A fish cools as its water cools. Which response does this show?

Conformity. Its internal temperature changes with its surroundings.

When does a desert cactus usually take in carbon dioxide?

At night. Opening stomata during cooler hours reduces water loss.

Grass covers half the sampled ground. What is its percentage cover?

50%. Half the sampled area is covered, so its percentage cover is 50%.

Drought reduces the grass available to deer. What happens to the habitat's carrying capacity?

It decreases. The support limit depends on available resources.

With fixed resources, what happens to offspring number when investment per offspring increases?

It decreases. Each offspring uses more of the limited resources.

What interaction occurs when one bird excludes another of its species from food?

Competition. They need the same resource, regardless of belonging to the same species.

What does a host lose to a feeding parasite even while remaining alive?

Resources. Parasitism costs the host resources without requiring immediate death.

A bird nests in a tree without affecting it. Which partner benefits?

The bird. The bird gains shelter, while the tree gains nothing and loses nothing.

Teach Organisms and Populations in eight steps.

For each idea: show it, ask before you tell, then say one line.

Gather moong, two dishes, cloth, water, string, ruler, paper and pencil. Locate a cactus and a plant patch.

Observe cactus spines without touching them.

Common misconceptions & worked visual for this chapter · 15 practice questions with answers

Why it matters

Dry seeds can resume growth when water becomes available.
Crowded plants compete for limited water, light and space.
Bees visiting flowers can help plants produce seeds and fruits.

What to teach

How to teach it

Organisms and their surroundings

Dry moong and sprouted moong in separate cloth-lined dishes.

1. Organism and Its Environment; Responses to Abiotic Factors

Conditions affect organisms, which respond in different ways.

Show: Two to four days beforehand, put moong on cloth in two dishes somewhere warm. Keep only one cloth damp. Compare them during the session. After the prediction, dampen the dry seeds. Check for sprouting over the following days.
Ask: Could these dry seeds still sprout after watering?
Say: These seeds could still sprout; seeds can suspend growth until water returns.
Used in: Dry seed storage delays sprouting, keeping seeds available for sowing.

If your child says…“Everything keeps its internal conditions constant.”

Ask: Does this fish keep its body temperature constant as the water cools?
What’s true: Many organisms change internally when their surroundings change.

2. Adaptations

Inherited features help organisms survive particular conditions.

Show: Observe an existing spiny cactus. Point out its thick green stem and spines without touching.
Ask: Which part could supply stored water between rains?
Say: Its stem stores water.
Used in: Plants adapted to dry conditions help gardens cope with scarce water.

If your child says…“Cactus pores open widest in the midday heat.”

Ask: Would cooler nights or midday heat favour less water loss through open pores?
What’s true: Desert cactus stomata usually open at night, reducing water loss during gas exchange.

Populations

String square enclosing separate plants of the same species.

3. Population Attributes

Density, birth rates and age proportions describe groups of one species.

Show: Find separate plants of one species. Mark a square with string, each side one metre. Count the enclosed plants. Record the count per square metre.
Ask: If more plants grow inside this same square, what will happen to their density?
Say: Their density will increase. Density links the population to its area.
Used in: Plant density helps nurseries plan spacing so seedlings have room to grow.

If your child says…“Density always means counting individual plants.”

Ask: How could you measure tangled grass when separate plants are difficult to count?
What’s true: Living mass or percentage cover can also measure population density.

Exponential growth rises steeply while logistic growth approaches the carrying-capacity line.

4. Population Growth

Limited resources slow growth near carrying capacity.

Show: In the diagram, follow both curves towards the carrying-capacity line. N means population size, and K means carrying capacity.
Ask: Looking at these curves, how would population growth change as food becomes limiting?
Say: As population size increases, growth per individual falls with the factor $1-\frac{N}{K}$ . At carrying capacity, this becomes zero, giving zero net growth.
Used in: Wildlife recovery plans match animal numbers to available resources.

If your child says…“A habitat can always support the same number.”

Ask: If drought removes much of the food, can the habitat still support the same number?
What’s true: Carrying capacity can change when food, water or other limiting resources change.

Orchid and coconut seeds contrast offspring number, size and stored food.

5. Life History Variation

Reproductive timing and investment affect how many offspring survive and reproduce.

Show: Look at the diagram's orchid seeds on the left and coconuts on the right. Compare seed size and the resources each seed receives.
Ask: With fixed resources, what would happen to seed numbers if each seed received more?
Say: Each coconut seed receives more resources than an orchid seed. With limited resources, investing more per seed usually means producing fewer seeds.
Used in: Plant propagation allows for seed losses so enough seedlings survive for planting.

