A herd of reindeer grazing on a wide yellow tundra plain in Alaska, with bare mountains behind

ENVIRONMENTAL SCIENCE · CHAPTER 5

Population Limits Lab

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

mrsajdak.com/populations

Photo: NOAA Central Library

BEFORE YOU START

Nothing grows forever

A small forest pond covered edge to edge with bright green duckweed

Fast at first

One plant makes two. Two make four. A pond can turn green in a few weeks.

An old black and white photo of a long line of reindeer crossing flat snowy ground on St. Paul Island

Then it stops

A herd grows fast too. But an island has only so much food.

The four cases

1. Duckweed on a pond

2. Reindeer on an island

3. Trout in tanks

4. Bears, robins and perch

A limiting factor is anything that keeps a population from growing bigger.

Every population can grow fast. So what holds each one back?

Photos: Paavo01, CC BY 4.0 · U.S. National Archives

BEFORE YOU START

Get set up

Open mrsajdak.com/populations on your device.

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

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

Start on Tab 1. Do not skip ahead.

The top of the lab page, showing the title and four tabs: Duckweed pond, Reindeer island, Trout tanks, Life strategies

Tab in the lab	Worksheet	Questions
1. Duckweed pond	Part 1	1 to 4
2. Reindeer island	Part 2	5 to 9
3. Trout tanks	Part 3	10 to 13
4. Life strategies	Part 4 and Wrap it up	14 to 16

Screenshot: mrsajdak.com/populations

A flooded forest where the water is covered with a solid green carpet of duckweed around the tree trunks

TAB 1 · QUESTIONS 1 TO 4

Part 1. Duckweed pond

Duckweed is a tiny floating plant. Each plant splits into a new one about every 2 days.

You will grow it on a pond four ways. Find out what stops it.

Photo: U.S. Geological Survey

PART 1 · TAB 1

Set the pond, then run it

Find the Settings box. Pick a Pond size. Pick the Nutrients in the water.

Check or uncheck Limits on.

Press Skip to the end. Or press Run to watch all 30 days.

Find Your runs. It keeps the numbers for every setting you try.

The Settings box in the lab: buttons for pond size and nutrients, check boxes for Limits on and Early frost on day 18, then Run, Skip to the end and Speed

A small round pond in the lab on day 14, partly covered with green dots of duckweed

Each dot on the pond stands for 5 plants.

The box in the corner shows the day, the plants and the density.

Change a setting and the run starts over.

Screenshots: mrsajdak.com/populations, Tab 1

PART 1 · QUESTION 1

Your four runs

Run	Limits	Pond size	Nutrients
1	OFF	200 m²	Normal
2	on	200 m²	Normal
3	on	400 m²	Normal
4	on	200 m²	Fertilizer runoff

The Your runs box in the lab: one row showing the settings, the plants on day 10 and the plants on day 30

Copy Day 10 and Day 30 for each run.

J or S? Look at the line on the graph. A J-curve keeps shooting up. An S-curve climbs, then levels off.

Screenshot: mrsajdak.com/populations, Tab 1

PART 1 · QUESTION 2

Where does it level off?

Run limits on, 200 m², Normal again. Look at the graph.

Find the dotted red line. The plants level off near it. Read its number on the left side.

The graph names the dotted line. That name is the second blank.

For population density, divide the plants on day 30 by the size of the pond.

The lab graph of duckweed plants over 30 days: the line climbs slowly, then fast, then flattens under a dotted red line

Example

1,000 ÷ 50 = 20

A pond with 1,000 plants on 50 m² has 20 plants per m². Not your numbers.

Screenshot: mrsajdak.com/populations, Tab 1

PART 1 · QUESTIONS 3 AND 4

What held the duckweed back?

QUESTION 3

Two limiting factors

A bigger pond raised the top number. So did fertilizer runoff.

What did each one give the plants more of? Name both.

QUESTION 4

6 in 10

An early frost

Check Early frost on day 18. Run it. Watch the line on day 18.

