🔗 Interactions — Environmental Interactions
Primary 6 Science — Summary Notes
Created by Miss Clarissa Ng · www.clartutors.com
1. Sustaining a Stable Ecosystem
- In their natural surroundings, the organisms and the physical environment they live in make up an ecosystem.
- A stable ecosystem is delicately balanced.
- Disrupting even one part of it can bring about devastating consequences for the whole community.
2. Food Chains
- A food chain shows how energy is transferred through an ecosystem.
- It always starts with a food producer, such as green plants or algae.
- An animal feeding on the producer may be a herbivore (eats plants only) or an omnivore (eats both plants and animals).
- An animal feeding on other animals may be an omnivore or a carnivore (eats animals only).
- An animal that hunts and eats another animal is the predator; the animal being hunted and eaten is the prey.
⭐️ KEY POINT
Herbivores cannot be predators, and plants cannot be prey.
3. Transfer of Energy in a Food Chain
- Energy transfer — from the Sun to photosynthetic organisms, and from each organism to its consumer — is extremely inefficient.
- At every position in the chain, energy is lost in waste, lost as heat, or used to sustain life processes. So the energy passed on decreases down the food chain.
- About 98% of the Sun's energy is lost — it is reflected, absorbed by water (causing evaporation), or trapped by the soil and atmosphere.
- Only about 2% is trapped by food producers, and only about 10% of that is passed on to the next organism.
- At each position, roughly 90% of the energy is lost (life processes, heat and waste) — so only about 10% reaches the next organism.
| Position in the food chain | Example |
|---|---|
| Food producer | Green plant |
| Primary consumer | Herbivore / omnivore |
| Secondary consumer | Carnivore / omnivore |
| Tertiary consumer | Carnivore / omnivore |
4. Ecological Pyramids
- An ecological pyramid shows the transfer of energy between organisms in a food chain.
- Food producers sit at the bottom, and the chain continues up to the apex predator at the top.
- General rule: there must be more food producers than consumers and more prey than predators, so that consumers and predators have enough energy for their life processes — this supports a stable ecosystem.
Ecological pyramid showing the energy available at each position
5. Other Types of Ecological Pyramids
- A pyramid of numbers shows the number of organisms at each position of a food chain.
- Its advantage is that the data needed is easily obtained.
- However, it does not account for the size (biomass) of each organism. For instance, a single banyan tree can support many insects, spiders and birds — giving an inverted pyramid.
Pyramid of numbers — standard and inverted
6. Food Webs
- Most animals do not depend on just one type of organism for food.
- When an organism is part of more than one food chain, those food chains link together to form a food web.
- A food web shows the relationships between all the organisms in an ecosystem.
7. Different Types of Relationships
| Type of relationship | Description | Example |
|---|---|---|
| Food relationships (producer–consumer or predator–prey) |
One organism benefits (it obtains food) while the other is harmed (it gets eaten). | cabbage → snail → bird The cabbage is the producer, the snail is the consumer of cabbage and the prey of the bird, and the bird is the consumer/predator of the snail. |
| Competition | Both organisms are harmed as they compete for limited resources. | Bees and butterflies compete for the same food source — the nectar in flowers. |
| Mutualism | Both organisms benefit from the relationship. | A cleaner fish feeds on the parasites living on a larger fish. The cleaner fish gets food, while the larger fish is freed of parasites and stays healthy. |
8. Effects of Changes in Population Size
- When the population size of one organism changes, the population sizes of the organisms that share a relationship with it — directly or indirectly — change too.
- Many factors affect the population size of an organism in a habitat:
- More food available → better chances of survival → population size increases over time.
- More predators, or the presence of disease or natural disasters → population size decreases.
- Since predators depend on their prey for food, the change in a predator's population size lags behind that of its prey.
Graph of the changes in population size of a predator and its prey
9. Common Question — A Change in Population Size in a Food Web
Food web — rice plant, grasshopper, caterpillar, sparrow and lizard
The two food chains in this web:
- rice plant → grasshopper → sparrow
- rice plant → caterpillar → sparrow and lizard
The sparrow and the lizard compete for caterpillars.
If the sparrow population decreased drastically — perhaps due to disease or hunting — what would happen?
In the short term
- The populations of grasshoppers and caterpillars increase, since there are fewer sparrows feeding on them.
- The population of lizards increases, as there are more caterpillars to feed on and fewer sparrows competing with them for caterpillars.
- The population of rice plants decreases, as the greater number of grasshoppers and caterpillars consume more of them.
In the long term
- The increased consumption of rice plants causes a food shortage for the grasshoppers and caterpillars, so they eventually starve and their numbers fall.
- With fewer grasshoppers and caterpillars to feed on, the sparrows and lizards may also starve.
10. Effect of Introducing Poisonous Chemicals in a Food Chain
- Insecticides and pesticides can lead to poisoning across a food chain.
- Many poisons are difficult to flush out of the body.
- Even if the target organisms do not take in enough poison to kill them, the poison remains in their bodies.
- Organisms higher up the food chain feed on many of these organisms, accumulating a higher concentration of poison.
- Hence, what was meant to kill organisms at a lower position may end up killing the predators at the top of the food chain.
11. Decomposition
- Decomposition is the process by which animal waste and dead matter are broken down by decomposers into simpler substances, returning nutrients to enrich the soil so plants grow more healthily.
- Only a finite amount of matter is available to living things, so decomposition allows this matter to be recycled.
12. Detritivores and Decomposers
| Group | What they do |
|---|---|
| Detritivores (termites, woodlice, millipedes, earthworms) |
Animals that feed on dead organic matter. They speed up decomposition by breaking dead matter into smaller pieces, which increases the surface area exposed to decomposers. They are not decomposers. |
| Decomposers (bacteria and fungi) |
They secrete digestive enzymes to break dead matter down into simpler substances. They absorb the nutrients they need, and the remaining nutrients leach into the soil to enrich it, so plants can absorb them and grow more healthily. They also prevent animal waste and dead matter from piling up on the surface of the earth. |
13. Decomposition and Food Preservation
Decomposition needs sufficient moisture, oxygen and warmth. Removing or reducing these factors helps to preserve food.
| Factor removed / reduced | How the food is preserved | Example |
|---|---|---|
| Oxygen | Food is vacuum-sealed to remove oxygen from the packaging. | Vacuum-packed nuts and coffee beans stay fresh for months |
| Warmth (temperature) | Food is kept in a freezer or refrigerator — the lack of warmth stops decomposers from growing. | Milk and cooked leftovers kept chilled last much longer |
| Moisture | Food is dried to remove moisture, so decomposers cannot grow. | Dried mango slices and dried mushrooms |