AQA GCSE Geography (8035) — Living with the physical environment
Think about your favourite meal—every single ingredient originally got its energy from the sun. This is because all ecosystems rely on producers to capture solar radiation. Through photosynthesis, these green plants and algae convert sunlight into glucose, forming the base of every food chain.
Organisms that cannot make their own food are called consumers. Primary consumers are herbivores that eat producers, such as pond snails. Secondary consumers and tertiary consumers are carnivores or omnivores that eat other animals, with tertiary consumers acting as apex predators.
When plants and animals die, decomposers like bacteria and fungi break down their dead organic matter. This vital process returns nutrients to the soil for producers to use again. Together, these living (biotic) and non-living (abiotic) parts rely on each other, creating a state of interdependence.
A food chain is a simple, linear diagram showing the direct feeding links between producers and consumers. In reality, animals eat more than one thing, so ecosystems are better represented by a food web, which shows multiple interconnected feeding relationships. Each stage in these diagrams is called a trophic level, with producers always sitting at Level 1.
Energy flows in one direction through these trophic levels, but it is highly inefficient. On average, only 10% of energy is passed from one level to the next. The remaining 90% is lost to the environment through respiration (heat), movement, excretion, or because parts like bones and roots are inedible.
Because so much energy is lost at each stage, there is less energy available to support organisms at the top of the food chain. As a result, the total weight of living material, known as biomass, always decreases as you move up the trophic levels.
We can calculate the efficiency of energy transfer between trophic levels using a simple formula.
Question: Grass in an ecosystem contains of energy. The rabbits that eat the grass store of energy in their biomass. Calculate the efficiency of the energy transfer between these two trophic levels.
Step 1: Identify the energy at each trophic level.
Step 2: Substitute the values into the formula.
Step 3: Calculate the final answer.
Unlike energy, which flows completely out of an ecosystem, nutrients must be constantly recycled to ensure survival. Nutrient cycling is the continuous movement of essential minerals between biotic and abiotic components. The Gersmehl Model is a visual framework used to represent this cycle, where the size of circles shows the amount stored and arrow thickness shows the speed of transfer.
This model features three main stores: the biomass (living plants and animals), the litter (dead organic matter on the surface), and the soil. Nutrients move between these stores through transfers like fallout (leaves dropping), decomposition (decaying litter), and uptake (roots absorbing minerals).
Ecosystems also gain and lose nutrients from the outside environment. Inputs arrive via rainwater and weathered rock. Outputs occur when nutrients are washed off the surface (runoff) or washed deep into the ground out of reach of plant roots, a process called leaching.
Different biomes have dramatically different nutrient cycles due to their climates. In a Tropical Rainforest (TRF), the biomass store is huge due to the lush vegetation, but the litter and soil stores are surprisingly small. The hot, wet climate causes rapid decomposition, meaning litter breaks down instantly and nutrients are either quickly taken back up by plants or lost to leaching, leaving behind infertile red soil called latosol.
By contrast, cold environments like the Tundra have a massive litter store. The freezing temperatures severely limit bacterial activity, preventing decomposition. Consequently, the biomass remains extremely small, as plants cannot access enough nutrients from the frozen soil to grow rapidly.
Students often assume Tropical Rainforest soil is incredibly fertile because the forest is so green, but it is actually nutrient-poor (latosol) because nutrients are locked in the biomass or washed away by leaching.
When drawing or interpreting food chains, always ensure the arrows point in the direction of energy flow (away from the organism being eaten and towards the eater).
In food web questions asking about the removal of a species, always trace the 'knock-on effect' through multiple trophic levels to secure top marks (e.g., 'Removing the apex predator increases primary consumers, which overgraze the producers').
If asked to evaluate the impact of deforestation using the Gersmehl model, explain that removing trees destroys the biomass store, which stops litter fallout, meaning the soil loses all nutrients via leaching.
Producer
An organism that converts energy from the environment (mainly sunlight) into sugars (glucose) through photosynthesis.
Solar radiation
Energy emitted by the sun, which is captured by producers to create biomass.
Consumer
An organism that gets its energy by eating other organisms (producers or other consumers).
Primary consumers
Herbivores that eat producers.
Secondary consumers
Carnivores or omnivores that eat primary consumers.
Tertiary consumers
Apex predators at the top of the food chain with no natural predators.
Decomposer
An organism, such as bacteria or fungi, that breaks down dead plant and animal material, returning nutrients to the soil.
Interdependence
The concept that all biotic (living) and abiotic (non-living) parts of an ecosystem rely on each other for survival.
Food chain
A simple linear diagram showing the direct feeding links between producers and consumers.
Food web
A complex diagram showing the multiple interconnected feeding relationships within an ecosystem.
Trophic level
The specific position an organism occupies in a food chain or food web.
Biomass
The total mass or weight of living organisms in a specific area at a given time.
Nutrient cycling
The continuous recycling of essential nutrients (e.g., nitrogen, phosphorus) between living and non-living components of an ecosystem.
Gersmehl Model
A visual framework representing the storage and transfer of nutrients using circles for stores and arrows for flows.
Litter
Dead organic matter on the surface of the ground, such as fallen leaves and animal waste.
Soil
A main store in the nutrient cycle representing nutrients held in the earth.
Decomposition
The process by which dead organic matter is broken down into simpler forms, releasing nutrients back into the soil.
Leaching
The process by which nutrients are washed deep out of the soil by water moving downwards.
Latosol
The nutrient-poor, iron-rich, red soil typically found in Tropical Rainforests.
Put your knowledge into practice — try past paper questions for Geography
Producer
An organism that converts energy from the environment (mainly sunlight) into sugars (glucose) through photosynthesis.
Solar radiation
Energy emitted by the sun, which is captured by producers to create biomass.
Consumer
An organism that gets its energy by eating other organisms (producers or other consumers).
Primary consumers
Herbivores that eat producers.
Secondary consumers
Carnivores or omnivores that eat primary consumers.
Tertiary consumers
Apex predators at the top of the food chain with no natural predators.
Decomposer
An organism, such as bacteria or fungi, that breaks down dead plant and animal material, returning nutrients to the soil.
Interdependence
The concept that all biotic (living) and abiotic (non-living) parts of an ecosystem rely on each other for survival.
Food chain
A simple linear diagram showing the direct feeding links between producers and consumers.
Food web
A complex diagram showing the multiple interconnected feeding relationships within an ecosystem.
Trophic level
The specific position an organism occupies in a food chain or food web.
Biomass
The total mass or weight of living organisms in a specific area at a given time.
Nutrient cycling
The continuous recycling of essential nutrients (e.g., nitrogen, phosphorus) between living and non-living components of an ecosystem.
Gersmehl Model
A visual framework representing the storage and transfer of nutrients using circles for stores and arrows for flows.
Litter
Dead organic matter on the surface of the ground, such as fallen leaves and animal waste.
Soil
A main store in the nutrient cycle representing nutrients held in the earth.
Decomposition
The process by which dead organic matter is broken down into simpler forms, releasing nutrients back into the soil.
Leaching
The process by which nutrients are washed deep out of the soil by water moving downwards.
Latosol
The nutrient-poor, iron-rich, red soil typically found in Tropical Rainforests.
