Sciences · Topic 2.1
Ecosystems and Classification
Classification organises all living things into groups based on shared characteristics. Ecosystems describe how organisms interact with each other and with their non-living environment. Energy flows through food chains and food webs, linking all organisms together.
What You'll Learn
- Use the seven-level hierarchy (Kingdom to Species) to classify organisms
- Describe the roles of producers, consumers, and decomposers in an ecosystem
- Interpret and draw food chains and food webs, including the direction of energy flow
- Write and explain the equation for photosynthesis
- Explain how changes in one population affect an entire ecosystem
- Distinguish between biotic and abiotic factors
IB Assessment Focus
Criterion A: Recall classification hierarchy, photosynthesis equation, and ecological roles.
Criterion B: Design investigations to test a variable affecting plant growth; identify independent and dependent variables.
Criterion C: Use correct scientific vocabulary; interpret data from food web diagrams.
Criterion D: Apply ecological knowledge to real-world scenarios (e.g., effect of removing a species from an ecosystem).
Sciences · Topic 2.1
Classification Systems
Taxonomy is the science of organising living things into groups. Carl Linnaeus developed the hierarchical system still used today.
The Seven Levels of Classification
Classification Hierarchy (Broadest → Most Specific)
Kingdom → Phylum → Class → Order → Family → Genus → Species
Mnemonic: King Philip Came Over For Good Spaghetti
- Kingdom: the broadest group (e.g., Animalia, Plantae, Fungi, Protista, Monera)
- Species: the most specific group — organisms that can interbreed and produce fertile offspring
Example: Classification of a Lion
| Level | Name | Explanation |
|---|---|---|
| Kingdom | Animalia | Multi-celled, consumes food |
| Phylum | Chordata | Has a backbone |
| Class | Mammalia | Warm-blooded, feeds young with milk |
| Order | Carnivora | Primarily eats meat |
| Family | Felidae | Cat family |
| Genus | Panthera | Big cats |
| Species | leo | Lion |
Key Vocabulary
| Term | Definition |
|---|---|
| Taxonomy | The science of classifying living organisms into groups |
| Kingdom | The broadest level of biological classification |
| Species | A group of organisms that can interbreed and produce fertile offspring |
| Binomial nomenclature | The two-part naming system using Genus + species (e.g., Homo sapiens) |
Key Rule: As you move down the hierarchy (Kingdom → Species), each level contains fewer and fewer organisms that share more and more characteristics. Species share the most similarities; Kingdom contains the most variety.
Sciences · Topic 2.1
Ecosystems and Food Webs
An ecosystem is a community of living organisms interacting with their non-living environment. Energy flows through the system via feeding relationships.
Biotic and Abiotic Factors
| Biotic Factors (living) | Abiotic Factors (non-living) |
|---|---|
| Plants, animals, fungi, bacteria | Temperature, sunlight, water, soil type, pH |
| Prey and predators | Wind, humidity, altitude |
| Competitors for resources | Availability of minerals and nutrients |
Trophic Levels
- Producers (Trophic Level 1): Plants and algae; make their own food via photosynthesis
- Primary consumers (Trophic Level 2): Herbivores that eat producers (e.g., rabbits, caterpillars)
- Secondary consumers (Trophic Level 3): Carnivores that eat primary consumers (e.g., foxes, frogs)
- Tertiary consumers (Trophic Level 4): Top predators that eat secondary consumers (e.g., eagles, sharks)
- Decomposers: Bacteria and fungi that break down dead matter at all levels
Food Chains vs Food Webs
- Food chain: Shows ONE path of energy transfer. Grass → Rabbit → Fox → Eagle
- Food web: Shows ALL possible feeding paths between organisms in an ecosystem
- Arrow direction: Arrows point FROM what is eaten TO what eats it (showing energy transfer direction)
Critical Rule: In a food web, arrows show the direction of energy transfer — they point FROM what is eaten TO what eats it. A food chain shows ONE energy path; a food web shows ALL paths. Energy is lost at each trophic level (approximately 90% lost as heat), which is why food chains rarely have more than 5 links.
