B. Life Systems · Interactions in the Environment

Biotic and Abiotic Components and How They Interact

B2.2 A1.2 A1.4 A1.5About 55 minutes

Learning goals

I can identify the biotic (living) and abiotic (non-living) components of an ecosystem.
I can describe how abiotic factors affect living things, and how living things change abiotic factors.
I can plan and carry out a fair test about an abiotic factor, and identify the variables.

Words to know

biotic
Living, or once living. Biotic components include plants, animals, fungi, bacteria, and dead leaves or logs.

abiotic
Non-living and never alive. Abiotic components include sunlight, water, air, temperature, rocks, minerals and snow.

factor
Something that has an effect on something else.

dissolved oxygen
Oxygen gas mixed into water. Fish and other water animals take it in through their gills or skin.

independent variable
In a fair test, the one thing you change on purpose.

dependent variable
In a fair test, the thing you measure to see the effect.

controlled variables
Everything you keep the same so the test is fair.

Salt on the roads

Every Canadian winter, trucks spread road salt to melt ice and keep drivers safe. In spring, the snowbanks melt, and the salty water runs into ditches, soil, streams and ponds.

Have you noticed brown, dying grass or shrubs right beside a busy road in spring? Salt is a non-living thing, but it can have a big effect on living things. Today you'll learn the words scientists use for these two kinds of components, and you'll test the effect of salt yourself.

A melting roadside snowbank with a salty crust, its meltwater trickling into a grassy ditch. — Salty meltwater from snowbanks is an abiotic factor that soaks into soil and flows into streams, where it can harm plants and animals. AI-generated image

Biotic and abiotic

Biotic components are living things, or things that were living: plants, animals, fungi, bacteria, protists, and also fallen leaves, dead wood and animal droppings.
Abiotic components are non-living things that were never alive: sunlight, temperature, water, air, rocks, minerals in soil, wind, snow and ice.

Tip: bio means life. A- means not. So abiotic means not living.

Soil is interesting: it is a mixture of abiotic parts (sand, silt, clay, water, air, minerals) and biotic parts (roots, earthworms, fungi, bacteria and decaying leaves). You studied soil in Grade 3.

Sorting the parts of an ecosystem. Soil sits in the overlap because it is a mixture: sand, water and minerals (abiotic) plus roots, earthworms, fungi and decaying leaves (biotic).

Abiotic factor	How it affects living things in Ontario
Sunlight	Plants need light to make food. Spring wildflowers like trilliums grow and bloom early, before the trees' leaves shade the forest floor.
Temperature	Cold triggers hibernation (groundhogs) and migration (many songbirds). Brook trout need cold water.
Water	All living things need water. Wetland plants like cattails need wet soil; others would rot there.
Air (oxygen and carbon dioxide)	Animals need oxygen. Plants need carbon dioxide to make food. Fish need oxygen dissolved in water.
Soil and minerals	Plants take minerals (nutrients) from the soil. Sandy soil dries out fast; clay soil holds water.
Snow and ice	Snow insulates small animals and plant roots. Ice on a lake blocks wind from mixing oxygen into the water.
Salt (in water or soil)	Too much salt makes it hard for many plants to take in water, and harms many freshwater animals.

Abiotic factors decide which organisms can live in a place, and how well they grow.

Living things change abiotic factors too

The arrows go both ways. Biotic components change the abiotic parts of their ecosystem.

Trees shade the ground, so it stays cooler and moister. Their roots hold soil so rain does not wash it away.
Plants take in carbon dioxide and release oxygen into the air and water.
Earthworms, ants and burrowing animals mix and loosen the soil so air and water can get in.
Decomposers break down dead things and add minerals back to the soil.
Beavers build dams that turn a stream into a pond (you saw this in Grade 6).
People change abiotic factors on a huge scale: we pave land, spread salt, warm the climate and add chemicals to water.

Worked example

Let's trace a chain of biotic and abiotic interactions

A row of trees grows along a stream where brook trout live. A landowner cuts down all the trees beside the stream. Predict what happens, and label each step biotic or abiotic.

Before: the trees (biotic) shade the stream, so the water (abiotic) stays cool.
Cool water can hold more dissolved oxygen (abiotic) than warm water. Brook trout (biotic) need cold water with plenty of oxygen.
Tree roots (biotic) hold the soil (abiotic) of the stream bank in place.
After cutting: sunlight (abiotic) now hits the water all day, and the water warms up. Warm water holds less oxygen.
Without roots, rain washes soil into the stream. The water becomes cloudy, and mud can cover the gravel where trout lay their eggs.
Brook trout may die or move away. Animals that eat trout, like herons and otters, also lose food.

Removing a biotic part (the trees) changed abiotic parts (temperature, oxygen, soil), and that harmed other biotic parts (trout, herons, otters). Biotic and abiotic components are tied together.

Testing an abiotic factor: a fair test

Scientists test one factor at a time in a fair test (an experiment).

The independent variable is the one thing you change on purpose.
The dependent variable is what you measure.
The controlled variables are everything you keep the same.

You also make a hypothesis: a prediction with a reason, such as "If the water has more salt, then fewer seeds will sprout, because salt makes it harder for seeds to take in water."

