Cambridge Biology Lower Secondary Review Notes 4th Term (Food
Webs and Decay)
Introduction to Food Chains and Food Webs
What is a Food Chain?
A food chain is a linear sequence of organisms where each organism is eaten by the
next one in the chain. It shows how energy and nutrients move through an
ecosystem.
Example of a Food Chain:
Sun → Grass → Rabbit → Fox
●​ Sun: Provides energy for plants through photosynthesis.
●​ Grass (Producer): Converts sunlight into energy.
●​ Rabbit (Primary Consumer): Eats the grass.
●​ Fox (Secondary Consumer): Eats the rabbit.
Key Terms:
●​ Producers: Organisms that make their own food through photosynthesis (e.g.,
plants, algae).
●​ Consumers: Organisms that eat other organisms for energy.
○​ Herbivores: Eat only plants (e.g., rabbits, deer).
○​ Carnivores: Eat only meat (e.g., lions, hawks).
○​ Omnivores: Eat both plants and animals (e.g., humans, bears).
●​ Decomposers: Break down dead organisms and recycle nutrients (e.g.,
bacteria, fungi).
What is a Food Web?
A food web is a complex network of interconnected food chains. It shows how
different food chains overlap and how energy flows through multiple pathways in an
ecosystem.
Why are Food Webs Important?
Food webs show the interdependence of organisms in an ecosystem. If one species
is removed, it can affect many other species in the web.
The Role of Decomposers
Decomposers, such as bacteria, fungi, and detritivores (e.g., earthworms), break
down dead plants and animals, returning nutrients to the soil. Without decomposers,
dead organisms would pile up, and nutrients would be locked away, preventing new
growth.
Experiment: Build a Food Chain Model
Objective: To understand how energy flows through a food chain.
Materials:
- Index cards
- Markers
- String
- Tape
Instructions:
1. Write the names of different organisms (e.g., sun, grass, rabbit, fox) on index
cards.
2. Arrange the cards in the correct order to form a food chain.
3. Use string to connect the cards, showing the flow of energy.
4. Discuss what happens if one organism is removed from the chain.
Trophic Levels and Energy Flow
What are Trophic Levels?
Trophic levels are the positions organisms occupy in a food chain.
●​ Level 1: Producers (e.g., plants, algae) – Convert sunlight into energy through
photosynthesis.
●​ Level 2: Primary Consumers (e.g., herbivores) – Eat producers.
●​ Level 3: Secondary Consumers (e.g., carnivores) – Eat primary consumers.
●​ Level 4: Tertiary Consumers (e.g., top predators) – Eat secondary consumers.
Energy Flow in Ecosystems
Energy flows from the sun to producers, then to consumers, and finally to
decomposers. However, only about 10% of the energy is passed on to the next
trophic level. The rest is lost as heat or used for the organism’s life processes (e.g.,
movement, growth).
Ecological Pyramids:
- Energy Pyramid: Shows how energy decreases as it moves up the food chain.
- Biomass Pyramid: Shows the total mass of living organisms at each level.
- Numbers Pyramid: Shows the number of organisms at each level.
Food Web Complexity and Interdependence
Interdependence in Food Webs
All organisms in a food web are connected. Removing one species can have a ripple
effect on the entire ecosystem.
Example: If wolves (a keystone species) are removed from an ecosystem, deer
populations may increase, leading to overgrazing and a decline in plant populations.
Keystone Species
A keystone species is a species that has a disproportionately large impact on its
environment relative to its abundance.
Example: Sea otters in kelp forests control sea urchin populations, preventing
overgrazing of kelp.
Experiment: Create a Food Web
Objective: To understand the complexity and interdependence of organisms in a food
web.
Materials:
- String
- Index cards
- Markers
Instructions:
1. Write the names of different organisms on index cards.
2. Arrange the cards on a table and connect them with string to show how they are
interconnected.
3. Remove one organism and discuss how it affects the rest of the web.
Human Impact on Food Webs
Pollution
Chemicals from factories, cars, and agriculture can enter ecosystems, harming
organisms and disrupting food chains.
