Why is Balance Important?

Ecosystems tend to have cycles that change around a state of equilibrium that can change when climate changes, when one or more new species appear as a result of immigration or when one or more species disappear.


Every living thing needs to be in balance with its environment. In northeastern Ohio, there are tens of thousands of deer. In some places the deer population has gotten too big, meaning that it’s out of balance. When there are too many deer, then they start eating peoples’ bushes, shrubs and grass. This results in unhealthy deer and unhappy people! In other words, the deer are out of balance with their environment; there isn’t enough food to support them all.

Equilibrium is about balance!

Equilibrium is when there is a balance between two or more things. In biology, when we talk about equilibrium, we’re talking about living things being spread out so that they can get their four basic needs met: food, water, shelter and space. If they are unable to get any of these basic needs met, then they are out of equilibrium with the environment. Even more importantly, if other organisms are unable to get any of their basic needs met because of the deer, then the deer are out of equilibrium with their environment. If you take too much or get too little from your environment, then you’re not in balance with your surroundings!

The example between the deer and peoples’ trees, shrubs and grass is a little silly: the deer don’t prevent us from getting our basic needs met. But there are areas of Ohio where deer are responsible for the destruction of crops (plants farmed for food) which does threaten that most basic of our needs, food.

Rabbits in Australia

The reason that there are so many deer in the first place is actually our society’s fault. Wolves and cougars hunt deer, but we hunted so many wolves and cougars that by 1900 the only ones around had fled north to Canada. Over the last hundred or so years, deer have been able to survive and reproduce because they have no natural predators (other than us). When species become extinct or if they emigrate (leave a particular area), it often leaves a gap in the food web. Other species can benefit, while others can be harmed in this change in equilibrium.

The opposite change can be just as destructive: when a new species is introduced, it can often be disastrous. Take, for example, rabbits in Australia. Rabbits were never native to Australia; they were introduced by the British who brought rabbits there in 1788. They were kept in cages until 1859, when just 24 rabbits were released into the wild. Because there were no natural predators in Australia, and the climate was perfect for rabbit reproduction, they reproduced so rapidly that they spread throughout the entire country. Within 10 years, hunters were able to kill 2 million rabbits in the wild, and it had no noticeable effect on the rabbit population!

The rabbits in Australia is an example of an invasive species (or non-native species). When invasive species enter a particular area, they usually change the equilibrium in their favor by pushing out one or more native species. When humans use invasive species to solve problems, it can often cause even bigger problems.

A mongoose eating a snake

Rats, not native to Hawaii, were accidentally introduced when they hitched a ride on boats headed to the islands. Since rats eat crops, some farmers had the idea to introduce an animal that could eat the rats: the mongoose. Well, mongooses do eat rats, but they also eat almost anything else; what’s more, the mongoose sleeps at night and rats sleep during the day. The mongooses didn’t end up hunting the rats, so the rat problem was not solved; it only created a new mongoose problem! Most of the Hawaiian native species (plant and animal) are now considered endangered species due to the destructiveness of the mongoose.

Species immigrate (move into a certain place) and emigrate quite often, but as we’ve learned, there are more than biotic factors in an ecosystem. Since the abiotic factors can often influence living things, climate changes often cause shifts in equilibrium. For the hundreds of millions of rabbits in Australia, it would be disastrous if the average temperature dropped significantly; suddenly, the rabbits would not be able to survive in such a cold climate, and Australia’s rabbit problem would disappear. However, Australia would end up with a lot of other problems!

1. Describe equilibrium in your own words.
2. Name three crops that you eat in some form.
3. What happens when non-native species are introduced into particular ecosystems?
Put it together
4. Choose an animal that doesn't live in East Cleveland. What do you think would happen if several of these animals were introduced to East Cleveland?
5. Give examples of both a biotic and an abiotic factor affecting the equilibrium of an ecosystem. Describe how changing the factor would affect the equilibrium.
Think about it
6. In Forest Hills, there are snakes and rabbits (among other animals). For each snake to survive a year, they need to eat 10 rabbits. However, since rabbits reproduce quickly, each rabbit produces three new rabbits every year. Snakes don't reproduce as quickly, each one only producing one new snake per year. You are going to find out what happens to the rabbit and snake populations of Forest Hills over the course of 10 years, starting in 2010.
To start off your populations, take the number of the month that you were born (January = 1, February = 2, March = 3, etc.) and multiply it by 5. Add to that number the day of the month that you were born. This number is the number of snakes in 2010. Enter it into the table below for Column A in 2010. Then multiply the number of snakes by 12 to get the number of rabbits in 2009. Enter it into the table below for Column B in 2010.
Due to the size of Forest Hills, if the number of snakes goes above 100, then there is not enough space in the ecosystem for the snakes. They die off in huge numbers, and there will only be 10 surviving snakes. If the number of rabbits goes over 10,000, then there is not enough food or space for the rabbits. They die off in huge numbers, so there will only be 100 surviving rabbits.
Year(A) # of Snakes(B) # of Rabbits(C) Surviving Snakes(D) Rabbits killed(E) Surviving Rabbits(F) New rabbits produced
Column C x 2 (from previous year) Column E + Column F (from previous year) This is equal to Column A unless A is over 100; then this is 10 Column C x 10 Column B - Column D (If it's over 10,000 this drops to 100) Column E x 10

Now create a graph for Columns A and E. Use one color for Column A, and another for Column E.
7. Define natural selection in your own words.

