause and Effect
    • Cause and effect relationships can be suggested and predicted for complex natural and human designed systems by examining what is known about smaller scale mechanisms within the system.
  • Stability and Change
    • Much of science deals with constructing explanations of how things change and how they remain stable.
    • Change and rates of change can be quantified and modeled over very short or very long periods of time. Some system changes are irreversible.
  • Systems and System Models
    • Models (e.g., physical, mathematical, computer models) can be used to simulate systems and interactions—including energy, matter, and information flows—within and between systems at different scales.

NSES

• NSES Life Science – The Interdependence of Organisms
  • Living organisms have the capacity to produce populations of infinite size, but environments and resources are finite. This fundamental tension has profound effects on the interactions between organisms.
  • Organisms both cooperate and compete in ecosystems. The interrelationships and interdependencies of these organisms may generate ecosystems that are stable for hundreds or thousands of years.
• NSES Life Science – Matter, Energy, and Organization in Living Systems
  • The distribution and abundance of organisms and populations in ecosystems are limited by the availability of matter and energy and the ability of the ecosystem to recycle materials.

Discussion: Setting the Stage

• What determines how large a population's size will be?

  Student answers will vary. Some factors: how much food there is, how large the starting population size is, birthrate, deathrate, types of food, etc.

• Why do organisms go extinct?

  Because they cannot survive in their environment anymore. (Students may provide many factors leading to extinction)

• Does the size of populations of organisms keep growing forever?

  No. At some point populations will reach a maximum size (carrying capacity) which depends on many factors, including food availability, water supply, environmental conditions, and available space.

Discussion: Formative Questions

• What do the factors: Initial-number, Grass-regrowth-rate, Gain-from-food, and Birthrate-% do to the model? How do they affect the population?

  This model is robust because students can see the complex factors that contribute to survival or extinction. A) Initial number: The starting population size can have profound effects on survival, especially if birthrates are lower or food supply is insufficient in nutritional value. B) Grass regrowth- A higher rate of regrowth will support a larger population. Ask students to look at the graph when the grass regrowth is set high. Initially, there will be a spike in population. Ranchers may have to consider removing a percentage of the population each year to allow the grass supply to remain stable.  C) Gain-from-food - how much energy sheep get from the grass they eat. The lower the number, the lower the population size which can be supported. This factor is especially important in environments where nutrient content of grass is lower. D) Birthrate-% - The ratio of births to the total population. The lower the number, the lower the population growth.

Extinction

• Was it easier for the population to go extinct by changing Initial-number or grass-regrowth rate? why?

  If the grass regrowth is high, populations <200 sheep can remain stable. Even at the highest population level (250 sheep) the population will decrease fairly rapidly, but will stabilize at about 150. Clearly, with the parameters of this model, the population could go extinct more easily by lowering the grass regrowth rate. 

• What factor do you think leads to extinction the fastest? At what level?

  (Answers will vary) After using the model, students should see that low birthrate can have a catastrophic effect on populations (below 50% will always result in eventual extinction). If the birthrate is under 60%, the environmental conditions had better be nearly perfect. Expect students to be surprised at the effects of gain-from-food on the sheep populations (this is a value that represents nutritional value of the food). Even if all other conditions are perfect, if the gain-from-food falls below 1.5, populations will die out. 

Population Balance

• As the amount of sheep decreases, what happens to the amount of grass?

  It increases.

• Describe the relationship between grass and sheep? How do they affect each other?

  As sheep numbers increase, this means more grass is eaten, resulting in a decline in the amount of grass. Still, the dynamics of overgrazing are a bit more complicated. If the land is overgrazed, plants become weakened and have reduced root length, and the pasture sod weakens. The reduced root length makes the plant more susceptible to death during dry weather and allows weeds to germinate and flourish. When the grassland weakens, sheep will die out because there isn't enough grass left to sustain the population. As sheep populations decline, grass populations will likely strengthen. This is a natural cycle. Ask students how ranchers can manipulate this dynamic to keep a stable sheep population (they can remove a certain number of sheep each year).  

• Which factor, at what level, leads to the smallest fluctuations?

  (Answers will vary, depending on the strength of the factor)

Population Explosion

• Which setting on the limited-grass switch is more realistic? On or off?

  On, because in real life grass regrows slowly, not instantly. This is a good time to discuss with students the benefits and limitations of models. A computer model will never be exactly like the phenomenon it's representing. We want students to evaluate a model to gauge whether its parameters are reasonable. In this case, the setting that provides instant grass regrowth is unreasonable. So why did the authors put this on/off switch in the model? (To show, computationally, what would happen to a population if its food supply were instantly available and unlimited. You would have an uncontrollable population explosion.)

• What is the reaper button supposed to simulate?

  It represents the removal of sheep from the population (could be due to death or to managed removal). 

• Keeping the settings at default, how high can the reaper rate be before the population goes extinct? How low before it grows out of control?

