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Biology - Ecosystems, Food Webs, and Adaptation

Grade 8IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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An Ecosystem consists of all the living (biotic) communities interacting with their non-living (abiotic) environment such as H2OH_2O, sunlight, and soil.

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Producers (Autotrophs) convert light energy into chemical energy via photosynthesis: 6CO2+6H2OightarrowC6H12O6+6O26CO_2 + 6H_2O ightarrow C_6H_{12}O_6 + 6O_2.

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Trophic Levels represent the position an organism occupies in a food chain. Energy decreases as it moves up from producers to primary, secondary, and tertiary consumers.

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Energy Transfer Efficiency: Only approximately 10%10\% of energy is passed from one trophic level to the next. The remaining 90%90\% is lost through respiration, movement, excretion, and as heat energy.

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Pyramids of Biomass represent the total dry mass of living material at each stage of a food chain. These are almost always pyramid-shaped, unlike pyramids of numbers which can be inverted.

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Bioaccumulation and Biomagnification: Toxins like DDTDDT or heavy metals increase in concentration as they move up the food chain, often reaching lethal levels in top predators.

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Adaptations are features that increase an organism's chance of survival. They can be Structural (e.g., a polar bear's thick layer of fat/blubber), Behavioral (e.g., desert animals being nocturnal), or Physiological (e.g., concentrated urine in camels to conserve H2OH_2O).

📐Formulae

Efficiency of Energy Transfer=Energy available after transferEnergy available before transfer×100\text{Efficiency of Energy Transfer} = \frac{\text{Energy available after transfer}}{\text{Energy available before transfer}} \times 100

6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2

Net Productivity=Gross Productivity−Respiration\text{Net Productivity} = \text{Gross Productivity} - \text{Respiration}

💡Examples

Problem 1:

In a food chain: Grass →\rightarrow Grasshopper →\rightarrow Frog →\rightarrow Snake, the Grass contains 50,000 kJ50,000 \text{ kJ} of energy. Calculate the energy available to the Snake, assuming a 10%10\% efficiency at each level.

Solution:

50 kJ50 \text{ kJ}

Explanation:

Energy at Grasshopper level = 50,000×0.10=5,000 kJ50,000 \times 0.10 = 5,000 \text{ kJ}. Energy at Frog level = 5,000×0.10=500 kJ5,000 \times 0.10 = 500 \text{ kJ}. Energy at Snake level = 500×0.10=50 kJ500 \times 0.10 = 50 \text{ kJ}.

Problem 2:

Explain why a 'Pyramid of Numbers' for an Oak tree supporting thousands of aphids might look 'top-heavy' or inverted.

Solution:

The Oak tree is a single large producer (n=1n=1), while the primary consumers (aphids) are very small and numerous (n>1000n>1000).

Explanation:

A pyramid of numbers only counts individuals. Because one Oak tree has a massive biomass, it can support many smaller organisms, resulting in a narrow base for the first trophic level.

Problem 3:

Identify one structural adaptation of a Xerophyte (desert plant) and explain how it helps the plant survive.

Solution:

Sunken stomata or a thick waxy cuticle.

Explanation:

A thick waxy cuticle reduces the rate of transpiration by providing a waterproof barrier, preventing H2OH_2O loss in high-temperature environments where the vapour pressure deficitvapour \ pressure \ deficit is high.