If your child says…“Making the most tiny offspring is always best.”

Ask: Would extra offspring help if none survived to reproduce?
What’s true: Offspring number alone does not determine how many survive and reproduce.

Interactions between organisms

Predator and prey beside two birds sharing a limited seed supply.

6. Predation and Competition

Predation consumes organisms; competition limits access to shared resources.

Show: In the diagram, follow food from the prey to the predator on the left. Then follow the limited seeds to both birds on the right.
Ask: If one bird eats more of these limited seeds, what happens to the other's food supply?
Say: The predator gains food by consuming its prey. Each bird's feeding leaves fewer seeds for the other, so both face competition.
Used in: Plant-eating insects can control invasive weeds, allowing farmland to return to production.

If your child says…“Only different species compete.”

Ask: Does sharing a species make enough food appear for both birds?
What’s true: Members of one species can compete for the same limited food.

Leafless amarbel winds around a leafy host, with an arrow showing resource transfer.

7. Parasitism

A parasite gains resources while its host bears a cost.

Show: Find the leafy host and leafless amarbel in the diagram. Follow the water-and-nutrients arrow from host to amarbel.
Ask: Which plant will gain resources through the attachment?
Say: Amarbel draws resources from the living host through specialised attachments. The host loses resources.
Used in: Removing parasitic weeds protects crop resources and helps reduce harvest losses.

If your child says…“A parasite must kill its host straight away.”

Ask: Can the parasite keep feeding while its host remains alive?
What’s true: A living host can keep supplying a parasite with resources.

A nesting bird benefits alone; lichen partners exchange resources and both benefit.

8. Commensalism and Mutualism

Mutualism benefits both partners. Commensalism benefits one without affecting the other.

Show: Find the nest in the diagram. Follow the arrows between the lichen partners.
Ask: If exchange stopped, what would each lichen partner lose?
Say: The bird gains shelter without affecting the tree, showing commensalism. Algae supply sugars; fungi supply water and minerals. Both benefit, showing mutualism.
Used in: Pollinating insects support fruit production by carrying pollen between flowers.

If your child says…“The tree benefits just as much as the nesting bird.”

Ask: What does the tree gain merely from supporting this nest?
What’s true: In this example, the bird gains shelter while the tree remains unaffected.

A real problem

Cactus-feeding caterpillars helped reclaim Australian farmland.

Invasive prickly pear threatened Australian grazing and agricultural land.

Australia began releasing Cactoblastis moths in 1926 to control it.

Their caterpillars ate the cactus interiors, leaving hollow shells.

The campaign brought previously infested land back into production.

National Museum of Australia University of California, Riverside Weeds Australia

Prickly-pear cactus with damaged pads beside pasture.

Concept Lab · Common misconception

The tempting mistake in Organisms and Populations

Open standalone misconception page →

Does a population grow exponentially forever?

Wrong

A population that doubles once should keep doubling indefinitely.

Why it feels right. I extend the initial pattern without accounting for available resources.

Right

Growth can slow as resources become limiting.

Worked example

This schematic population curve rises rapidly, then levels off as nutrients become limiting.

A small population in a closed culture grows rapidly at first.
As nutrients are used up, the number added per hour falls and the growth curve levels off.

Ask your child eight quick questions.

One per step. The answer and the reason are given.

A fish cools as its water cools. Which response does this show?

Conformity. Its internal temperature changes with its surroundings.
When does a desert cactus usually take in carbon dioxide?

At night. Opening stomata during cooler hours reduces water loss.
Grass covers half the sampled ground. What is its percentage cover?

50%. Half the sampled area is covered, so its percentage cover is 50%.
Drought reduces the grass available to deer. What happens to the habitat's carrying capacity?

It decreases. The support limit depends on available resources.
With fixed resources, what happens to offspring number when investment per offspring increases?

It decreases. Each offspring uses more of the limited resources.
What interaction occurs when one bird excludes another of its species from food?

Competition. They need the same resource, regardless of belonging to the same species.
What does a host lose to a feeding parasite even while remaining alive?

Resources. Parasitism costs the host resources without requiring immediate death.
A bird nests in a tree without affecting it. Which partner benefits?

The bird. The bird gains shelter, while the tree gains nothing and loses nothing.

15 more questions with answers

Teach Organisms and Populations in eight steps.

Why it matters