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6.1: Cycling in Biological Communities

Difficulty Level: At Grade Created by: CK-12
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How do resources cycle in a forested watershed?

You've seen that biological communities tend to reuse the resources that they have. Natural resources recycle constantly. This section will help you to learn about biological recycling.

“Life on Earth represents a continual process of birth and death, decay and rebirth as the building materials are used over and over again by all living things.”

Steve Van Matre

The Earth Speaks

Nutrient cycles in a biological community naturally recycle and reuse materials. It's easy to say that, but it makes a lot more sense when you can actually observe nutrient cycles in nature. A group of ecologists in New Hampshire demonstrated nutrient cycles in an experiment that will help you observe the natural recycling and reuse of materials.

Figure 5.1 A rain gauge captures both the water and the nutrients that are found in rain.

These ecologists wanted to find out how nutrients in a forest were recycled. They wanted to try to answer three questions: What was being cycled? How much was being recycled? And how fast did the materials complete the cycle? They tackled these questions in a very clever way. They realized they wouldn't be able to study all parts of every nutrient cycle. They would have to study the whole Earth to do that. So they decided to study one specific area. They selected one stream called Hubbard Brook. Then their plan was to study everything that was happening in the part of the forest that surrounded Hubbard Brook.

The ecologists realized they had to do some planning before they began their study. They knew they would have to be able to measure all the materials, such as the water and nutrients, brought into their piece of the forest by raindrops. So they set up rain gauges like the one illustrated in Figure 5.1 to catch the precipitation coming in. They also wanted to measure all of the nutrients leaving the forest. They solved this problem by defining the edges of the study area as the watershed of Hubbard Brook.

Figure 5.2 The top figure shows a sample of a small watershed drained by a creek. The shaded area on the map of the United States in the bottom figure shows the huge watershed drained by the Mississippi River.

What is a watershed? A watershed is the area of land drained by a stream or river. Watersheds can be relatively small or very large. For example, the watershed of a creek is the land drained by the creek and might include only part of one or two hillsides. On the other hand, the watershed of the Mississippi River is all of the area drained by the Mississippi and all of the streams that drain into the Mississippi. The Mississippi watershed includes most of the middle of the United States. So you see that watersheds can be small or large. Compare the watersheds in Figure 5.2.

Did You Know?

A hectare is 10,000 square meters or 2.47 acres.

The watershed around Hubbard Brook was fairly small. It was only about 16 hectares or slightly more than 32 football fields. The ecologists chose this watershed to study for a good reason. They knew the only way most nutrients could leave the forest was by flowing out with the water of Hubbard Brook. They also knew all of the water of Hubbard Brook flowed past one point. So all they had to do was measure the concentration of nutrients at that one place. They built a small dam at that place where they could collect samples of water easily. The drawing in Figure 5.3 shows how the dam was shaped to make water collection easy for the ecologists.

Figure 5.3 All of the water that drains from the Hubbard Brook watershed flows past a dam like this one. The V-shaped notch makes it easier for scientists to take samples and measure the rate of water flow.

Now that you have a good idea of what makes up a watershed, use a map and describe the watershed closest to your community. Include the name of the river and/or creeks that are a part of your local watershed.

So what did the ecologists find out? There were some surprises and some puzzles. First, they observed that a lot of rainwater fell on the forest. The surprise was that not very much of the water left the watershed at the bottom of the hill where they built the dam. They made several conclusions from their observations and measurements. They concluded that three things happened to the water coming into the watershed: The trees used a lot of the rainwater to grow and photosynthesize. Some rainwater evaporated, and some rainwater soaked into the ground.

The ecologists also knew that the trees in the forest needed more than rain to survive and grow. The trees needed other nutrients, including carbon dioxide, nitrogen, potassium, and calcium. So the ecologists had another question to answer: Where were the trees getting these nutrients?

The trees could get the required carbon dioxide from the atmosphere. But the other nutrients the trees needed had to come from within the watershed of Hubbard Brook.

The next thing the ecologists did was pretty clever. They reasoned that, if the trees were recycling these nutrients, they could stop the recycling by cutting down the trees. This may sound drastic as a part of an experiment. But it was a good way to find out what happens to a functioning forest when it is clear-cut by foresters.

Figure 5.4 A clear-cut watershed

What do you think happened when the ecologists clear-cut the trees? The flow of water running by the dam at the bottom of Hubbard Brook increased by 40 percent! This change meant that all of the extra water was used by the trees before they were cut down. The ecologists also noticed that the flow of nutrients past the dam went up dramatically. You can see these results in Figure 5.5 on page 30. The arrow in each chart points to the time when the scientists removed all the trees. You can see what happened to the nitrogen flow. The nitrogen (in the form of nitrates) that flowed past the dam increased more than 5,000 percent! It's hard to imagine a 5,000 percent increase of anything, isn't it? You can see that the flows of calcium and potassium increased, too.

The ecologists wanted to make sure they could support their results. So they used a control watershed where the trees were not clear-cut. The white, dotted line in Figure 5.5 represents the flow of a nearby watershed that the ecologists used as their control. The scientists compared the two sites. They concluded that the changes in flow occurred because of the tree cutting and not because of something else, such as a period of extra-heavy rain.

