Grade by Grade
B. Life Systems · Habitats and Communities

What Helps or Harms a Community?

B2.2 A1.2 A1.4 A1.5About 40 minutes
Learning goals
  • I can identify natural factors, like water, temperature and sunlight, that affect a community.
  • I can identify living factors, like food supply, predators, disease and invasive species.
  • I can identify human factors that affect a local community.
  • I can plan and carry out a fair test about a factor that affects small animals.

Words to know

factor
Something that has an effect on what happens.
drought
A long time with little or no rain.
invasive species
A species brought to a new place (often by people by accident) that spreads quickly and harms the species already living there.
disease
An illness that can spread through a population.
fair test
An experiment where you change only one thing and keep everything else the same.
variable
Something in an experiment that can change.

Remember from the last lesson

A community is all the species that live together and interact in one habitat. Today's question: what makes it easier or harder for a community to survive?

The dry summer

One summer it hardly rains for six weeks. The pond in the park shrinks to a muddy puddle.

  • The cattails at the edge dry out.
  • Tadpoles that have not yet turned into frogs have less and less water to live in.
  • Dragonflies lay their eggs in water, so there are fewer places to lay them.
  • Herons fly away to find fish somewhere else.

One factor, a drought, changed the whole community. Scientists sort factors like this into three groups.

A pond shrunk to a small puddle in the middle of dry, cracked mud, surrounded by dried-out cattails.
A drought is a non-living factor. With so little water left, the cattails dry out and frogs, dragonflies and herons lose their habitat. AI-generated image

1. Natural non-living factors

These are parts of the environment that are not alive:

  • Water: too little (drought) or too much (flooding).
  • Temperature: a very hot summer, a late spring frost, a very cold winter.
  • Sunlight: a shady forest floor has different plants than a sunny meadow.
  • Soil: sandy, clay or rich soil decides which plants can grow, and that decides which animals can live there.
  • Big events: storms, ice storms and forest fires can change a habitat quickly.

2. Living factors

These come from other living things:

  • Food supply: if acorns are scarce one year, squirrels and mice have a hard winter.
  • Predators: more predators can mean fewer prey animals.
  • Disease: an illness can spread through a population.
  • Competition: too many organisms needing the same thing.
  • Invasive species: species from somewhere else that spread quickly. In Ontario, the emerald ash borer (a beetle) has killed huge numbers of ash trees, and zebra mussels crowd out native mussels in the Great Lakes.

3. Human factors

People change habitats too, sometimes in harmful ways and sometimes in helpful ways:

  • Harmful: building roads and houses, pollution, litter, road salt washing into streams, cutting down too many trees.
  • Helpful: planting native trees and flowers, cleaning up shorelines, protecting wetlands, building turtle crossings.

You will learn much more about human impacts later in this unit.

Natural, non-livingdrought or floodlate frost, ice stormsunlight and soilLivingfood supplypredators, diseaseinvasive speciesHumanroads, pollutionplanting native treesprotecting wetlands
Scientists sort the factors that affect a community into three groups. Human factors can harm a habitat or help it.
FactorTypePossible effect on a forest community
ice storm breaks many branchesnatural, non-livingless shelter for birds; fallen wood feeds decomposers
a big crop of acornsliving (food supply)more food: chipmunks, mice and deer do well
emerald ash borer arrivesliving (invasive species)ash trees die; animals that used them lose food and shelter
a new parking lot is builthumanhabitat is lost; many organisms must move or die
Examples of factors that affect a community.

Let's test a factor: do pill bugs prefer damp or dry places?

Pill bugs and their close cousins, sow bugs (both are kinds of woodlice), live under logs and rocks. Scientists can do a fair test to find out which conditions they need. In a fair test we change only one variable and keep everything else the same.

Worked example

Let's plan a fair test

Question: Do pill bugs prefer damp or dry places?

