A sea otter floating on its back in calm water

ENVIRONMENTAL SCIENCE · CHAPTER 5

Sea Otters and Kelp Lab

Follow along with your worksheet, one question at a time.

mrsajdak.com/otters

Photo: Mike Baird, CC BY 2.0

BEFORE YOU START

Who eats whom

A sea otter holding a purple sea urchin in its paws

Sea otter

Eats sea urchins.

eats

Purple sea urchins in a rocky tide pool

Sea urchin

Eats kelp.

eats

Tall kelp plants seen from below with sunlight coming through

Kelp

Makes its food from sunlight.

Otters never eat kelp. So why would kelp need otters?

Photos: matt knoth, CC BY 2.0 · James St. John, CC BY 2.0 · NOAA Photo Library

BEFORE YOU START

Two seafloors

A thick kelp forest reaching up toward the surface

Kelp forest

Thick kelp. Few urchins.

Many sea urchins on bare rock with almost no kelp left

Urchin barren

Bare rock. Urchins everywhere.

Same kind of rocky reef. Today you find out what makes the difference.

Photos: NOAA Photo Library, Channel Islands · Shaun Lee, CC BY 4.0, New Zealand

BEFORE YOU START

Get set up

1

Open mrsajdak.com/otters on your device.

2

Write your name, the date and your period on the worksheet.

3

Find the three tabs at the top of the lab. Each tab goes with one part of the worksheet.

4

Start on Tab 1. Do not skip ahead.

The top of the lab page, showing the title and three tabs: Survey the seafloor, Keystone coast, California comeback
Tab in the labWorksheetQuestions
1. Survey the seafloorPart 11 to 5
2. Keystone coastPart 26 to 9
3. California comebackPart 3 and Wrap it up10 to 16

Screenshot: mrsajdak.com/otters

A scuba diver on the seafloor counting inside a square frame called a quadrat

TAB 1 · QUESTIONS 1 TO 5

Part 1. Survey the seafloor

A quadrat is a square frame, 1 m on each side. Divers set it on the seafloor and count what is inside.

You will count 10 quadrats on each of two islands.

Photo: Olivier Dugornay, Ifremer, CC BY 4.0

PART 1 · TAB 1

Count a quadrat

1

Pick Island A. Start with quadrat 1.

2

Tap every purple urchin inside the white square.

3

Tap every kelp plant. Tap the dark knot in the middle. That is its holdfast.

4

Check the tally. Press Record this quadrat.

5

Count two quadrats by hand on each island. Then you may press Count the rest of this island for me.

The lab screen for Island A, quadrat 1: a map of the transect on the left and the quadrat on the right with four kelp plants circled

A white ring means you marked it. Tap again to undo. Rocks do not count.

Screenshot: mrsajdak.com/otters, Tab 1

PART 1 · QUESTION 1

Fill in your data table

1

Find the Your data box in the lab.

2

Copy all 40 counts onto your worksheet.

3

Add up each column. Write the total.

4

Find each average per m². Divide the total by 10.

5

Press Show averages to check your math.

The Your data box in the lab: a table with rows for quadrats 1 to 10, a total row and an average row, for urchins and kelp on Island A and Island B

Example

A column adds up to 40 urchins in 10 quadrats.

40 ÷ 10 = 4

That is 4 urchins per m².

These are not your numbers. Use your own totals.

Screenshot: mrsajdak.com/otters, Tab 1

PART 1 · QUESTIONS 2 TO 4

Use your averages

QUESTION 2

B ÷ A

How many times as many?

Take the urchin average for Island B. Divide it by the urchin average for Island A.

Show the division.

QUESTION 3

× 100

Scale it up

A patch of reef is 10 m × 10 m. That is 100 m².

Multiply each urchin average by 100. Do it for both islands.

QUESTION 4

10, not 1

Why so many quadrats?

Look down one column of your table. Are all 10 counts the same?

What if you had counted only one square?

PART 1 · QUESTION 5

Which island has otters?

Claim

Pick one island. Write A or B in the blank.

Evidence

Give two numbers from your table. Say what each number counts.

Reasoning

Explain why those numbers point to otters. Remember what otters eat.

Last, press your answer in Your claim. The lab tells you the real names of both islands. Write them on the last line.

A rocky green island coast with waves, in the Aleutian Islands of Alaska

Both islands are in the Aleutian chain of Alaska. Same cold sea, same depth, same rock.

Photo: U.S. Fish and Wildlife Service

Two killer whales leaping out of the sea

TAB 2 · QUESTIONS 6 TO 9

Part 2. Keystone coast

Now you run a model of a whole coast for 50 years.

You choose what happens to the otters. Fur hunters. Killer whales. Or otters swimming back in.

Photo: Robert Pittman, NOAA

PART 2 · TAB 2

Predict, then run

1

In On this coast, check the boxes for your run. Uncheck everything else.

