B. Life Systems · Biodiversity

Living Together: Interrelationships That Sustain Biodiversity

B2.5 A2.1 A3.2 A3.3About 50 minutes

Learning goals

I can describe interrelationships within a species, between species, and between species and their environment.
I can identify mutualism, commensalism, parasitism, predation and competition.
I can explain how these interrelationships sustain biodiversity.
I can trace a coded model of a predator and its prey.

Words to know

interrelationship
A connection between living things, or between living things and their environment, in which each affects the other.

symbiosis
A close, long-term relationship between two different species.

mutualism
A relationship in which both species benefit.

commensalism
A relationship in which one species benefits and the other is not helped or harmed.

parasitism
A relationship in which one species (the parasite) benefits and the other (the host) is harmed.

predation
When one animal (the predator) hunts and eats another (the prey).

competition
When organisms need the same limited resource, such as food, water, light or space.

A pond made by a rodent

A beaver cuts down trees and builds a dam across a stream. The water backs up into a pond. Soon, ducks nest in the reeds, frogs lay eggs in the shallows, moose wade in to eat water plants, and fish swim in the deep water. Dead trees standing in the pond become homes for woodpeckers.

One species changed its environment, and dozens of other species gained a home. Living things are connected in many ways. Today you will learn the names for those connections, and why they keep biodiversity going.

A beaver pond with a stick lodge, a dam, reeds, standing dead trees and two ducks swimming. — The beaver's dam made this pond, which is now home to ducks, fish, frogs and plants. Dead trees in the water also give woodpeckers places to nest. AI-generated image

Relationships within a species

Members of the same species affect each other.

Cooperation: grey wolves live and hunt in packs, so together they can catch large prey like deer. Honey bees in a hive divide the work: some gather nectar, some care for young, some guard the entrance. Canada geese often fly in a V formation, which can help the birds behind save energy on long flights.
Competition: members of a species need the same things. Seedlings under a maple compete for light. Male white-tailed deer compete during mating season.

Competition within a species means the individuals best suited to the conditions are more likely to survive and reproduce.

Relationships between species

Predation: a predator eats its prey. A Canada lynx hunts snowshoe hares.
Competition: different species need the same resource. Red squirrels and blue jays may both eat seeds from the same trees.
Symbiosis is a close, long-term relationship between two species. There are three kinds:

Kind	Who benefits?	Examples
Mutualism	both species benefit (+ / +)	bees get nectar while carrying pollen between flowers; the fungus and alga in a lichen; bean plants and the bacteria in their roots
Commensalism	one benefits, the other is not helped or harmed (+ / 0)	a bird nests in a tree; burdock burrs hitch a ride on a dog's fur to a new place
Parasitism	the parasite benefits, the host is harmed (+ / −)	a blacklegged tick feeds on a deer's blood; a tapeworm lives in an animal's intestine

The three kinds of symbiosis.

The three kinds of symbiosis, shown with signs: + means the species is helped, 0 means it is not helped or harmed, and − means it is harmed.

Relationships between species and their environment

Living things also affect, and are affected by, the non-living parts of their environment:

Beavers build dams that turn streams into ponds and wetlands, creating habitat for many other species.
Tree roots hold soil in place, and fallen leaves add nutrients to the soil.
Decomposers (fungi and bacteria) return nutrients from dead things to the soil and water.
Plants take in carbon dioxide and release oxygen, and shade from trees keeps streams cool for fish like brook trout.
The environment shapes living things too: temperature, water and soil decide which species can live in a place.

How interrelationships sustain biodiversity

These connections help keep many species alive together:

Pollinators let flowering plants make seeds, so plant species continue, and the fruits and seeds feed many animals.
Seed dispersers spread plants to new places. Blue jays and squirrels bury acorns to eat later; the ones they forget can grow into new oak trees.
Predators keep prey populations from growing so large that they eat all the plants. That leaves room and food for many other species.
Habitat makers like beavers create homes for other species.
Decomposers recycle nutrients so new plants can grow.

Remove one link and many others can be affected.

Just for fun

Beavers are serious builders. The longest beaver dam ever found, in Wood Buffalo National Park in northern Alberta, is about 850 m long. Scientists spotted it in satellite pictures!

Predator and prey: a famous pattern

For many years, the Hudson's Bay Company kept records of the furs brought to its trading posts, many of them trapped by First Nations and Métis trappers. Scientists later studied these records and saw a pattern: snowshoe hare numbers rise and fall in a cycle of about ten years, and Canada lynx numbers rise and fall a little after them.

When hares are plentiful, lynx have lots of food, so more lynx survive and raise young.
With more lynx (and less food for the hares to eat), hare numbers fall.
With fewer hares, lynx go hungry, and lynx numbers fall.
With fewer lynx, hare numbers rise again, and the cycle repeats.

A sketch of the lynx and hare pattern (not real data). The lynx line rises and falls a little after the hare line.

