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

A food chain shows a single linear pathway of energy transfer from one organism to another, such as grass → rabbit → fox. A food web is a more realistic representation that shows multiple interconnected food chains, illustrating how organisms may have multiple feeding relationships. Food webs better demonstrate the complexity of ecosystems and the impact of changes in one population on others.

How do you calculate the efficiency of energy transfer between trophic levels?

Efficiency is calculated using the formula: (energy available at the higher trophic level / energy available at the lower trophic level) × 100. For example, if producers have 10,000 kJ of energy and primary consumers have 1,000 kJ, efficiency = (1,000 / 10,000) × 100 = 10%. Always show your working and include units in your answer.

What is the role of decomposers in the carbon cycle?

Decomposers, such as bacteria and fungi, break down dead organic matter (e.g., dead plants and animals) and release carbon dioxide back into the atmosphere through respiration. They also return nutrients to the soil, making them available for producers. Without decomposers, carbon would remain locked in dead matter and the cycle would stop.

Why do pyramids of biomass sometimes appear inverted?

Pyramids of biomass represent the total mass of organisms at each trophic level at a given time. In aquatic ecosystems, the biomass of phytoplankton (producers) can be less than that of zooplankton (primary consumers) because phytoplankton reproduce rapidly and are consumed quickly. This leads to an inverted pyramid, but the energy flow still follows the 10% rule over time.

What is the difference between primary and secondary succession?

Primary succession occurs in areas with no soil, such as bare rock after a volcanic eruption. Pioneer species like lichens colonise, break down rock, and form soil. Secondary succession happens in areas where soil already exists, such as after a fire or deforestation. It is faster because soil and seeds are already present.

How does deforestation affect the carbon cycle?

Deforestation reduces the number of trees available to absorb carbon dioxide through photosynthesis. It also releases stored carbon when trees are burned or decompose. This increases atmospheric CO₂ levels, contributing to the greenhouse effect and climate change. Additionally, deforestation disrupts habitats and reduces biodiversity.

Ecosystems

PEARSON

A-Level

This topic covers nutrient cycles, specifically the carbon and nitrogen cycles. Learners will understand the processes involved and the role of microorganisms in cycling nutrients.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Subtopics in this area

Nutrient Cycles

Energy Flow

Succession

Topic Overview

Ecosystems form a fundamental component of A-Level Biology, exploring the intricate relationships between living organisms and their physical environment. This topic covers the flow of energy through food chains and webs, the cycling of nutrients such as carbon and nitrogen, and the dynamic nature of populations within habitats. Understanding ecosystems is crucial for grasping how biodiversity is maintained and how human activities can disrupt these delicate balances, leading to issues like climate change and habitat loss.

In the Pearson A-Level specification, ecosystems are studied under the theme of 'Energy and Ecosystems' and 'Nutrient Cycles'. You will learn how energy is transferred from producers to consumers, with only about 10% passing between trophic levels due to losses through respiration and waste. The carbon and nitrogen cycles are examined in detail, highlighting the roles of decomposers, bacteria, and human interventions such as fertiliser use and deforestation. This knowledge is not only examinable but also essential for understanding contemporary environmental challenges.

Mastering ecosystems allows you to connect concepts from photosynthesis, respiration, and evolution. It provides a systems-thinking approach that is vital for ecologists, conservationists, and anyone interested in sustainability. By the end of this topic, you should be able to calculate energy transfer efficiencies, interpret pyramids of biomass and numbers, and evaluate the impact of human activities on ecosystem stability.

Key Concepts

Core ideas you must understand for this topic

→Energy flow and trophic levels: Understand that energy enters ecosystems as sunlight, is converted to chemical energy by producers, and passes through trophic levels with only about 10% being transferred to the next level. The rest is lost as heat through respiration or in waste.
→Nutrient cycles: The carbon cycle involves photosynthesis, respiration, decomposition, and combustion; the nitrogen cycle includes nitrogen fixation, nitrification, assimilation, and denitrification. Know the roles of bacteria like Rhizobium and Nitrosomonas.
→Ecological succession: The process of change in species composition over time, from pioneer species to climax community. Primary succession occurs on bare rock, secondary on cleared soil. Understand seral stages and how biodiversity increases.
→Population dynamics: Factors affecting population size, including birth rate, death rate, immigration, and emigration. Carrying capacity and limiting factors (density-dependent and density-independent) are key.
→Measuring biodiversity: Species richness and evenness, using indices like Simpson's Diversity Index. Understand how sampling methods (quadrats, transects) are used to estimate population size and distribution.

What You Need to Demonstrate

Key skills and knowledge for this topic

Describe the stages of the carbon cycle.
Describe the stages of the nitrogen cycle.
Explain the role of microorganisms in decomposition and nitrogen fixation.
Identify human impacts on nutrient cycles.
Define trophic levels and give examples.
Explain why energy transfer between trophic levels is inefficient.
Construct and interpret food chains and food webs.
Describe the role of decomposers in energy flow.

