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Carrying Capacity

Carrying capacity refers to the number of predators and prey that can live in an area. Too many predators and not enough prey leads to predators starving and dying because they can't find enough food. Too many prey and not enough predators leads to the spread of disease and depletion of resources for the prey species and others that live in their habitat.

In this game, students will examine the impacts of varying the numbers of predators and prey on an area's ability to sustain their populations, the carrying capacity.

When resources are scarce, only the healthiest and specialized individuals will manage to reproduce and make it to the next season. The sick, very old, and very young will be unable to get their portion of food before it is removed by the stronger individuals. This is survival of the fittest, the process that drives evolution.

Nature thrives on balance. Features that help prey from getting eaten are beneficial to prey, and features that help predators catch prey are beneficial to predators. Animals born without the special feature that makes their species successful will be less likely to survive long enough to make it to the next season.

This is what is meant by natural selection. Generally speaking, due to genetic variation in a population, some features will cause some animals to survive and reproduce more successfully than others.

A Case Study:

To better understand, let's look at a population of bugs that are of the same species. Some of the bugs are born with black bodies and others are born with grey bodies (just like some humans are born with blue eyes and others with brown eyes).

  • Grey bugs are more likely to get eaten by birds since they're not as well camouflaged against the dirt as the black ones. This means that they won't live long enough to reproduce and pass on their lighter-colour genes.
  • Black bugs, on the other hand, will reproduce and have more black bugs.
  • As a result, the next generation will have more dark-coloured bugs than grey bugs.
  • If the environmental conditions changed and the dirt became lighter, the grey bugs would have better camouflage and be less likely to be eaten than the black bugs.

This is the reason why genetic variation is important in a species. It helps prevent one type of species from being wiped out if the environmental conditions change. Those who survive will reproduce and pass on their advantageous feature to their young, survival of the fittest.

Objectives

  • Assess survival needs and interactions between animals and the environment.

Materials

  • Per Class or Group:
    timer
    whistle
    4 long ropes or several cones to delineate the playing area’s boundaries (Approx. 15m x 15m. the playing field size can be adjusted to accommodate the age and ability of the students)
    30 blue poker chips (one or two chips per student)
    30 green poker chips (one or two chips per student)
    2 hula hoops
    30 red bandanas (or other means of identifying the predators)
    30 green bandanas (or other means of identifying the prey)

Key Questions

  • Season 1: Normal Predator-Prey Conditions What happened to the predator population after Season 1? What happened to the prey population after Season 1? Was there enough food and water to go around?
  • Season 2: Too Much Predation What happened to the predator population after Season 2? What happened to the prey population after Season 2? Was there enough food and water to go around?
  • Season 3: Not Enough Predation What happened to the predator population after Season 3? What happened to the prey population after Season 3? Was there enough food and water to go around?
  • In nature, which prey are likely to get caught by predators? Which predators are likely to catch prey?
  • When the resources are low (less prey for predators or less food and water for prey), which ones will succeed in reproducing?
  • What animal species could this game represent?
  • What is meant by “carrying capacity” of an area?

What To Do

Setup

  1. Divide students into predators and prey. The relative numbers of each will depend on the conditions present (see Parts 1, 2, 3). Give the predators a red bandana to wear on their head. Give the prey a green bandana to wear on their head.
  2. The playing field will have 4 boundaries: water, food, predator den (at one end of game area), prey shelter (at opposite end of game area).
  3. In the middle of the water boundary, place blue poker chips within a hula hoop. These will represent portions of water for the prey.
  4. In the middle of the food boundary, place green poker chips within a hula hoop. These will represent portions of food for the prey.

Game
Each season represents different sets of predator-prey conditions. The numbers are based on a class of 25 students.

Season 1: Normal Predator-Prey Conditions

  1. The game starts with 6 predators and 19 prey.
  2. The prey start as a group behind the shelter line, and the predators start as a group behind the den line. The game begins at the blast of the teacher’s whistle. The predators and prey will have 2 minutes to reach their goals.

The Predator’s Goal 

  • The predator must catch a prey and bring it back to its den.
  • It only catches one prey per round (since it takes a while to eat and digest it).
  • If a predator does not manage to catch a prey, it dies of starvation and does not make it to the next season.

The Prey’s Goal

  • The prey must grab a portion of water (blue chip) and a portion of food (green chip) before returning to its shelter to reproduce, all without getting caught by a predator.
  • If there is still time, the prey can go back out and try to get more portions of water and food, but will this time grab the number of portions it needs for itself and its young (e.g. two portions of food).
  • If the prey does not get enough food/water, it dies of starvation or dehydration and does not make it over the winter to the next season.
  1. At the end of the round, the food and water chips are returned to their stations, indicating the renewal of the resource.

Season 2: Too Much Predation

  1. The game starts with 15 Predators and 10 Prey.
  2. Play the game as Season 1.

Season 3: Not Enough Predation

  1. The game starts with 2 Predators and 23 Prey.
  2. Play the game as Season 1 and 2.

Extensions

  • Add in other features: As a group, the prey will choose a feature and a simple way to act it out. This will be made clear to everyone, including the predators. Examples of features include camouflage, the use of horns or tusks, etc. When a prey uses its feature, the predator must freeze for 3 seconds giving the prey time to escape. Have the prey practice their feature and the predators practice freezing.
  • The Prey’s use of feature: If a prey sees a predator approaching it, it can use its feature to defend itself. The predator must freeze for 3 seconds, giving the prey time to escape. The prey cannot use its feature after the predator has tagged it. The prey cannot use its feature while drinking nor eating. This mimics how prey must often let their guard down momentarily while eating and drinking.
  • Add one or two prey that were born without the special feature that the others have:
  • How would it affect their chances of survival?
  • What special features would you want if this game was set in the woods? On a beach? In the snow?
  • At the beginning and end of each round, write down the number of predators and prey (including offspring). Graph the results when you return to class.
  • Consider carrying capacity in familiar terms by considering a schoolyard “ecosystem”. What happens if only 3 students are outside at recess (e.g. you can use whatever you want, there is little sharing/waiting, but you can’t play kickball)? What happens if two other schools showed up and there were 300 more students outside at recess?