Frost kills 6 of every 10 plants. Does it matter how crowded the pond is?

Two kinds of limits

Density-dependent: it hits harder when the population is crowded.

Density-independent: it hits the same, crowded or not.

Pick one for frost. Say how you know.

Brown treeless tundra sloping down to a grey sea, with snow patches on low hills, on St. Matthew Island

TAB 2 · QUESTIONS 5 TO 9

Part 2. Reindeer island

This is St. Matthew Island, Alaska. In 1944 the Coast Guard let 29 reindeer loose here.

The island had thick lichen to eat and no wolves. Watch what the herd did next.

Photo: Laura McDuffie, U.S. Geological Survey

PART 2 · QUESTION 5

Run the island. Stop at three years.

Go to Tab 2. Keep the island on St. Matthew, 1944.

Press Run. Watch the box in the corner. It shows the year and the reindeer.

Press Pause when the year is 1957. Write the reindeer in the Model row.

Press Keep going. Pause again at 1963 and at 1966.

Missed a year? Let it finish. Then press Run again.

The Settings box in the lab: buttons for St. Matthew, 1944 and St. Paul, 1911, check boxes for Wolves on the island and The hard winter of 1963 to 1964, then Run, Skip to the end and Speed

St. Matthew Island in the lab in 1950: a long pale island with a few dark dots for reindeer, and a box showing the year, reindeer, density and lichen left

Each dot stands for 10 reindeer.

The black dots on the graph are the real counts.

Keep Speed on Normal. Fast is hard to pause.

Screenshots: mrsajdak.com/populations, Tab 2

PART 2 · QUESTION 6

How far past the limit?

Find Your runs. Read the number under Highest (year). That is the highest count.

Divide it by 128 square miles. That is the density.

Look under the graph. The orange dotted line is the carrying capacity. Read its number.

Divide the highest count by the carrying capacity.

The lab graph for St. Paul Island: the reindeer line climbs to a peak and crashes, the green lichen line falls, and an orange dotted line near the bottom marks the carrying capacity

This graph is the other island, St. Paul. Yours will look different.

Example

3,000 ÷ 200 = 15

A herd of 3,000 on land that can feed 200 is 15 times too big. Not your numbers.

Screenshot: mrsajdak.com/populations, Tab 2

PART 2 · QUESTIONS 7 TO 9

Find the real limit

QUESTION 7

Lichen

The food

Run St. Matthew again. Watch Lichen left in the corner box.

What happened to the lichen before the crash? Read the note under the picture. What did the island not have?

QUESTION 8

Winter

Take the winter away

Uncheck The hard winter of 1963 to 1964. Run it. Pause at 1966.

Did the herd still crash? Was the real limit the winter or the food?

QUESTION 9

Wolves

Add a predator

Check the hard winter again. Check Wolves on the island. Run it.

What shape is the reindeer line now? What happens to the lichen? Why?

A brook trout with pale spots and red fins swimming over round stones in a dark stream

TAB 3 · QUESTIONS 10 TO 13

Part 3. Trout tanks

The brook trout is New York’s state fish.

pH tells how acid water is. Low pH is acid. You will test 13 tanks. Only the pH is different.

Photo: Eric Engbretson, U.S. Fish and Wildlife Service

PART 3 · QUESTION 10

Run the tanks for 30 days

Go to Tab 3. There are 13 tanks. Each one starts with 20 trout.

Press Run. Or press Skip to the end.

Wait for Day 30 of 30 in the corner box.

Read the number above each tank. Or use the table in Your runs.

Write how many are alive under each pH.

Thirteen fish tanks in the lab on day 8, in two rows, each labelled with its pH and how many trout are alive

This picture is day 8, not day 30. Gray fish at the top have died.

Screenshot: mrsajdak.com/populations, Tab 3

PART 3 · QUESTION 11

The range of tolerance

Optimum range

The pH where the trout do best.

Zone of stress

Some trout live. Some die. The pH is a little too low or too high.