A coded model

Scientists use computer models to test their ideas. Here is a very simple predator–prey model. It asks the user how many years to run.

set hares to 100

set lynx to 10

ask "How many years should the model run?" and save the answer as years

repeat years times

if lynx >= 12 then change hares by -20

else change hares by 20

if hares >= 100 then change lynx by 2

else change lynx by -2

say hares and lynx

A simple predator–prey model. Inside the loop, the hares change first, then the lynx.

Worked example

Let's trace the model for 4 years

Start: hares = 100, lynx = 10. Follow the code one year at a time.

Year 1: lynx is 10, which is not ≥ 12, so hares go up by 20 → 120. Hares are now 120, which is ≥ 100, so lynx go up by 2 → 12.
Year 2: lynx is 12, which is ≥ 12, so hares go down by 20 → 100. Hares are 100 (≥ 100), so lynx go up by 2 → 14.
Year 3: lynx is 14 (≥ 12), so hares go down → 80. Hares are 80, not ≥ 100, so lynx go down by 2 → 12.
Year 4: lynx is 12 (≥ 12), so hares go down → 60. Hares are 60, so lynx go down → 10.

After 4 years: 60 hares and 10 lynx. Hares peaked first (year 1: 120), lynx peaked one year later (year 2: 14), just like the real pattern.

Many ways of knowing

Many First Nations, Métis and Inuit people teach that all living things are relatives, connected to each other and to the land, water and sky. The phrase "all my relations", used by many Indigenous people, expresses this idea. Western science describes these connections with words like food web and mutualism. Using both ways of knowing can give a fuller picture of how nature works.

Hands-on activity

Relationship detective

You need: a notebook, a pencil, 20 minutes outside in a yard, park or schoolyard (or a window view), a grown-up.

Look for at least five interrelationships. Ideas: a bee on a flower, a bird in a tree, a squirrel burying a nut, a spider web with prey, plants growing in a tree's shade, lichen on bark, burrs on your socks, ants on a dead insect.
For each one, write: who is involved, what is happening, and the kind of relationship (mutualism, commensalism, parasitism, predation, competition, cooperation, or living thing and environment).
Choose one and explain how it helps sustain biodiversity.
Model it (optional): change the starting numbers in the coded model (for example, 100 hares and 14 lynx) and trace or run it. What happens?

Safety: stay with your grown-up, watch bees and wasps from a distance, and don't touch wild animals. Check for ticks after time in tall grass.

What to look for: five observations with correct relationship names, and one clear explanation, for example: "The bee pollinates the flower so it makes seeds, which keeps that plant species growing here."

Your turn

Name the relationship: a bee drinks nectar and carries pollen to another flower.

Name the relationship: a tick feeds on a deer's blood.

Name the relationship: a robin builds a nest in a maple tree.

Give one example of cooperation within a species.

How does a beaver affect its environment and other species?

How do predators help sustain biodiversity?

In the lynx-hare pattern, which peaks first, hares or lynx?

More practice: warm-up

What does mutualism mean?

Name a predator and its prey.

What are two plants competing for when they grow close together?

What do decomposers return to the soil?

In the code, what does the "ask" line do?

More practice: on your own

Explain how a blue jay burying acorns can help oak trees. What kind of relationship is it?

Continue tracing the model: after year 4 there are 60 hares and 10 lynx. What are the numbers after year 5?

Why do lynx numbers fall after hare numbers fall?

A cowbird lays its eggs in a warbler's nest, and the warblers raise the cowbird chicks instead of only their own. What kind of relationship is this? Explain.

Explain one way a non-living part of the environment affects which species live in a place.

Challenge

Wolves were removed from an area long ago, and the deer population grew very large. Predict two effects on biodiversity in the forest.

Change the model so the hares start at 140 and the lynx at 10. Trace two years.

Real hare and lynx populations are affected by more than each other. Name two things the simple model leaves out, and explain why scientists still find simple models useful.

Check your understanding

Quiz

Bees and flowers both benefit from their relationship. This is...

A tick feeding on a deer is an example of...

Which is an example of cooperation within a species?

How do beavers help biodiversity?

In the lynx and hare pattern...

Hares = 100, lynx = 14. Using the model, after one year there will be...

For parents and teachers

Big idea: living things are connected by interrelationships within species (cooperation, competition), between species (predation, competition, mutualism, commensalism, parasitism) and with their environment. These connections, like pollination, seed spreading, predators keeping prey in balance, and habitat building, help sustain biodiversity.

How to help at home:

Go on the "relationship detective" walk and name what you see together.
Trace the coded model with pencil and paper, or build it in Scratch. Changing the starting numbers is a great experiment.
Talk about relationships in your own family or team (cooperation, competition). It helps the words stick.

Safety: watch bees and wasps from a distance, don't handle wild animals, and do a tick check after walks in tall grass.

Common difficulties:

Mixing up commensalism (one benefits, other unaffected) and mutualism (both benefit).
Thinking predators are "bad" for biodiversity. Predators help keep communities balanced.
In the code, remember the hares change first, and then the lynx rule uses the new hare number.