Populations in Balance

Initiate a class discussion prior to beginning the activity.

Display image of forest fire.

Paraphrase:

Are ecological disturbances good or bad?

It depends.

The situation you read about on Guam is an example of a disturbance of an ecosystem. A disturbance is when something happens that messes up the normal way an ecosystem functions. We tend to think that disturbances are bad, and in the case of the snakes on Guam, it does indeed look like a difficult situation that is causing serious problems.

But in other cases, ecosystems depend on disturbances to maintain their health.

Today we’re looking disturbance of an ecosystem. A disturbance is when something happens that messes up the normal way an ecosystem functions. We tend to think that disturbances are bad, but in some cases, ecosystems depend on disturbances to maintain their health.

Trees are left blackened and largely dead after a fire. Animals have no food and homes. So fires are a bad thing, correct?

 

Are fires good or bad?

 

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In this section, students learn the definition of a disturbance and discover that not all disturbances are bad; some ecosystems, like redwood forests in California, depend on a disturbance to maintain their health. This is the case for redwood forests.

Show Slide #1 (an image of a redwood ecosystem, sequoia trees in Muir Woods).

Paraphrase:

Let's look at the case of fires in California.

In California, redwood trees don’t get killed by forest fires. In fact, the tiny cones on the tree (like pine cones) drop their seeds during fires because that’s the best time for the the seeds to sprout into new trees. Seeds need sunlight to shine all the way to the forest floor to have a chance of growing into trees.

Ask

Can you think of a disturbance that might both help and harm a forest ecosystem?

 

Listen for possible responses and record on the board:

A beaver fells a tree to build a dam, providing a habitat for other species.

Rain brings water that plants need, but too much can result in floods, washing away homes of people and animals, uprooting plants too.

Write the students’ responses on the board.

If a student mentions forest fires, proceed to Slide #2. Or, proceed to Slide #2 after students have contributed a few ideas to the list with no mention of fire.

Show Slides #2 and #3 (images of a redwood forest on fire. Images by the US Forest Service).

Say

Let’s consider forest fires. In California, redwood trees don’t get killed by forest fires. In fact, the entire ecosystem depends on fire.

 

Show Slide #4. Paraphrase:

Here’s a quick timeline of a fire. Try to complete the captions to explain how fire benefits the redwood tree and its ecosystem.

Possible responses:

• On a ridge about a kilometer away from a redwood grove, lightning strikes the dry grass and a fire is ignited.
• The fire spreads to the trees within the redwood grove.

• The tiny redwood cones react to the heat of the fire and release seeds that fall to the ground.

• Since the forest floor is cleared of brush, sunlight can shine through and allow for the seeds to sprout.

 

To summarize, paraphrase:

When a fire clears away all the dense underbrush, seeds have a much better chance of growing into trees. Only then does sunlight shine all the way to the forest floor.

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Display slide #1.

Paraphrase:

A disturbance is when something happens that messes up the normal way an ecosystem functions.

Sometimes disturbances to ecosystems are caused by humans, but other times they occur naturally.

Sometimes disturbances benefit ecosystems, but other times they are harmful.

Read these statements with a partner and see if you can think of an example to add in the final column.

Display slide #2.

Paraphrase:

After a disturbance, a recovery usually follows.

An ecosystem that goes through a disturbance is usually able to “fix itself” over time. This process is called a recovery.

Sometimes recoveries occur naturally.

Sometimes recoveries involve humans who are trying to help.

Read the chart below with a partner and see if you can think of an example to add in the final column.

 

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