  (Answers will vary)

• Keeping the settings at default, how high can the birthrate be before the pop. goes extinct? How low before it grows out of control?

  Even if all environmental conditions are perfect, populations will go extinct if the birthrate is  below 50%. It might take many generations at a birthrate of 47%, but it's a statistical certainty. Populations can spiral out of control very quickly in this model, given the on/off switch for grass. If the food supply is unlimited with instantly replenished grass (switch turned to "Off") the population will explode at a very rapid rate. 

Discussion: Wrapping Up

Extinction

• What leads to the extinction of species?

  A variety of factors. Limited resources (food, shelter), birth rate, poor nutrient quality of food supply, starting population size.

Population Balance & Population Explosion

• Do populations sizes continue to increase forever?

  No, because at some point they will reach a carrying capacity. This depends on the amount of individuals an ecosystem can support. Teachers: This could be a good time to introduce human population growth. Students may have the erroneous belief that human population can sustain itself forever at its present rate of growth. Ask them how the models in this unit could relate to human population. How are the models unrelatable to human population growth? Obviously, we're not going to implement a "Reaper" system to stabilize human population growth. What do your students suggest might be done?

Additional Background

NOTE: Each of the activities utilizes concepts from previous activities so it is important the teacher is familiar with all of the background material, even if they’re not using all of the activities.

A large variety of species have gone extinct throughout the history of Earth. A variety of factors contribute to species going extinct as well. Climate change, meteors, and shifting plates has played a role in extinction.

Most textbooks categorize species with certain strategies of “r” or “K”, which refers to exponential or logistic growth respectively. R strategists produce many offspring, have short lives, and have low survivorship while K strategists have the opposite. A common misconception is that only K strategists work within the confines of a carrying capacity (K), however all organisms are susceptible to restrictions on population size. Sheep are categorized as K-strategists.

Analysis

Extinction

1. What would each of the factors represent in an actual population of a sheep? a. Initial-number; b. Grass-regrowth-rate; c. Gain-from-food; d. Birthrate-%

   a. Starting # of sheep in the population. b. how fast the grass grows. c. how much energy sheep get from the grass. d. ratio of births to the total population

2. Which factors seem to lead most strongly to extinction?

   All contribute but gain-from-food factors most strongly.

3. Do you think extinction is a chance event or a certainty for a given situation?

   Responses can vary. Some environmental factors are predictable and lend themselves to statistical models. But some events can be random, such as a volcanic eruption, large-scale wildfire, or tsunami. Ask students to share insights gained from the models. Example responses might be, "If birthrates fall below 50% and remain that way, extinction is a certainty." "If the energy value of the food supply is too low, the population will have to migrate or face extinction."  Ask students if there are times when a chance event can cause extinction. (Example: chance introduction of an invasive species can kill out a population.)

4. Do you think we should care about species going extinct? Why or why not?

   Most will say yes. A healthy population is measured by biodiversity (# of species in a pop) so if one goes extinct it means there may be something unhealthy. However, extinction can also just be a product of natural selection. Human interference is a reason species are extinct and why some have not gone extinct.

Population Balance

1. How does the initial number of sheep affect the carrying capacity, that is, the long-term average stable population?

   Depending on whether the initial # is above or below the carrying capacity (K) will affect how long it takes for it to stabilize. (Example: If K is 60 sheep and the initial sheep # is 60 it won't fluctuate too much or suffer large decrease or increase in numbers.)

2. How does the grass regrowth rate affect the carrying capacity?

   Lower grass retrowth rate lowers the carrying capacity.

3. Does the gain from food affect the carrying capacity? Why?

   Higher gain from good raises the carrying capacity.

4. How does the birthrate affect the carrying capacity?

   Yes. If the birthrate increases, more individuals are introduced into the population, using more resources. This will typically lower the carrying capacity. The reverse is also true. 

Population Explosion

1. Many animals - for example, mosquitoes and fish - may lay thousands of eggs in one year. Why might this be a good survival strategy? Why don't they take over the world?

   Because it means there's a chance some offspring will survive and it's not an issue if a large number don't survive. They can't take over because they still are affected by carrying capacity, meaning if there are too many fish then they will compete with each other and limit their own resources.

2. For some animals, as the population gets denser, the reproduction rate goes down. For example, mice reduce the size of their litter if the food supply is limited. Why would this be useful?

   Because if there was limited food and a mouse still had a large litter, there wouldn't be enough food for the offspring. Either some mice would get enough food and some will starve or all the mice will not get enough food. Also, it's evolutionarily disadvantageous to use a lot of energy making a lot of babies if they won't survive.

Further Investigation

After these activities it would be a great opportunity to model population growth by using a mathematical model, such as modeling bell pepper growth based off how many seeds they produce and making an excel spreadsheet. Students can also begin learning about r and k strategists who have different “survival” strategies.