What is the difference between a food chain and a food web?

A food chain is a linear sequence showing who eats whom, e.g., grass → rabbit → fox. A food web is a network of interconnected food chains, showing the complex feeding relationships in an ecosystem. Food webs are more realistic because most organisms have multiple food sources and predators.

Why is only 10% of energy transferred between trophic levels?

Energy is lost at each trophic level primarily through respiration (used for metabolism and released as heat), as well as through uneaten parts, excretion, and movement. This is known as the 10% law (Lindemann). It explains why there are fewer organisms at higher trophic levels and why food chains rarely exceed 4-5 levels.

How do human activities affect nutrient cycles?

Human activities like deforestation, agriculture, and burning fossil fuels disrupt nutrient cycles. For example, deforestation reduces biomass and soil carbon storage, releasing CO₂. Overuse of fertilisers adds excess nitrogen and phosphorus to waterways, causing eutrophication—algal blooms that deplete oxygen and harm aquatic life.

What is ecological succession and why is it important?

Ecological succession is the gradual change in species composition over time, from pioneer species to a climax community. It is important because it shows how ecosystems develop and recover after disturbances (e.g., volcanic eruptions, fires). Understanding succession helps in conservation and habitat restoration.

What are the main stores and flows in the carbon cycle?

Main stores: atmosphere (CO₂), oceans (dissolved carbon), biomass (living and dead organic matter), soils, and fossil fuels. Main flows: photosynthesis (CO₂ → biomass), respiration (biomass → CO₂), decomposition, combustion, and ocean uptake/release. Human activities like burning fossil fuels add CO₂ to the atmosphere, enhancing the greenhouse effect.

How do abiotic factors affect ecosystem structure?

Abiotic factors like temperature, precipitation, light, soil type, and pH determine which species can survive in an area. For example, low rainfall limits plant growth, leading to deserts; high rainfall supports rainforests. These factors create distinct biomes and influence biodiversity, productivity, and nutrient cycling.

The structure and functioning of ecosystems

WJEC

A-Level

This topic explores the ecosystem concept, focusing on energy flows, nutrient cycling, and the role of limiting factors in primary productivity. It examines the interdependence of abiotic and biotic elements and the impact of human activity on ecosystem structure and functioning.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Ecosystems are dynamic systems where living organisms (biotic components) interact with each other and their non-living environment (abiotic components). This topic explores the structure of ecosystems, including trophic levels, food webs, and nutrient cycles, and how energy flows through them. Understanding these interactions is crucial for grasping how ecosystems function, maintain balance, and respond to changes like human impact or climate change.

In the WJEC A-Level Geography specification, this topic forms the foundation for understanding global ecosystems, biodiversity, and sustainability. You will examine key processes such as photosynthesis, respiration, decomposition, and the cycling of carbon and nitrogen. These concepts are not only central to physical geography but also link to human geography through topics like resource management and environmental degradation.

Mastering this topic enables you to analyse real-world issues such as deforestation, eutrophication, and climate change impacts on ecosystems. It also develops your ability to evaluate ecosystem services, conservation strategies, and the role of feedback loops. By the end, you should be able to explain how energy and matter move through ecosystems and why this matters for planetary health.

Key Concepts

Core ideas you must understand for this topic

→Trophic levels and energy flow: Energy enters ecosystems via photosynthesis (producers) and is transferred through consumers (primary, secondary, tertiary) and decomposers. Only about 10% of energy passes between levels due to respiration and heat loss (Lindemann's 10% law).
→Nutrient cycles: Carbon and nitrogen cycles are essential. Carbon is stored in biomass, soils, oceans, and the atmosphere; processes include photosynthesis, respiration, combustion, and decomposition. Nitrogen fixation by bacteria converts atmospheric N₂ into usable forms for plants.
→Food webs vs. food chains: Food chains show linear energy flow, but real ecosystems are complex webs with multiple interactions. Understanding food webs helps explain ecosystem stability and the impact of species removal.
→Biotic and abiotic factors: Biotic factors include predation, competition, and disease; abiotic factors include temperature, light, water, and soil pH. These interact to determine species distribution and ecosystem dynamics.
→Ecological succession: The process of change in species composition over time, from pioneer species to climax community. Primary succession occurs on bare rock; secondary succession on cleared soil. This links to ecosystem resilience and management.

