Grade by Grade
B. Life Systems · Habitats and Communities

Why Every Habitat Has Limits

B2.7 A2.1 A3.2 A1.5About 40 minutes
Learning goals
  • I can explain why a habitat can only support a certain number of plants and animals.
  • I can name the limits in a habitat: food, water, shelter and space.
  • I can describe what happens when a population grows too big for its habitat.
  • I can follow and change a program that models a population, and read its graph.

Words to know

limit
The most that something can hold or support.
carrying capacity
The largest population of a species that a habitat can support over time.
resources
The things organisms use to survive, like food, water, shelter and space.
overpopulation
When there are more organisms than the habitat's resources can support.
model
A simpler version of something real that helps us understand it, like a drawing, a game or a computer program.

The crowded bird feeder

In winter you fill a bird feeder with seeds. At first, 3 chickadees visit. Soon there are 10 chickadees, 4 blue jays and 6 sparrows. By noon the feeder is empty. The birds that came late get nothing.

The feeder has a limit: it can only feed so many birds. Every habitat is like that feeder.

Five black-capped chickadees crowd around a wooden bird feeder full of seeds in a snowy backyard.
The feeder holds only so many seeds. When too many birds come, the food runs out, just like resources in any habitat. AI-generated image

Why habitats have limits

Every habitat has a certain amount of resources:

  • Food: a meadow grows only so much clover; a pond has only so many insects.
  • Water: a stream or pond holds only so much water, and it may shrink in a dry summer.
  • Shelter: there are only so many hollow trees, burrows and hiding places.
  • Space: each animal needs room to find food, raise young and stay away from rivals. Plants need room for their roots and leaves to get water, nutrients and light.

Because resources are limited, a habitat can only support a certain number of each species. Scientists call this number the habitat's carrying capacity.

What happens when a population gets too big?

If a population grows bigger than the habitat can support (overpopulation):

  • animals compete harder for food, water and shelter,
  • some go hungry, become weak or get sick, and diseases spread more easily in crowded groups,
  • some move away to find a new habitat,
  • fewer babies survive.

So the population goes back down toward what the habitat can support. Predators also help keep populations from growing too large.

Worked example

Let's think about deer in winter

In a snowy winter, many white-tailed deer gather in a sheltered cedar forest (called a deer yard). The forest has enough twigs to feed 100 deer for the winter, but 150 deer arrive. What will happen?

  1. Compare: 150 deer, but food for only 100. That is 50 deer too many.
  2. The deer compete for twigs. They eat every branch they can reach, damaging the young trees.
  3. Some deer become weak from hunger. Some may move to other areas; some may not survive the winter.
  4. By spring the herd is smaller, closer to what the forest can feed. The damaged cedars will feed fewer deer next winter until they grow back.

A habitat cannot support more animals than its resources allow. Too many deer can also harm the habitat for the future.

Food for100 deerDeer thatarrive: 15050 deer too manyEach picture = 10 deer (or food for 10 deer).
The deer yard has twigs for 100 deer, but 150 arrive. The 5 extra pictures in the dashed box are the 50 deer with no food.

Let's model it with code

A model is a simpler version of something real. Here is a program that models a rabbit population in a meadow. The meadow has enough food for 40 rabbits. Each year, the number of rabbits doubles (if there is enough food). The program makes two kinds of output: a bar graph and a message.

set rabbits to 4
set limit to 40
draw a bar for year 0
repeat 5 times
set rabbits to rabbits × 2
if rabbits > limit then
set rabbits to limit
say "Not enough food! Some rabbits must leave or will not survive."
draw a bar for this year

A population model. Year by year: 4, 8, 16, 32, then the limit of 40.

4Y08Y116Y232Y340Y440Y5limit 40year
The program's bar graph. The rabbits double each year until they reach the meadow's limit of 40 (the dashed line). Y0 means year 0, Y1 means year 1, and so on.
Worked example

Let's trace the program

Follow the code year by year. When does the message appear?