Marking Points

Key points examiners look for in your answers

Describe the stages of the carbon cycle.
Describe the stages of the nitrogen cycle.
Explain the role of microorganisms in decomposition and nitrogen fixation.
Identify human impacts on nutrient cycles.
Define trophic levels and give examples.
Explain why energy transfer between trophic levels is inefficient.
Construct and interpret food chains and food webs.
Describe the role of decomposers in energy flow.
Describe the stages of ecological succession.
Explain the difference between primary and secondary succession.
Identify pioneer species and climax communities.
Give examples of each type.

Examiner Tips

Expert advice for maximising your marks

💡Draw and label diagrams of both cycles.
💡Use mnemonics to remember the order of processes.
💡Understand the difference between aerobic and anaerobic decomposition.
💡Use diagrams to support explanations of energy flow.
💡Remember the 10% rule for energy transfer.
💡Link energy flow to the concept of biomass.
💡Use diagrams to show stages.
💡Mention examples like sand dunes or abandoned farmland.
💡Explain the concept of 'climax community'.
💡When answering questions on energy transfer, always show your working and include units (e.g., kJ m⁻² yr⁻¹). Use the formula: efficiency = (energy transferred to next level / energy received) × 100. Don't forget to account for energy lost in respiration and waste.
💡For nutrient cycles, draw clear diagrams and label processes with specific bacteria names where relevant. For example, in the nitrogen cycle, mention Rhizobium for nitrogen fixation, Nitrosomonas for nitrification, and Pseudomonas for denitrification.
💡In questions about succession, describe the changes in species diversity, biomass, and soil depth over time. Use terms like 'pioneer species', 'climax community', and 'seral stage'. Explain how each stage alters the environment to make it suitable for the next.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing nitrogen fixation with nitrification.
Forgetting that respiration and photosynthesis are key to carbon cycle.
Not mentioning the role of bacteria in the nitrogen cycle.
Confusing energy flow with nutrient cycling.
Assuming energy is recycled within an ecosystem.
Misidentifying producers and consumers in a food chain.
Confusing primary and secondary succession.
Omitting the role of soil formation.
Thinking succession always leads to forest.
Misconception: Energy is recycled in ecosystems. Correction: Energy flows one way through ecosystems and is not recycled; it is lost as heat at each trophic level. Only nutrients are recycled.
Misconception: Decomposers are not important in food chains. Correction: Decomposers (bacteria and fungi) are vital for breaking down dead matter and returning nutrients to the soil, enabling nutrient cycling. They are often omitted from simple food chains but are essential.
Misconception: The 10% rule means exactly 10% of energy is transferred. Correction: The 10% is an average; actual efficiency varies. Also, energy transfer efficiency is calculated as (energy available at trophic level n / energy available at trophic level n-1) × 100.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Photosynthesis: Understanding how producers convert light energy into chemical energy is essential for grasping energy flow.
•Respiration: Knowledge of aerobic and anaerobic respiration helps explain energy losses as heat.
•Cell structure: Familiarity with bacteria and fungi is useful for understanding decomposers and nitrogen-fixing bacteria.

Key Terminology

Essential terms to know

Carbon cycle: photosynthesis, respiration, decomposition, combustion
Nitrogen cycle: nitrogen fixation, nitrification, denitrification
Mycorrhizae and root nodules
Human impact on cycles
Producers, consumers, decomposers
Gross primary productivity (GPP), net primary productivity (NPP)
Energy losses between trophic levels
Ecological pyramids
Pioneer species, climax community
Primary succession on bare rock
Secondary succession after disturbance
Conservation and management

Likely Command Words

How questions on this topic are typically asked

Describe

Explain

Identify

Outline

State

Construct

Interpret

Compare

Ready to test yourself?

Practice questions tailored to this topic

Ecosystems

Subtopics in this area

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Key Terminology

Likely Command Words

Ready to test yourself?

Related Topics in PEARSON A-Level Biology

Biological Molecules

Classification and Biodiversity

Biological Molecules

Cells, Viruses and Reproduction of Living Things

How to Revise Ecosystems — Pearson A-Level Biology

Examiner Tips for Ecosystems

Common Mistakes in Ecosystems

Key Marking Points

Ecosystems

Subtopics in this area

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?

How do you calculate the efficiency of energy transfer between trophic levels?

What is the role of decomposers in the carbon cycle?

Why do pyramids of biomass sometimes appear inverted?

What is the difference between primary and secondary succession?

How does deforestation affect the carbon cycle?

Before You Start

Key Terminology

Likely Command Words

Ready to test yourself?

Related Topics in PEARSON A-Level Biology

Biological Molecules

Classification and Biodiversity

Biological Molecules

Cells, Viruses and Reproduction of Living Things