Ch 2 · Food Chains and Food Webs · O Level Biology 5090
Topic 2 of 4 in Organisms and their Environment — O Level Biology 5090 — 48 lessons.
Introduction
Food chains and food webs map who eats whom in an ecosystem, and pyramids of numbers and biomass summarise them. Questions are predictable: define the terms, pick out organisms from a web, predict what happens when one is removed, and draw or criticise a pyramid.
Key ideas — Food chains and the key terms
A food chain shows the transfer of energy from one organism to the next, beginning with a producer. Arrows point in the direction of energy flow, from the eaten to the eater:
🧑🏫 Teacher's explanation
Think of energy like money in an economy. A producer (like a plant) is like a factory that creates new money using sunlight. When a consumer eats that plant, they are 'spending' that energy to live. However, just like when you exchange currency and lose a little bit to transaction fees, energy is lost at every step. You don't get 100\% of the energy from your lunch; much is lost as heat or through waste.
📖 Definitions
🔍 Worked example — Easy
A food chain is represented as: Algae arrow Small fish arrow Large fish. Identify the trophic level of the 'Small fish'.
🔍 Worked example — Standard
In a field, there are 500 blades of grass, 50 grasshoppers, and 5 frogs. Calculate the percentage decrease in the number of organisms from the producer level to the secondary consumer level.
🔍 Worked example — Exam-style
A food web contains: Grass arrow Rabbit arrow Fox and Grass arrow Mouse arrow Owl. If the population of mice is removed due to disease, explain the effect on the owl population.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Food webs and what happens when they change
A food web is a network of interconnected food chains. Most animals eat more than one food, so an organism can sit at different trophic levels in different chains.
🧑🏫 Teacher's explanation
Think of a food web like a complex spider web or a social network of energy. In a single food chain, energy flows in one direction, like a one-way street. But in nature, most animals are not picky eaters; they eat multiple things. This creates a web where everything is connected. If you pull on one thread of the web, the whole structure vibrates and changes.
📖 Definitions
🔍 Worked example — Easy
In a food web, a grasshopper eats grass, and a frog eats grasshoppers. If the population of frogs decreases significantly due to disease, describe the effect on the grasshopper population and the grass population.
🔍 Worked example — Standard
A farmer introduces a new species of bird into a field. This bird eats the same insects that the local birds eat. Predict the effect on the local bird population.
🔍 Worked example — Exam-style
Calculate the percentage decrease in biomass if the biomass at Trophic Level 1 is 10,000 kg and the biomass at Trophic Level 2 is 1,000 kg.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Pyramids of numbers and biomass
Ecological pyramids show the trophic levels of one chain as horizontal bars, producer at the bottom, each bar's width proportional to the quantity it represents.
🧑🏫 Teacher's explanation
Think of an ecological pyramid like a budget spreadsheet for a company. A 'Pyramid of Numbers' is like counting the number of employees: you might have one CEO (the producer) and 500 interns (the consumers). The spreadsheet looks huge at the bottom, but that doesn't tell you how much actual work or 'value' is being done. A 'Pyramid of Biomass' is like looking at the total salary paid out at each level. Even if there is only one CEO, their 'mass' of influence/salary is huge compared to an intern.
📖 Definitions
🔍 Worked example — Easy
A food chain consists of: Grass arrow Grasshopper arrow Frog arrow Snake. If there are 500 grasshoppers, 50 frogs, and 5 snakes, sketch the shape of a pyramid of numbers.
🔍 Worked example — Standard
In a woodland ecosystem, the biomass of producers is 10,000 kg/ha, the biomass of herbivores is 1,000 kg/ha, and the biomass of carnivores is 100 kg/ha. Calculate the percentage of biomass transferred from the producer level to the herbivore level.
🔍 Worked example — Exam-style
An ecosystem has 1 large Oak tree (biomass 500 kg) and 2,000 caterpillars (biomass 0.1 kg each). Compare the shape of a pyramid of numbers with a pyramid of biomass for this food chain.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Inefficient transfer and human impacts
Energy transfer and how humans disturb food webs. Only about 10 % of energy passes from one trophic level to the next, because not all of the organism is eaten, some of what is eaten is egested undigested, some energy is lost in excretion, and most is released by respiration as heat (lesson 24.19.1). Each level therefore supports a smaller biomass than the one below.
🧑🏫 Teacher's explanation
Think of energy like money in a business. If a company starts with \1000 but spends \900 on rent, electricity, and staff wages, they only have \100 left to reinvest. In biology, 'rent' is the energy used for life processes like movement and respiration.
📖 Definitions
🔍 Worked example — Easy
A producer level contains 50,000 kJ of energy. If only 10\% of this energy is transferred to the primary consumer, calculate the energy available to the primary consumer.
🔍 Worked example — Standard
Explain why a pyramid of numbers for a single large tree supporting many insects might look 'inverted' (narrower at the bottom) even though the tree is the producer.
🔍 Worked example — Exam-style
A food web contains a sequence: Algae arrow Small Fish arrow Large Fish arrow Shark. If humans overharvest the Large Fish, predict the effect on the population of Small Fish and Algae.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. In a pond, pondweed is eaten by water snails and tadpoles; snails and tadpoles are eaten by small fish; small fish are eaten by herons. A disease kills most of the small fish.
🔍 Worked example — Easy
Identify the trophic level of a hawk in the following food chain: Grass arrow Grasshopper arrow Frog arrow Snake arrow Hawk.
🔍 Worked example — Standard
Calculate the percentage of energy transferred from a producer to a primary consumer if the producer contains 10,000 kJ of energy and the consumer contains 1,000 kJ of energy.
🔍 Worked example — Exam-style
In a field, there are 500 blades of grass, 50 grasshoppers, and 5 birds. Calculate the percentage change in the number of grasshoppers if the number of grasshoppers increases to 75.
⚠️ Common errors
Common mistakes
Practice
Answers