  1. Year 0: rabbits = 4.
  2. Year 1: 4 × 2 = 8. Is 8 > 40? No.
  3. Year 2: 8 × 2 = 16. No.
  4. Year 3: 16 × 2 = 32. No.
  5. Year 4: 32 × 2 = 64. Is 64 > 40? Yes! Set rabbits to 40 and show the message.
  6. Year 5: 40 × 2 = 80. Yes, too many again. Set to 40 and show the message.

The message appears in year 4 and year 5. The population cannot grow past the meadow's limit.

A model is simpler than real life

Real rabbit populations do not double neatly. Foxes, owls, weather and disease all change the numbers. A model leaves out details so we can see one big idea clearly: habitats have limits.

Just for fun

What if a bird feeder had a sign? "Maximum 10 chickadees. Blue jays, please take a number!" Every habitat has a limit, even a feeder.

Hands-on activity

Musical habitats game

You need: 4 or more players (family or friends), sheets of paper to be "habitat squares", music.

  1. Spread out 5 sheets of paper. Each sheet is a patch of habitat with enough food, water and shelter for one animal.
  2. Play music. Players walk around like animals. When the music stops, each player must stand on a sheet.
  3. Players without a sheet must "move away" (sit out) this round. Record how many animals found a habitat.
  4. Change the habitat: remove 2 sheets (a new parking lot was built). Play again. Then add 3 sheets (a meadow was restored). Play again.
  5. Make a table: round, number of habitat sheets, number of animals that survived.
  6. Talk about it: "What decided how many animals could stay?"

Coding extension: In a block-coding app, build the rabbit model. Change the starting number or the limit, and see how the graph changes.

Grown-up note: Clear the floor so no one trips or slips on paper.

Your turn

1

Why does every habitat have a limit to the number of organisms it can support?

2

Name the four main resources that limit a habitat.

3

What is carrying capacity?

4

What can happen to animals when a population gets too big?

5

Why do plants also have limits?

6

In the rabbit program, what is the limit?

7

In the program, what does the line "set rabbits to rabbits × 2" model?

More practice: warm-up

1

Is space a resource?

2

A pond has food for 20 ducks. 30 ducks arrive. Is the pond over its limit?

3

What is a model?

4

In the bar graph, how many rabbits were there in year 2?

5

What do predators help do to prey populations?

More practice: on your own

1

The deer yard had food for 100 deer, but 150 came. How many deer were over the limit? What fraction of the deer could the yard feed?

2

Why might a hollow tree be a limit for owls or woodpeckers that nest in tree holes?

3

Change the program: start with 5 rabbits instead of 4. Write the numbers for years 0 to 5.

4

In the musical habitats game, what happened when sheets were removed? What does it model?

5

Too many deer can harm a forest. Explain how.

Challenge

1

Change the program so the meadow can support 60 rabbits, starting with 4. In which year does the message first appear?

2

The rabbit model is simpler than real life. Name two things it leaves out.

3

Wolves were removed from an area long ago, and the deer population grew very large. Explain what likely happened to the plants, and then to the deer.

Check your understanding

Quiz

1.

Why can't a habitat support an unlimited number of animals?

2.

The largest population a habitat can support over time is called its...

3.

When a population grows too large for its habitat...

4.

In the rabbit program, the population stopped at 40 because...

5.

A meadow is paved for a parking lot. What happens to the number of animals it can support?

6.

Which is not a resource that limits a habitat?

For parents and teachers

This lesson explains why habitats have limits (carrying capacity) and uses a simple coding model and bar graph to show a population levelling off.

Common difficulties:

  • Thinking populations can grow forever. Use the bird feeder or lunch-table example: there is only so much to share.
  • Reading the code line by line. Trace it with a pencil, writing the value of "rabbits" after each line.

Note: The deer and rabbit numbers in this lesson are made-up examples for learning, not real counts.

At home: Look for limits around you: how many people fit at the dinner table? How many birds fit at a feeder?