Example: Pesticides can kill insects, which are food for birds, leading to a decline in
bird populations.
Deforestation
Cutting down forests destroys habitats for many species, reducing biodiversity and
disrupting food webs.
Example: Amazon Rainforest deforestation affects animals like monkeys, jaguars,
and birds.
Overfishing
Removing too many fish from oceans disrupts marine food webs and can lead to the
collapse of fish populations.
Example: Overfishing of cod has led to a decline in cod populations, affecting other
marine species.
Decay and Decomposers
The Process of Decomposition
Decomposition is the process by which dead organisms are broken down into
simpler substances by decomposers.
Role of Decomposers:
●​ Bacteria and Fungi: Break down dead plants and animals, releasing nutrients
like carbon and nitrogen back into the soil.
●​ Detritivores: Organisms like earthworms and beetles that feed on dead
organic matter, speeding up decomposition.
Experiment: Decomposition Observation
Objective: To observe the process of decomposition and the role of decomposers.
Materials:
- Clear plastic bags
- Fruit slices
- Soil
Instructions:
1. Place fruit slices in clear plastic bags with a small amount of soil.
2. Seal the bags and observe them daily for changes.
3. Record observations and discuss the role of decomposers in breaking down the
fruit.
Factors Affecting Decomposition
a.​ Temperature
Warmer temperatures speed up decomposition because decomposers are
more active.
b.​ Moisture
Decomposers need water to survive, so moist environments speed up decay.
c.​ Oxygen
Most decomposers need oxygen to break down organic matter. In
waterlogged or anaerobic (oxygen-free) environments, decomposition slows
down.
Experiment: Decomposition Rate Experiment
Objective: To investigate how different conditions affect the rate of decomposition.
Materials:
- Four containers
- Fruit slices (e.g., banana or apple)
- Water
- Plastic wrap
- Dark box or cupboard
- Labels
Instructions:
1.​ Set Up the Containers:
-​ Container 1: Sealed (Low Oxygen)
Place a few fruit slices in a container and seal it tightly with plastic wrap
to limit oxygen.
-​ Container 2: Exposed to Air (Normal Oxygen)
Place fruit slices in an open container exposed to air.
-​ Container 3: Humid Environment (High Moisture)
Place fruit slices in a container and add a small amount of water to
create a humid environment.
-​ Container 4: Dry Environment (Low Moisture)
Place fruit slices in a dry container and keep it in a dry place (e.g., near
a heater or in a sunny spot).
2.​ Observe and Record:
-​ Check the containers daily for changes in the fruit slices.
-​ Record the rate of decomposition in each container over several days.
3.​ Discussion:
-​ Compare the rate of decomposition in each container.
-​ Discuss how temperature, moisture, and oxygen affect the activity of
decomposers.
Carbon and Nitrogen Cycles
The Carbon Cycle
The carbon cycle is the movement of carbon between the atmosphere, plants,
animals, and decomposers.
Key Processes:
-​ Photosynthesis: Plants absorb CO₂ from the atmosphere and convert it into
glucose.
-​ Respiration: Animals and plants release CO₂ back into the atmosphere when
they breathe.
-​ Decomposition: Decomposers break down dead organisms, releasing CO₂.
-​ Fossil Fuels: Burning coal, oil, and gas releases stored carbon into the
atmosphere, contributing to climate change.
The Nitrogen Cycle
The nitrogen cycle is the movement of nitrogen through the environment.
Key Processes:
-​ Nitrogen Fixation: Bacteria in the soil convert atmospheric nitrogen into forms
plants can use.
-​ Assimilation: Plants absorb nitrogen from the soil and use it to grow.
-​ Consumption: Animals get nitrogen by eating plants.
-​ Decomposition: Decomposers break down dead organisms, returning nitrogen
to the soil.
Experiment: Carbon Cycle Simulation
Objective: To simulate the carbon cycle and observe how plants absorb CO₂.
