Ecology

Ecosystems and Biomes: Structure and Energy Flow

5th Year · 6th Year (Leaving Cert)

✓By the end of this lesson students will be able to define and distinguish between abiotic and biotic factors within an ecosystem.
✓By the end of this lesson students will be able to explain the concept of trophic levels and describe the flow of energy through an ecosystem.
✓By the end of this lesson students will be able to apply the 10% rule to calculate energy transfer between trophic levels and construct energy pyramids.
✓By the end of this lesson students will be able to understand the significance of energy loss at each trophic level and its impact on ecosystem structure.

Key concepts

Ecosystem

A community of living organisms (biotic factors) interacting with their non-living environment (abiotic factors) in a particular area.

Biotic Factors

The living or once-living components of an ecosystem. Examples include plants, animals, fungi, bacteria, and protists.

Abiotic Factors

The non-living physical and chemical components of an ecosystem. Examples include temperature, light intensity, water availability, pH, soil type, and oxygen levels.

Trophic Level

The position an organism occupies in a food chain. Organisms at the same trophic level obtain energy from the same part of the food chain.

Producers (Autotrophs)

Organisms that produce their own food, typically through photosynthesis, converting light energy into chemical energy. They form the base of all food chains (e.g., plants, algae).

Consumers (Heterotrophs)

Organisms that obtain energy by feeding on other organisms. They are categorised by what they eat: Primary Consumers (herbivores), Secondary Consumers (carnivores/omnivores that eat primary consumers), Tertiary Consumers (carnivores/omnivores that eat secondary consumers).

Decomposers

Organisms (e.g., bacteria, fungi) that break down dead organic matter, recycling nutrients back into the ecosystem. They are crucial for nutrient cycling but are not typically assigned a trophic level in energy flow diagrams.

Energy Flow

The unidirectional movement of energy through an ecosystem, typically from producers to consumers. Energy is lost at each transfer, primarily as heat.

10% Rule

On average, only about 10% of the energy from one trophic level is transferred to the next trophic level. The remaining 90% is lost, primarily as heat during metabolic processes (respiration), or is not consumed, or not assimilated (e.g., excreted).

Energy Pyramid

A graphical representation showing the amount of energy at each trophic level in an ecosystem. It is always broad at the base (producers) and narrows at successive levels, reflecting the significant energy loss at each transfer.

Key facts to remember

1Ecosystems are composed of interacting biotic (living) and abiotic (non-living) components.
2Producers (autotrophs) form the base of all food chains and webs, converting light energy into chemical energy.
3Energy flows unidirectionally through an ecosystem, from producers to consumers, and is not recycled.
4Trophic levels represent an organism's feeding position in a food chain, starting with producers at the first trophic level.
5The 10% rule states that only about 10% of energy is transferred between successive trophic levels.
6Approximately 90% of energy is lost at each trophic transfer, mainly as heat during respiration, or due to unconsumed/undigested biomass.
7Energy pyramids are always upright because the amount of energy decreases at each higher trophic level.
8Decomposers play a crucial role in nutrient cycling by breaking down dead organic matter and returning essential nutrients to the soil or water.

Worked examples

Example 1

If producers in a grassland ecosystem contain 75,000 kJ of energy, how much energy would be available to secondary consumers?

IIdentify the trophic levels involved: Producers → Primary Consumers → Secondary Consumers.

IIApply the 10% rule from Producers to Primary Consumers: Energy for Primary Consumers = 75,000 kJ × 0.10 = 7,500 kJ.

IIIApply the 10% rule from Primary Consumers to Secondary Consumers: Energy for Secondary Consumers = 7,500 kJ × 0.10 = 750 kJ.

Answer

750 kJ

Remember to apply the 10% rule for each step up the food chain.

Example 2

A tertiary consumer in a marine food chain has 15 kJ of energy available. Assuming the 10% rule applies, calculate the amount of energy that was present at the producer level.

IIdentify the trophic levels involved: Producers → Primary Consumers → Secondary Consumers → Tertiary Consumers.

IIWork backwards from Tertiary Consumers to Secondary Consumers: Energy for Secondary Consumers = 15 kJ / 0.10 = 150 kJ.

IIIWork backwards from Secondary Consumers to Primary Consumers: Energy for Primary Consumers = 150 kJ / 0.10 = 1,500 kJ.

IVWork backwards from Primary Consumers to Producers: Energy for Producers = 1,500 kJ / 0.10 = 15,000 kJ.

Answer

15,000 kJ

When working backwards, divide by 0.10 (or multiply by 10) for each step down the food chain.

Example 3

Explain why an energy pyramid is always upright, even though pyramids of numbers or biomass can sometimes be inverted.

IState the 10% rule: Only approximately 10% of the energy from one trophic level is transferred to the next.

IIExplain the fate of the lost energy: The remaining 90% of energy is lost at each transfer, primarily as heat during metabolic processes (e.g., respiration), or is contained in uneaten parts, or undigested waste products.

IIIRelate energy loss to pyramid shape: This significant and consistent energy loss means that each successive trophic level must contain substantially less total energy than the level below it.

IVConclude: Therefore, the producer level (base) will always represent the largest amount of energy, and the amount of energy decreases progressively at higher trophic levels, resulting in an upright pyramid shape.

Answer

An energy pyramid is always upright because energy is lost at each successive trophic level, primarily as heat during metabolic activities and through unconsumed or undigested matter. According to the 10% rule, only about 10% of the energy from one trophic level is transferred to the next. This significant energy loss means that the total energy available decreases substantially at higher trophic levels, ensuring the producer level always has the greatest amount of energy, forming the wide base of the pyramid.

Common mistakes

✗Confusing biotic and abiotic factors (e.g., listing sunlight as a biotic factor or animals as abiotic).
✗Assuming 100% energy transfer between trophic levels, or using an incorrect percentage for the 10% rule.
✗Not explaining *why* energy is lost at each trophic level (e.g., simply stating 'energy is lost' without mentioning heat, respiration, or unconsumed parts).
✗Drawing inverted energy pyramids, which is incorrect for energy flow.
✗Mixing up the definitions of food chains (linear) and food webs (interconnected chains).

Exam tips

★Clearly define and provide specific examples for both biotic and abiotic factors relevant to a given ecosystem.
★When calculating energy transfer, always apply the 10% rule consistently. For each step up a food chain, multiply by 0.10; for each step down, divide by 0.10.
★Be prepared to explain the implications of the 10% rule, such as why food chains are typically short and why top carnivores are less numerous.
★Practice drawing and labelling energy pyramids accurately, ensuring the base is widest and each subsequent level is narrower to reflect energy loss.

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