Sciences · Topic 2.1
Photosynthesis
Photosynthesis is the process by which plants convert light energy into chemical energy stored in glucose. It is the foundation of almost all food chains on Earth.
Word Equation
Carbon dioxide + Water → Glucose + Oxygen (in the presence of light and chlorophyll)
Symbol Equation
6CO&sub2; + 6H&sub2;O → C&sub6;H&sub1;&sub2;O&sub6; + 6O&sub2;
Key Components
| Component | Role |
|---|---|
| Sunlight | Provides the energy to drive the reaction |
| Chlorophyll | The green pigment in chloroplasts that absorbs light energy |
| Carbon dioxide (CO&sub2;) | Absorbed from the air through tiny pores called stomata |
| Water (H&sub2;O) | Absorbed from the soil through roots |
| Glucose | The product — used for energy, growth, and reproduction |
| Oxygen | Released as a by-product into the atmosphere |
Factors Affecting Photosynthesis Rate
- Light intensity: More light → faster photosynthesis (up to a limit)
- CO&sub2; concentration: More CO&sub2; → faster photosynthesis (up to a limit)
- Temperature: Warmer (up to ~35°C) → faster; too hot destroys enzymes
- Water availability: Without sufficient water, photosynthesis stops
Sciences · Topic 2.1
Ecological Roles
Every organism in an ecosystem plays a specific role. Understanding these roles helps explain how ecosystems function and what happens when they are disrupted.
Role of Decomposers
Decomposers (bacteria and fungi) break down dead organisms and organic waste. This process:
- Recycles nutrients back into the soil where plants can absorb them
- Prevents accumulation of dead matter
- Allows nutrients to re-enter the food chain via producers
Without decomposers, nutrients would remain locked in dead matter and ecosystems could not sustain life over time.
Effects of Population Changes
Example: If foxes (secondary consumers) are removed from a food chain: Grass → Rabbits → Foxes → Eagles
- Rabbit population increases (no predation)
- Grass population decreases (over-grazing by more rabbits)
- Eagle population may decrease (less fox to eat) or may switch prey
Nutrient Cycling
- Producers take minerals (e.g., nitrates) from the soil
- Consumers get these minerals by eating producers
- When organisms die, decomposers break down their bodies
- Minerals are returned to the soil, completing the cycle
Sciences · Topic 2.1
Worked Examples
These examples show the depth of response expected at Grade 7. Notice how answers use specific scientific vocabulary and explain WHY, not just WHAT.
EXAMPLE 1Describe the role of decomposers in an ecosystem. Why are they essential?
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Full Solution
Decomposers (such as bacteria and fungi) break down the bodies of dead organisms and organic waste. This process recycles nutrients back into the soil, where they can be absorbed by plants and re-enter the food chain. Without decomposers, nutrients would remain locked in dead matter and ecosystems could not sustain life over time. Decomposers are therefore essential for maintaining the nutrient cycle that supports all life in the ecosystem.
EXAMPLE 2Write the word equation for photosynthesis and explain its significance.
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Full Solution
Carbon dioxide + Water → Glucose + Oxygen (using sunlight and chlorophyll)
Photosynthesis is significant because it converts light energy from the sun into chemical energy stored in glucose. This provides the starting point for almost all food chains on Earth — plants are the primary producers. The oxygen released is essential for the respiration of most living organisms.
Photosynthesis is significant because it converts light energy from the sun into chemical energy stored in glucose. This provides the starting point for almost all food chains on Earth — plants are the primary producers. The oxygen released is essential for the respiration of most living organisms.
EXAMPLE 3State the correct order of the classification hierarchy from broadest to most specific.
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Full Solution
Kingdom → Phylum → Class → Order → Family → Genus → Species
Memory aid: King Philip Came Over For Good Spaghetti
Kingdom is the broadest group (e.g., Animalia contains all animals). Species is the most specific — organisms in the same species can interbreed to produce fertile offspring.
Memory aid: King Philip Came Over For Good Spaghetti
Kingdom is the broadest group (e.g., Animalia contains all animals). Species is the most specific — organisms in the same species can interbreed to produce fertile offspring.