  1. Prediction: "I think more pill bugs will go to the damp side, because they live under logs where it is damp."
  2. The one thing we change: one side of a tray has a damp paper towel; the other side has a dry paper towel.
  3. What we keep the same: same tray, same light on both sides, same temperature, same paper towel, same starting spot (the middle).
  4. What we measure: after 10 minutes, count how many pill bugs are on each side.
  5. Repeat the test 3 times to be sure the result was not luck.
  6. Results (one class's test with 8 pill bugs): 7 on the damp side, 1 on the dry side.

Most pill bugs chose the damp side. Moisture is a factor that affects where pill bugs can survive.

damp paper toweldry paper towelafter 10 minutes
Results after 10 minutes: 7 pill bugs on the damp side (left) and 1 on the dry side (right).
Just for fun

Pill bugs aren't really bugs at all. They are crustaceans, more closely related to shrimp and crabs than to beetles. No wonder they love damp places!

Hands-on activity

Your own pill bug fair test

You need: a shallow tray or shoebox lid, 2 paper towels, water, a timer, 6 to 10 pill bugs, a small soft paintbrush or spoon.

  1. With a grown-up, find pill bugs under a log, a rock or a flowerpot.
  2. Lay a damp paper towel on one half of the tray and a dry one on the other half.
  3. Use the paintbrush or spoon to gently place all the pill bugs in the middle.
  4. Wait 10 minutes in a spot with even light. Count the pill bugs on each side. Record your results in a table.
  5. Repeat 2 more times. Did you get the same pattern?
  6. Write a conclusion: "Pill bugs prefer ___ places, so a habitat that is ___ would help them survive."
  7. Return the pill bugs to exactly where you found them.

Grown-up note (safety and care): Handle animals gently with a brush or spoon, never squeeze them. Keep them out of direct sun. Return them as soon as you finish (within about an hour), and keep them on damp paper towel between tests. Everyone washes hands with soap afterward.

Your turn

1

Name two non-living factors that can affect a pond community.

2

Name two living factors that can affect a community.

3

What is an invasive species? Give an Ontario example.

4

How could a forest fire affect a forest community?

5

In the pill bug test, what was the one thing that changed?

6

Why did we put all the pill bugs in the middle at the start?

7

Name one human action that helps a local community.

More practice: warm-up

1

Is temperature a living or non-living factor?

2

Is a disease a living or non-living factor?

3

What is a drought?

4

Is a predator a factor that affects prey animals?

5

In a fair test, how many things do you change?

More practice: on your own

1

A late spring frost kills the flowers on wild cherry trees. How might this affect animals that eat cherries in summer?

2

Why do zebra mussels harm native mussels?

3

Road salt washes into a stream every winter. Is this a natural or human factor? How could it affect the stream community?

4

Sam tested pill bugs with a damp towel in the dark and a dry towel in bright light. Is this a fair test? Explain.

5

A meadow is very sunny. A forest floor is shady. Why do they have different plant communities?

Challenge

1

The emerald ash borer kills most of the ash trees in a forest. Explain how this affects at least two other members of the community.

2

In the class test, 7 of 8 pill bugs went to the damp side. What fraction is that? What does it suggest?

3

Design a fair test to find out whether earthworms prefer light or dark. Say what you change, what you keep the same and what you measure.

Check your understanding

Quiz

1.

Which is a non-living factor?

2.

The emerald ash borer is an example of...

3.

Which human action helps a habitat?

4.

In a fair test, you should...

5.

Most pill bugs moved to the damp paper towel. What can we conclude?

6.

A lot of acorns fall one autumn. Which animals will most likely do well that winter?

For parents and teachers

This lesson covers factors that affect a community's ability to survive, sorted into natural non-living, living and human factors, and practises a fair test.

Common difficulties:

  • Changing more than one thing in an experiment. Ask: "What is the ONE thing we are changing?"
  • Seeing only harmful human factors. Point out helpful ones too.

Safety and care for animals: Supervise the pill bug test. Handle the animals gently with a soft brush or spoon, keep them out of the sun, return them where you found them as soon as the test is done (within about an hour), and wash hands with soap afterward.