Salt in 250 mL of water	Seeds that sprouted (out of 10) after 5 days
0 mL (none)	9
5 mL (1 teaspoon)	6
15 mL (1 tablespoon)	1

Sample results from one student's test (practice data, yours may differ).

The sample results as a bar graph. As the amount of salt (an abiotic factor) goes up, the number of seeds that sprout goes down: 9, then 6, then 1.

Worked example

Let's analyse the sample results

Look at the sample results in the table. What do they show? Is the conclusion trustworthy?

Independent variable: the amount of salt in the water (0, 5 or 15 mL).
Dependent variable: the number of seeds that sprouted.
Controlled variables: the same kind of seed, the same number of seeds (10), the same amount of water, the same temperature, light and number of days.
Pattern: as the salt increased, fewer seeds sprouted: 9, then 6, then 1. Changing to percent: 90%, 60%, 10%.
Trustworthy? The pattern is clear, but each amount was tested only once with 10 seeds. Repeating the test (more trials, more seeds) would make the conclusion stronger.

The sample data support the hypothesis: more salt meant fewer seeds sprouted. Repeating the test would make us more confident.

Just for fun

Sunlight is the ultimate influencer. It never posts a single thing, yet every plant in the forest leans toward it and follows it all day.

Hands-on activity

Fair test: does salt affect seeds sprouting?

You need: 30 radish or bean seeds (from a seed packet), table salt, 3 clear zip-top bags or jars, paper towels, a measuring cup and measuring spoons, masking tape, a marker.

Write your hypothesis.
Mix three solutions in clean cups: 250 mL water with no salt, with 5 mL salt, and with 15 mL salt. Stir until the salt dissolves. Label each cup.
Fold a paper towel into each bag. Pour the same amount (about 30 mL) of one solution onto each towel. Label each bag.
Place 10 seeds on each towel and seal the bags. Keep all bags together in the same warm spot.
Each day for 5 to 7 days, count the seeds that have sprouted. Record your results in a table.
Make a bar graph. Write a conclusion: was your hypothesis supported? What would you change to make the test better?

Safety: do not taste the seeds or the solutions (seed packets may be treated with chemicals). Wash your hands after handling seeds. Throw out mouldy bags without opening them.

What to look for: a clear hypothesis, the three variables named correctly, a data table, a labelled graph, and a conclusion that uses the data. Results often show fewer seeds sprouting with more salt.

Careers connection

Environmental technicians measure abiotic factors such as water temperature, dissolved oxygen and salt levels in lakes and streams. Road maintenance crews and city planners try to use just enough salt to keep roads safe in winter while protecting nearby streams, for example by using brine (salty water) before a storm or calibrating spreaders.

Your turn

Sort into biotic and abiotic: sunlight, moss, a fallen log, snow, a mushroom, carbon dioxide, a deer, sand.

What does the prefix a- mean in abiotic?

Why is soil a mix of biotic and abiotic parts?

Give an example of an abiotic factor affecting an animal.

Give an example of a living thing changing an abiotic factor.

In the salt fair test, what is the independent variable? the dependent variable?

Name two controlled variables in the salt test.

More practice: warm-up

Is a dead leaf biotic or abiotic?

Is wind biotic or abiotic?

What do plants need from the air to make food?

Which holds more dissolved oxygen: cold water or warm water?

In a fair test, how many variables do you change on purpose?

More practice: on your own

Trilliums bloom in early spring in Ontario forests. Use an abiotic factor to explain why blooming early helps them.

Ice and snow cover a lake all winter. Why might oxygen in the water get low by late winter?

A student tested salt on seeds, but put the no-salt bag on a sunny windowsill and the salty bags in a dark cupboard. What is wrong with the test?

In the sample results, what percent of seeds sprouted with 5 mL of salt?

Explain how beavers are biotic components that change abiotic components.

Challenge

Design a fair test to find out if the amount of light affects how tall bean seedlings grow. Name the independent, dependent and two controlled variables, and write a hypothesis.

A town is deciding how much road salt to use. Using biotic and abiotic components, explain the trade-off.

Explain how a single change, a hot summer drought, could affect both biotic and abiotic components of a wetland. Give at least three linked effects.

Check your understanding

Quiz

Which is a biotic component?

Which is an abiotic factor?

Cutting the trees beside a stream will most likely make the water...

In a fair test of salt on seeds, the dependent variable is...

Which shows a living thing changing an abiotic factor?

Why should you keep light and temperature the same for all bags in the seed test?

For parents and teachers

Big idea: ecosystems have biotic (living or once living) and abiotic (never living) components, and they affect each other in both directions.

How to help at home:

Set up the salt-and-seeds test together. Help your child measure carefully and count every day. Ask: What did you keep the same? Why?
On walks, play "biotic or abiotic?" Point to a rock, a stump, a puddle, a mushroom.
In spring, look for salt damage on plants beside roads and talk about the trade-off between safety and nature.

Safety: seed packets can contain treated seeds; no tasting, and wash hands. Discard mouldy bags without opening them.

Common difficulties:

Calling dead things (logs, leaves) abiotic. They are biotic because they were once living.
Mixing up the independent (what I change) and dependent (what I measure) variables.
Changing more than one thing in a test.