The differences observed in the experiment demonstrated that the forest had been recycling the nutrients. Younger trees growing in the area absorbed the nutrients from trees that died and decayed before the area was clear-cut. Tree roots absorbed nutrients from decaying leaves and wood on the ground. The next activity will help you to find out what went into the ecologists' study of the Hubbard Brook Watershed. You are actually going to build a watershed model yourself.

Why doesn't the concentration of nutrients flowing past the dam jump immediately after the trees have been cut down?

How Do Scientists Know? The Hubbard Brook Watershed study is an example of how scientists use the scientific method to find answers to their questions about how the world works. How do they do it? Look at the graphs of nutrients found in Hubbard Brook before and after deforestation and discuss the following questions:

  • What question did the scientists ask?
  • What methods did they use?
  • What data did they collect?
  • What conclusions did they reach?
  • Do you think these conclusions are valid?
  • What are some questions that could follow up on this research?

Figure 5.5 The solid, pink line up to the arrow shows the concentration of nutrients flowing past the dam at Hubbard Brook before the trees were clear-cut. The dark arrow indicates when the trees were clear-cut. The solid, pink line shows the concentration of nutrients flowing after the clear-cut. The dotted, pink line shows the concentration of nutrients from the control watershed where the trees were not clear-cut.

What Do You Think?

Look at Figure 5.5. The solid, white line represents the concentration of nutrients flowing past the dam at the bottom of the watershed clear-cut by the scientists. The dotted, white line is the flow past a dam at the bottom of a watershed that was not clear-cut. Explain why you think the scientists measured the nutrients at the “control” watershed.

Activity 5-1: Go with the Flow: Hubbard Brook Watershed


What role do trees play in the cycling of water and nutrients within a community?

Nutrients dissolved in water don't just drain through a forest. They enter the soil, pass through trees, and pass through the forest as runoff, too. In this activity you investigate the effects of clear-cutting a forested area and observe the impact that clear-cutting has on the cycling of nutrients in the surrounding area.


  • 8 to 10 Half-liter milk cartons, rinsed and opened
  • One 1-liter pitcher
  • Graduated cylinder
  • Water
  • Large tray with sides at least 4cm high
  • Stapler or tape
  • Large piece of sturdy cardboard-1 meter square
  • Resource 1
  • Resource 2
  • Activity Report


Step 1 Read about the Hubbard Brook watershed. You are going to construct a model of this watershed.

  • The large piece of cardboard represents Hubbard Hill.
  • The milk cartons represent the trees that cover the hill as a thick forest.
  • The bucket or large tray is the stream at the bottom of Hubbard Hill.
  • The pitcher of water represents a rainstorm that will shower Hubbard Hill.

Step 2 Open the milk cartons. Then attach the open milk cartons to the cardboard using staples or tape. The empty milk cartons must be completely open, clean, and not leaking. Place the cartons evenly over the cardboard. Attach them so that they are flush with the cardboard and so that no water will pass between the milk cartons and the cardboard.

Step 3 Place the bottom of the cardboard in the tray. Lean the top of the cardboard up against a wall.

Figure 5.6 Your watershed model should look like this.

Step 4 Fill one pitcher with water. Measure and record the amount of water in the full pitcher on Question 1 of the Activity Report.

Step 5 A rainstorm is approaching Hubbard Hill. You're about to see the effect of the rain on the hillside and trees. Standing over the cardboard, slowly pour all the water from the pitcher onto the cardboard, moving from side to side to cover the whole hillside with water. On Question 2 of the Activity Report, describe what you observe.

Step 6 Carefully remove the cardboard from the tray. Measure and record the amount of water that collected in the tray. Answer Questions 3, 4, and 5 on the Activity report.

Step 7 A local logging company has clear-cut Hubbard Hill. The workers have removed all of the trees from the hillside. How could you use your model to simulate the effects of deforestation on your hill? Design a simulation to observe the effects of deforestation on the hill. Explain your design in Question 6 of the Activity Report.

Step 8 Repeat steps 4 and 5 of the procedure. Use exactly the same amount of water. Record this amount in Question 7 of the Activity Report. Pour the water from the pitcher as you did in Step 5.

Step 9 Carefully remove the cardboard from the tray. Measure and record the amount of water that collected in the tray on Question 8 of the Activity Report. Then answer Question 9.

step 10 Analyze the graphs on Resource 2. These graphs were produced from data that was collected by the scientists who studied the flow of nitrogen in the Hubbard Brook watershed. In Questions 10 to 12 on the Activity Report describe what happened to the amount (or concentration) of each of the nutrients before and after the area was clear-cut.

Review Questions

  1. What is a watershed?
  2. What happens to most of the nutrients, such as water and nitrogen, in a forest that is undisturbed by humans?
  3. What happens to these nutrients if all of the trees in the forest are cut down?

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Date Created:
Feb 23, 2012
Last Modified:
Aug 06, 2016
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