2

In Predict first, pick what urchins and kelp will do by year 50.

3

Draw the same arrows on your worksheet. Up, down, or = for the same.

4

Press Run. Or press Skip to the end.

5

Find Your runs. Copy the three numbers at year 50.

The lab's right-hand panel on Tab 2: the On this coast checkboxes, the Predict first buttons and the Run button
The model coast at year 0: otters floating at the surface, tall kelp, and a few urchins on the seafloor

The picture shows the coast. The box in its corner shows the year and the numbers.

A wrong prediction is fine. Make it before you run.

Screenshots: mrsajdak.com/otters, Tab 2

PART 2 · QUESTION 6

Your four runs

RunRow on your worksheetBoxes to check in the lab
1Otters at the start (nothing else)Sea otters live here at the start
2Otters at the start + killer whalesSea otters live here at the start, and Killer whales start eating otters
3Otters at the start + fur huntersSea otters live here at the start, and Fur hunters kill otters
4No otters at the start + a few swim inA few otters swim in. Uncheck Sea otters live here at the start.

For every run: predict, run, then write otters per km², urchins per m² and kelp % at year 50.

PART 2 · QUESTIONS 7 TO 9

Explain what you saw

QUESTION 7

Fill the chain

Look at your killer whale run.

Write up or down in each box: otters, then urchins, then kelp.

QUESTION 8

Keystone species

A keystone species has a much bigger effect than its numbers suggest.

Pick two numbers from your table. Use one run with otters and one run without them.

QUESTION 9

Is the kelp back?

Look at your fur hunter run at year 50.

Compare the otter number with the kelp %. What does that tell you about how fast a kelp forest comes back?

A change at the top of a food chain that passes down, level by level, is called a trophic cascade.

A large group of southern sea otters floating together in blue water on the California coast

TAB 3 · QUESTIONS 10 TO 14

Part 3. California comeback

In 1938 about 50 sea otters were left in California.

Scientists have counted them every spring since 1982. You will test what holds them back today.

Photo: U.S. Fish and Wildlife Service, Pacific Southwest Region

PART 3 · QUESTION 10

Read the graph

Black dots: the real count each spring.

Red dashed line: the carrying capacity. It is the most otters this coast could hold.

Gold dashed line: the recovery goal.

For question 10

Find four counts: 1938, 1985, the highest year, and 2019.

Tip: press Run and watch the box on the map. It shows the real count for each year.

The lab's graph of California sea otters from 1938 to 2040: black dots for real counts, a red dashed line for carrying capacity and a gold dashed line for the recovery goal
The lab's map of the California coast in 1938, with a box showing the year and the number of otters

The map shows where the otters live. The yellow band grows as the model runs.

Screenshots: mrsajdak.com/otters, Tab 3 · Counts: U.S. Geological Survey

PART 3 · QUESTION 11

Make the model fit the dots

1

Keep the growth rate at 10%.

2

Leave Range can spread unchecked.

3

Check the boxes for your row: none, disease, shark bites, or both.

4

Press Skip to the end.

5

In Your runs, copy 2019, 2040 and Average miss.

6

Last row: try settings of your own.

The lab's Settings box on Tab 3: growth rate buttons with 10% picked, three checkboxes for shark bites, disease and range, and the Run and Skip to the end buttons
A great white shark swimming among smaller fish

Average miss is how far the model is from the real dots. Smaller is a better fit.

Screenshot: mrsajdak.com/otters, Tab 3 · Photo: Terry Goss, CC BY 2.5

PART 3 · QUESTIONS 12 TO 14

What holds the otters back?

QUESTION 12

Best fit

Smallest miss wins

Find the row with the smallest Average miss. Name its settings. Give the number as your proof.

QUESTION 13

6,500

The carrying capacity

Where does your best model level off? Write that number. Then name the cause and its effect.

QUESTION 14

3,090

The recovery goal

Does your best fit reach the goal and stay there by 2040? What would have to change?

WRAP IT UP · QUESTIONS 15 AND 16

Ask a question. Argue with numbers.

The lab's Your question box on Tab 3, with drop-down menus to pick a factor and something to measure, and a Put it together button

QUESTION 15

Build a question you could test

In Your question, pick a factor. Pick something to measure. Press Put it together, then Save my question.

Copy it to your worksheet. It must name one thing to change or compare and one thing to measure.

QUESTION 16

"The coast is full." Is it?

Agree or disagree with the classmate. Use at least two numbers from Part 3 as your evidence.

Screenshot: mrsajdak.com/otters, Tab 3

A sea otter in the water holding a purple sea urchin

LAST CHECK

Before you hand it in

Name, date and period are on top.

All three tables are full.

Every math answer shows the work.

Every claim uses numbers from your own tables.

Photo: matt knoth, CC BY 2.0