Science notes, unit 4
Interaction among living things
Textbook Science Year 6 (DLP, Semakan 2017), pages 51–66
What this chapter covers
- You can explain what interaction among living things means (4.1.1)
- You can describe the prey-predator interaction among animals and give examples (4.1.1)
- You can state the factors of competition among animals and tell intraspecies from interspecies competition (4.1.2)
- You can explain mutualism, parasitism and commensalism among animals with examples (4.1.3)
- You can investigate how space, water, sunlight and nutrients cause plants to compete (4.2.1)
- You can describe commensalism and parasitism among plants with examples (4.2.2)
- You can explain why interaction among living things is important to the ecosystem (4.2.3)
No living thing lives alone. Animals and plants in a habitat meet each other every day. In this unit you will learn how they affect each other, and why these interactions keep nature in balance.
4.1Interaction among animals
Interaction among living things is a relationship among living things that depend on each other to obtain their basic needs so that they can continue living.
There are three types of interaction among animals: prey-predator, competition and symbiosis.
Prey-predator
A predator is an animal that hunts other animals for food. The animal that is hunted or eaten is the prey.
Figure: Science Year 6 textbook, p.52.
The prey-predator interaction also happens between animals and plants. The Venus flytrap is a predator of small insects such as flies.
Example: who is the prey?
An eagle catches a rabbit. The eagle is the predator and the rabbit is the prey. A spider eats a grasshopper: spider (predator), grasshopper (prey).The prey is the animal being eaten, not the animal that is bigger. A spider is small but it is the predator of the grasshopper.
Competition among animals
When basic needs are in short supply, animals compete to survive.
| Type | Who competes | Example |
|---|---|---|
| Intraspecies competition | Animals of the same species | Hyenas compete for food. Male mynahs compete for a female mate. |
| Interspecies competition | Animals of different species | Vultures compete with lions for food. Zebras compete with gnus for water. |
The factors of competition among animals are food, water, shelter or territory and mate. Mate is a factor only for intraspecies competition, because animals mate with their own species.
An oystercatcher chases a seagull out of its territory. Is this intraspecies or interspecies competition? What is the factor?
Show answerHide answer
Interspecies competition, because the two birds are different species. The factor is territory (shelter).
Symbiosis among animals
Symbiosis is a close relationship between two species. There are three types.
| Type | What happens | Example |
|---|---|---|
| Mutualism | Benefits both organisms | Crocodile and bird; clownfish and sea anemone |
| Parasitism | Benefits one (the parasite) and harms the other (the host) | Tick and cat; tapeworm and human |
| Commensalism | Benefits one, the other is not harmed and not helped | Barnacles and crab; remora and shark |
Figure: Science Year 6 textbook, p.56.
Figure: Science Year 6 textbook, p.57.
Figure: Science Year 6 textbook, p.57.
Do not mix up parasitism and commensalism. In both, one organism gains. Ask: is the other one harmed? If yes, parasitism. If it is not affected, commensalism.
Clownfish hide among the tentacles of a sea anemone. The anemone's sting protects them, and they chase away the anemone's predators. Which type of symbiosis is this?
Show answerHide answer
Mutualism, because both the clownfish and the sea anemone benefit.
4.2Interaction among plants
There are two types of interaction among plants: competition and symbiosis.
Competition among plants
Plants compete when their basic needs are limited. The factors are space, water, sunlight and nutrients.
| Factor | Investigation (green bean seeds) | What you see |
|---|---|---|
| Space | 15 seeds in a small pot and 15 in a big pot | Seedlings in the small pot are crowded and grow poorly. |
| Water | 10 seeds in pot A, 3 seeds in pot B | Roots in pot B grow healthier. Seedlings in pot A share limited water. |
| Sunlight | 15 seeds in pot A, 3 seeds in pot B, both covered by a box with a small hole on top | Seedlings in pot A grow taller as they compete to reach the light. |
| Nutrients | Fertiliser added to pot B only | Seedlings in pot A compete for limited nutrients. |
Figure: Science Year 6 textbook, p.61.
In a fair test, only one thing changes (the number of seeds, or the pot size, or the fertiliser). Everything else, such as the amount of water each day, stays the same.
Symbiosis among plants
| Type | What happens | Example |
|---|---|---|
| Commensalism | One plant gains support and sunlight. The host plant is not harmed. | Bird's-nest fern and orchid growing on a host plant |
| Parasitism | The parasite takes water and nutrients and harms the host plant. | Rafflesia; Indian willow |
Figure: Science Year 6 textbook, p.62.
Figure: Science Year 6 textbook, p.62.
Why is Rafflesia a parasite?
Show answerHide answer
It cannot carry out photosynthesis, so it grows on a host plant and absorbs water and nutrients from it. This harms the host.
4.3Importance of interaction to the ecosystem
Figure: Science Year 6 textbook, p.63.
- Control the population of organisms: competition and prey-predator stop any one population from growing too large.
- Ensure the survival of species: in a pond, large fish depend on small fish, and small fish depend on tadpoles and water weeds.
- Maintain natural resources: the most resilient plants survive and produce strong seedlings.
- Restore the balance of nature.
Example: what if the owls disappear?
Fewer predators mean the rats multiply. They eat more oil palm fruit and damage the plantation. The balance is lost.Give one way in which competition between plants helps an ecosystem.
Show answerHide answer
The strongest, best-adapted plants survive and multiply, producing good seedlings, so natural resources are maintained. (It also controls the plant population.)