No survivors

The pH is much too low or too high. No trout live.

Check Show the zones on the graph. Read the pH numbers under each zone. Fill in the blanks.

The Settings box in the lab for the trout tanks, with a check box that says Show the zones on the graph

The lab bar graph of trout alive in each tank on day 8, one bar for each pH from 4.0 to 10.0

The zones show up as color bands behind the bars.

Screenshots: mrsajdak.com/populations, Tab 3

PART 3 · QUESTIONS 12 AND 13

One cause, and a real lake

A long narrow mountain lake between a steep rock cliff and a forested slope in the Adirondacks, with clouds reflected in the water

QUESTION 12

Why is this good evidence?

Read the note under the tanks. List what was the same in every tank.

How many things were different? Why does that matter?

QUESTION 13

Acid rain in the Adirondacks

Acid rain has pushed some Adirondack lakes down to about pH 4.8.

Find the two tanks closest to 4.8. Use their numbers to predict what happens to the trout.

Photo: G. Edward Johnson, CC BY 4.0

A black bear mother standing in tall grass between her two cubs, one black and one cinnamon brown

TAB 4 · QUESTIONS 14 AND 15

Part 4. Life strategies

Some animals have a few young and take care of them. Some have thousands and leave.

You will follow 1,000 newborns of three species. Then you will see which one bounces back fastest.

Photo: Neal Herbert, National Park Service

PART 4 · TAB 4

Three ways to raise young

A black bear mother standing in a grassy field with two small cubs beside her

Black bears

2 or 3 cubs, every other year. Cubs stay with mom about 17 months.

An American robin on a lawn pulling a worm out of the grass

American robins

6 to 12 eggs a year. Chicks are fed for about a month.

Yellow perch

10,000 to 40,000 eggs a year. No care from parents.

r-selected: many young, little care. K-selected: few young, lots of care. Which is which?

Photos: U.S. Fish and Wildlife Service

PART 4 · QUESTION 14

Follow 1,000 newborns

Go to Tab 4. Under What to watch, pick Survivorship. Press Skip to the end.

In Your runs, copy Alive at 10% and at 50% for each species.

Pick Recovery. Press Skip to the end. Copy the years to get back to 90%.

Look at the three lines on the Survivorship graph. Name each curve type.

Three boxes of 1,000 dots in the lab, one each for black bears, American robins and yellow perch, part way through a survivorship run; coloured dots are alive and gray dots have died

Late loss	Most live to old age.
Constant loss	About the same share die at every age.
Early loss	Most die very young. A few live long.

Screenshot: mrsajdak.com/populations, Tab 4

PART 4 · QUESTION 15

Who is most at risk?

The question

A disease kills most of the animals in all three populations in one year. Which species is most likely to go extinct?

Your numbers

Look at the recovery column in your Question 14 table. Compare the slowest species with the fastest.

One life fact

Find a fact in the Life facts box that explains why that species is so slow to come back.

The Life facts box in the lab: a table for black bears, American robins and yellow perch with rows for young per year, first breeds at, care from parents and longest life

Screenshot: mrsajdak.com/populations, Tab 4

WRAP IT UP · QUESTION 16

Can any population grow forever?

The claim

“No population can grow forever.”

Evidence 1

Pick one tab. Give a number that shows the population stopped growing, or crashed.

Evidence 2

Pick a different tab. Give a number from that one too.

Reasoning

What do the limits have in common? Think about what every habitat has only so much of.

A long line of caribou walking across flat white snow under a grey sky

A caribou herd in Alaska. Caribou and reindeer are the same species.

Photo: U.S. Fish and Wildlife Service, Alaska

A bare green and brown island with cliffs dropping to the sea, and a second island faint in the mist behind it

LAST CHECK

Before you hand it in

Name, date and period are on top.

All four tables are full.

Questions 2, 6 and 11 have every blank filled in.

Question 16 uses numbers from two different tabs.

Photo: Anne Morkill, U.S. Fish and Wildlife Service