Materials:
- Clear container (e.g., a large glass jar or aquarium)
- Water
- Baking soda (sodium bicarbonate)
- Vinegar (acetic acid)
- Small plant (e.g., a sprig of Elodea or any aquatic plant)
- pH indicator (e.g., bromothymol blue)
- Stirring rod
Instructions:
1.​ Set Up the Experiment:
-​ Fill the clear container with water.
-​ Add a small amount of baking soda to the water and stir until dissolved. The
baking soda will provide a source of CO₂ in the water.
-​ Add a few drops of pH indicator (bromothymol blue) to the water. The
indicator will turn yellow in the presence of CO₂ (acidic) and blue in the
absence of CO₂ (basic).
2.​ Add the Plant:
-​ Place the small plant (e.g., Elodea) into the container.
-​ Seal the container to prevent CO₂ from escaping.
3.​ Observe and Record:
-​ Observe the color of the water over time. Initially, the water should be yellow,
indicating the presence of CO₂.
-​ As the plant absorbs CO₂ during photosynthesis, the water should gradually
turn blue, indicating a decrease in CO₂ levels.
4. Discussion:
-​ Discuss how plants absorb CO₂ during photosynthesis and how this process
is part of the carbon cycle.
-​ Explain how the pH indicator helps visualize the absorption of CO₂ by the
plant.
Climate Change and Ecosystems
What is Climate Change?
Climate change refers to long-term changes in temperature and weather patterns,
often caused by human activities like burning fossil fuels and deforestation.
Impact on Food Webs
-​ Melting Ice: Polar ecosystems are affected as ice melts, reducing habitats for
animals like polar bears and penguins.
-​ Desertification: Dry regions become even drier, reducing plant growth and
affecting herbivores and carnivores.
-​ Deforestation: Cutting down forests reduces habitats and disrupts food
chains.
Biodiversity and Ecosystem Resilience
What is Biodiversity?
Biodiversity refers to the variety of life in an ecosystem, including the number of
species, their genetic diversity, and the variety of ecosystems.
Ecosystem Resilience
Ecosystem resilience is the ability of an ecosystem to recover from disturbances like
fires, floods, or human activities.
Importance of Biodiversity: High biodiversity makes ecosystems more stable and
resilient. If one species is lost, others can fill its role.
Experiment: Biodiversity Survey
Objective: To observe and record biodiversity in a local ecosystem.
Materials:
- Notebook
- Pencil
- Magnifying glass
Instructions:
1. Visit a Local Ecosystem:
- Choose a local park, garden, or natural area to conduct your survey.
2. Record Observations:
- Observe and record the different types of plants and animals you see.
- Use a magnifying glass to examine small organisms like insects or fungi.
3. Discussion:
- Discuss how biodiversity contributes to ecosystem resilience.
- Compare the biodiversity of different areas (e.g., a forest vs. a grassland).
Sustainable Practices
Conservation
Conservation involves protecting ecosystems and species from harm. This can
include creating protected areas, restoring damaged ecosystems, and protecting
endangered species.
Sustainable Practices
Sustainable practices are actions that reduce human impact on the environment,
such as recycling, reducing pollution, and protecting habitats.
Experiment: Composting Project
Objective: To observe how composting reduces waste and recycles nutrients.
Materials:
- Compost bin
- Kitchen scraps (e.g., fruit peels, vegetable waste)
- Soil
Instructions:
1. Set Up the Compost Bin:
- Start a compost bin with kitchen scraps and soil.
2. Mix Regularly:
- Mix the compost regularly to aerate it and speed up decomposition.
3. Observe and Record:
- Observe how the scraps decompose over time.
- Record changes in the compost, such as the appearance of worms or fungi.
4. Discussion:
- Discuss how composting reduces waste and recycles nutrients back into the soil.
Final Project
Objective:
-​ Create a multimedia project, poster, or model showcasing food webs,
decomposition, and conservation.
Presentation:
-​ Present your project to the class.
-​ Receive feedback from peers and teachers.