EXAMPLE 4A food web shows: Grass → Rabbits → Foxes; Grass → Mice → Owls; Foxes → Owls. If the rabbit population doubles, predict the effect on foxes and grass.
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Full Solution
If the rabbit population doubles:
• Foxes: With more prey (rabbits) available, the fox population would likely increase. More food means more foxes can survive and reproduce.
• Grass: More rabbits means more grazing, so the grass population would likely decrease. If grazing exceeds growth rate, grass could be severely reduced.
This illustrates how changes at one trophic level have cascading effects throughout the food web.
• Foxes: With more prey (rabbits) available, the fox population would likely increase. More food means more foxes can survive and reproduce.
• Grass: More rabbits means more grazing, so the grass population would likely decrease. If grazing exceeds growth rate, grass could be severely reduced.
This illustrates how changes at one trophic level have cascading effects throughout the food web.
EXAMPLE 5Explain the difference between a food chain and a food web.
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Full Solution
A food chain shows ONE linear path of energy transfer from producer to top consumer, e.g., Grass → Rabbit → Fox. A food web shows ALL the interconnected feeding relationships in an ecosystem — it is a network of multiple food chains overlapping. Food webs are a more realistic representation because most organisms eat multiple things and are eaten by multiple predators. The arrows in both always point FROM what is eaten TO what eats it.
EXAMPLE 6Identify TWO biotic and TWO abiotic factors in a pond ecosystem.
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Full Solution
Biotic factors (living):
• Fish (consumers that eat smaller organisms)
• Algae (producers that photosynthesise in the water)
Abiotic factors (non-living):
• Water temperature (affects the rate of photosynthesis and metabolism of all organisms)
• Dissolved oxygen level (essential for aquatic animals to respire)
• Fish (consumers that eat smaller organisms)
• Algae (producers that photosynthesise in the water)
Abiotic factors (non-living):
• Water temperature (affects the rate of photosynthesis and metabolism of all organisms)
• Dissolved oxygen level (essential for aquatic animals to respire)
EXAMPLE 7Why does energy decrease at each level of a food chain?
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Full Solution
At each trophic level, only about 10% of the energy is transferred to the next level. The other 90% is lost as heat during metabolic processes (movement, growth, keeping warm), in waste products, and in parts of the organism that are not eaten. This is why food chains rarely have more than 5 levels — by the 5th level, there is so little energy remaining that it cannot support another trophic level.
Sciences · Topic 2.1
Practice Q&A
Attempt each question before revealing the model answer. Use specific scientific vocabulary in your responses.
IDENTIFYAt what level of the classification hierarchy is classification most specific?
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Model Answer
Species is the most specific level. Organisms in the same species can interbreed and produce fertile offspring.
EXPLAINExplain what is meant by "an organism's niche" in an ecosystem.
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Model Answer
An organism's niche is its role or occupation within the ecosystem — including what it eats, when it is active, where it lives, and how it interacts with other organisms. No two species can occupy exactly the same niche in the same ecosystem — they would compete until one is eliminated or adapts to a different niche.
DESCRIBEDescribe what happens to photosynthesis rate if a plant is placed in darkness.
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Model Answer
Without light, photosynthesis stops completely. Light is needed to provide the energy that powers the conversion of CO&sub2; and H&sub2;O into glucose. The plant will still respire (using glucose), but no new glucose will be produced until light is available again.
COMPARECompare a producer and a consumer. Give an example of each.
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Model Answer
A producer makes its own food through photosynthesis (e.g., grass, oak tree). It converts light energy into chemical energy stored in glucose. A consumer cannot make its own food — it obtains energy by eating other organisms (e.g., rabbit eats grass; fox eats rabbit). Producers are always at the start of food chains; consumers are at higher trophic levels.
APPLYWhy does a rainforest have greater biodiversity than a desert?
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Model Answer
A rainforest has greater biodiversity because it has more favourable abiotic conditions: high rainfall, warm temperatures, and abundant sunlight. These conditions support high rates of photosynthesis, producing large amounts of plant matter. This provides food and habitat for a large variety of herbivores, which in turn support many carnivores. A desert has extreme temperatures and very low water availability, limiting the types of organisms that can survive there.