What You Need to Demonstrate

Key skills and knowledge for this topic

Explanation of the ecosystem concept including energy flows
Comparison of nutrient cycling between two biomes (stores and rates of flow)
Analysis of primary productivity levels linked to limiting factors (temperature, moisture, light, nutrient availability)

Marking Points

Key points examiners look for in your answers

Explanation of the ecosystem concept including energy flows
Comparison of nutrient cycling between two biomes (stores and rates of flow)
Analysis of primary productivity levels linked to limiting factors (temperature, moisture, light, nutrient availability)

Examiner Tips

Expert advice for maximising your marks

💡Ensure you can define and apply the concept of 'limiting factors' to different environments
💡Use specific examples of nutrient stores and flows when comparing biomes
💡Focus on the 'functioning' aspect—how energy and nutrients move through the system rather than just describing the plants and animals
💡Use specific terminology: In exam answers, always use correct terms like 'gross primary productivity (GPP)', 'net primary productivity (NPP)', and 'biomass'. This shows precision and gains marks.
💡Draw and label diagrams: For questions on energy flow or nutrient cycles, a clear, annotated diagram can earn you marks. Include arrows for energy flow and labels for processes like respiration and decomposition.
💡Link to case studies: When discussing human impacts, refer to specific examples like deforestation in the Amazon (affecting carbon cycle) or eutrophication in the Norfolk Broads (nutrient cycle). This demonstrates application of knowledge.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the ecosystem concept with specific biome characteristics
Failing to explicitly link primary productivity to the specific limiting factors mentioned in the specification
Generalizing nutrient cycling without providing comparative detail between two distinct biomes
Misconception: 'Energy is recycled in ecosystems.' Correction: Energy flows one-way through ecosystems and is lost as heat; it is not recycled. Only matter (nutrients) is cycled.
Misconception: 'Food chains are simple and linear.' Correction: In reality, most organisms eat multiple prey and are eaten by multiple predators, forming complex food webs. Simple chains are models, not reality.
Misconception: 'Decomposers are not important.' Correction: Decomposers (bacteria, fungi) are vital for breaking down dead matter and returning nutrients to the soil, enabling nutrient cycling. Without them, ecosystems would collapse.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Basic understanding of photosynthesis and respiration from GCSE Biology or Geography.
•Familiarity with the concept of systems (inputs, outputs, stores, flows) as used in physical geography.
•Knowledge of the water cycle and carbon cycle at a foundational level.

Likely Command Words

How questions on this topic are typically asked

Explain

Compare

Analyze

Assess

Ready to test yourself?

Practice questions tailored to this topic

The structure and functioning of ecosystems

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in WJEC A-Level Geography

Aeolian, fluvial and biotic processes, the characteristics and formation of landforms in coastal environments

Biodiversity under threat

Carbon stores in different biomes

Catchment hydrology – the drainage basin as a system

Topic Synopsis

Key Concepts & Core Principles

Exam Tips & Revision Strategies

Common Misconceptions & Mistakes to Avoid

Examiner Marking Points

The structure and functioning of ecosystems

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between a food chain and a food web?

Why is only 10% of energy transferred between trophic levels?

How do human activities affect nutrient cycles?

What is ecological succession and why is it important?

What are the main stores and flows in the carbon cycle?

How do abiotic factors affect ecosystem structure?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in WJEC A-Level Geography

Aeolian, fluvial and biotic processes, the characteristics and formation of landforms in coastal environments

Biodiversity under threat

Carbon stores in different biomes

Catchment hydrology – the drainage basin as a system