9. What are the five kingdoms of living things?
10. What can radiometric dating tell scientists?
Human Activities

From your own experiences or others’ experiences, identify one human activity that has a negative impact on the environment. Respond in complete sentences:

  1. What is it?
  2. What is the environmental impact of doing this?
  3. Why do people do it?
  4. What could people do in order to avoid making such a negative environmental impact?
  5. What can you do to help?
  6. Ask two others in the class for their responses, and write at least one paragraph each about what could be done to prevent this negative impact on the environment.
Equilibrium: Natural Selection
  1. Take one disaster card and one climate card. Based on the disaster card, eliminate half of your food web, but keep enough organisms so that there is still at least one on each level.
    1. For each organism eliminated, describe two characteristics of this organism that caused it to die
    2. For each organism that survived, describe two characteristics of the organism that allowed it to survive
  2. Using the climate card, describe for each of the surviving organisms:
    1. Which characteristics are favored for this new climate
    2. What these organisms might look like after ten generations in this new climate
Classroom Equilibrium
  1. Create your own food web of eight organisms; it should have at least two at each level (producer, primary consumer, secondary consumer).
  2. If you had to fit this habitat in a place the size of the science room, estimate the numbers of each organism (in your food web) that there could be. Be realistic: don’t include 10 lions, because they would never be able to live together and find food in such a small space!
  3. Now that you’ve established equilibrium(a balance in the ecosystem), what effect on the numbers of organisms would the following have:
    1. Twice as much rainfall
    2. Half as much sunlight
    3. Monthly fires
    4. Steady increase in air pollution (carbon dioxide)
Benefit & Harm
For #2 Purple Loose Strife Locusts Box Turtles
What Benefits
What is Harmed
For #3 and #4 (producer) (primary consumer) (secondary consumer)
What Benefits
What is Harmed
  1. Make a food web for the temperate deciduous forest. Include at least eight organisms.
  2. For each of the following organisms, pick one organism from your food web that benefits and one that is harmed and fill in the chart:
    1. Purple Loose Strife: An invasive plant that is not native to northeastern Ohio, but takes the place of other native species; not poisonous to herbivores
    2. Locusts: Beetle-like flying insects that swarm and damage crops
    3. Box Turtles: Small turtles that are omnivorous
  3. From your food web choose one producer, one primary consumer and one secondary consumer. Fill in the chart above with your choices.
  4. Assume that each organism that you just chose leaves your food web. You will fill in the chart with one organism that benefits and one that is harmed due to this organism leaving your food web.
  5. What kinds of organisms (producer, primary consumer, secondary consumer) benefit and which are harmed when the following are introduced:
    1. An invasive producer
    2. A carnivore
    3. An omnivore
  6. What kinds of organisms (producer, primary consumer, secondary consumer) benefit and which are harmed when the following leave the ecosystem:
    1. A producer
    2. An herbivore
    3. A carnivore
Modeling Predator-Prey Interactions

One type of interaction between organisms in an ecosystem is predation. A predator is an animal that captures and feeds on another animal, its prey. Predation affects the size of a prey population, and the size of a prey population affects the size of the predator population. In this activity, snakes will be the predators and mice will be the prey. You will record and interpret changes in the populations of both the predators and prey.


  • 200 1-inch red squares
  • 50 3-inch blue squares


In this activity, your desk represents an ecosystem. Use the red squares to represent mice. Use the blue squares to represent snakes—predators that feed on mice. You will work with a partner. Decide which one of you will be the predator and which one will be the prey. As you do the activity, record your results in the data table on the next page.

  1. Toss three red squares on your desk. In the data table, write 3 in the column labeled Number of Prey for Generation 1.
  2. Toss one blue square on your desk. Write 1 in the column labeled Number of Predators for Generation 1.
  3. If the blue square is touching one or more red squares, the snake has eaten one or more mice. Remove any red squares that the blue square is touching. Write the correct number of red squares remaining in the column labeled Prey Remaining and 1 in the column labeled Predators Remaining.
  4. If the blue square is not touching a red square, the snake did not eat. A predator that does not eat will die. Remove the blue square. Write 3 under Prey Remaining and 0 under Predators Remaining.
  5. The number of prey doubles in each generation. Multiply the Prey Remaining at the end of Generation 1 by 2. Enter that number under Number of Prey for Generation 2. Add enough red squares to your desk to equal that number. NOTE: When you add squares, toss them on the desk to keep the results random.
  6. In this activity, if a predator dies, a new predator takes its place. If your predator died, add a blue square to your desk. Write 1 under Number of Predators for Generation 2.
  7. The number of surviving predators doubles each generation. If your predator survived, add another blue square to your desk. Write 2 under Number of Predators for Generation 2.
  8. Repeat Steps 3 through 7 for a total of ten generations. NOTE: In this activity, if two predators capture the same prey, only one predator can survive.

Data Table: Results of Activity


Number of Prey

Number of Predators

Prey Remaining

Predators Remaining











Analyze and Conclude

  1. Why does it make sense to start the activity with a ratio of 3 prey to 1 predator? Hint: Think in terms of a food pyramid.
  2. At what point in the activity did the ratio of predators to prey change? What happens at that point?
  3. In well-established communities, the populations of a predator and its primary prey usually increase and decrease in predictable cycles. Why is that the case?
  4. Does the predator-prey relationship provide an advantage for the predator population, the prey population, or both populations? Explain your thinking.
  5. What would happen to a snake population if all of the mice died of disease?

Build Science Skills

Owls are predators of mice and snakes. How would you expect populations of mice and snakes to change when owls join a community?