EXPLAINWhy are sharks and dolphins classified in different classes despite both living in the ocean?
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Model Answer
Classification is based on biological characteristics, not just habitat. Sharks are fish (Class Chondrichthyes) — they are cold-blooded, breathe through gills, and lay eggs. Dolphins are mammals (Class Mammalia) — they are warm-blooded, breathe air through lungs, and give birth to live young that they nurse with milk. Habitat is an abiotic factor and does not determine classification.
DESCRIBEDescribe the raw materials needed for photosynthesis and state where each comes from.
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Model Answer
Carbon dioxide: absorbed from the air through tiny pores called stomata on the surface of leaves.
Water: absorbed from the soil through the plant's roots and transported up the stem through xylem vessels.
Light energy: captured by chlorophyll (the green pigment) in the chloroplasts of leaf cells.
Water: absorbed from the soil through the plant's roots and transported up the stem through xylem vessels.
Light energy: captured by chlorophyll (the green pigment) in the chloroplasts of leaf cells.
APPLYIf all decomposers were removed from an ecosystem, what would happen over time?
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Model Answer
Without decomposers, dead organisms and waste would accumulate in the environment. Nutrients (such as nitrates and phosphates) locked in these dead organisms could not be recycled back into the soil. Plants would gradually run out of these essential nutrients and fail to grow. Without producers, all consumer populations would eventually collapse. The entire ecosystem would break down, as the nutrient cycle is essential for sustaining life.
Sciences · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What is taxonomy?
The science of classifying living organisms into hierarchical groups based on shared characteristics.
Tap to reveal
State the classification hierarchy from broadest to most specific.
Kingdom → Phylum → Class → Order → Family → Genus → Species. (King Philip Came Over For Good Spaghetti)
Tap to reveal
What is a species?
A group of organisms that can interbreed with each other and produce fertile offspring.
Tap to reveal
Write the word equation for photosynthesis.
Carbon dioxide + Water → Glucose + Oxygen (using sunlight energy and chlorophyll)
Tap to reveal
Write the symbol equation for photosynthesis.
6CO&sub2; + 6H&sub2;O → C&sub6;H&sub1;&sub2;O&sub6; + 6O&sub2;
Tap to reveal
What is a producer? Give an example.
An organism that makes its own food through photosynthesis. Examples: grass, oak tree, algae.
Tap to reveal
What is a decomposer and why is it important?
An organism (bacteria or fungi) that breaks down dead matter, recycling nutrients back into the soil for plants to use.
Tap to reveal
In a food web diagram, which direction do arrows point?
Arrows point FROM what is eaten TO what eats it — showing the direction of energy transfer.
Tap to reveal
What is the difference between a food chain and a food web?
A food chain shows ONE path of energy flow. A food web shows ALL interconnected feeding relationships in an ecosystem.
Tap to reveal
What is a primary consumer?
An organism that feeds directly on producers (plants). Also called herbivores. Example: rabbits eat grass.
Tap to reveal
Give two biotic and two abiotic factors in a forest ecosystem.
Biotic: trees (producers), deer (consumers). Abiotic: temperature, rainfall.
Tap to reveal
Why do food chains rarely have more than 5 trophic levels?
About 90% of energy is lost as heat at each trophic level. By the 5th level, so little energy remains that it cannot support another trophic level.
Tap to reveal
What does chlorophyll do?
Chlorophyll is the green pigment in chloroplasts that absorbs light energy to power photosynthesis.
Tap to reveal
What are the five kingdoms of life?
Animalia (animals), Plantae (plants), Fungi, Protista (single-celled eukaryotes), Monera (bacteria). Some classifications use 6 kingdoms.
Tap to reveal
What happens to rabbit population if all foxes are removed?
Rabbit population increases (no predation). This then causes overgrazing, reducing the grass population. It affects the whole ecosystem.
Tap to reveal
Sciences · Practice Test
Practice Test — 20 Questions
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