Question 1

What is the difference between a habitat and an ecosystem?

Accepted Answer

A habitat is the specific place where an organism lives, such as a pond or a forest floor. An ecosystem includes the habitat plus all the living organisms (community) and the non-living factors (abiotic) that interact within that area. So, an ecosystem is a broader concept that encompasses the habitat and the interactions between biotic and abiotic components.

Question 2

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

Accepted Answer

Energy is lost at each trophic level primarily through respiration (as heat), and also through uneaten parts (e.g., bones, fur) and waste products (e.g., faeces). Only the energy stored in biomass that is consumed and assimilated by the next trophic level is transferred. This inefficiency limits the number of trophic levels in a food chain to typically 4 or 5.

Question 3

How do you calculate net primary production (NPP)?

Accepted Answer

Net primary production (NPP) is the amount of energy available to primary consumers after the producers have used some for respiration. It is calculated as NPP = GPP - R, where GPP is gross primary production (total energy fixed by photosynthesis) and R is the energy lost through respiration by producers. NPP represents the biomass gained by producers.

Question 4

What is the role of decomposers in the nitrogen cycle?

Accepted Answer

Decomposers (saprobionts) break down dead organic matter (e.g., dead plants, animals, and waste) into ammonia (NH₃) through a process called ammonification. This ammonia is then available for nitrifying bacteria to convert into nitrites and nitrates, which plants can absorb. Without decomposers, nitrogen would remain locked in organic compounds and not be recycled.

Question 5

What is the difference between primary and secondary succession?

Accepted Answer

Primary succession occurs on newly formed or exposed land with no soil, such as bare rock after a volcanic eruption. It begins with pioneer species like lichens that break down rock to form soil. Secondary succession occurs on land that previously supported vegetation but has been disturbed (e.g., after a fire or farming), where soil already exists. Secondary succession is faster because soil and seed banks are present.

Question 6

How do abiotic factors affect population size?

Accepted Answer

Abiotic factors such as temperature, light intensity, pH, and water availability can limit population size. For example, low temperatures may reduce enzyme activity in organisms, slowing growth and reproduction. These factors determine the carrying capacity of an environment. When abiotic conditions are optimal, populations can grow; when conditions are extreme, populations may decline or go extinct.

Ecosystems

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

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Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

How to Revise Ecosystems — OCR A-Level Biology

Examiner Tips for Ecosystems

Common Mistakes in Ecosystems

Key Marking Points