Trophic levels (AQA GCSE Biology): Revision Notes
Trophic levels
What are trophic levels?
Trophic levels show the different feeding positions in a food chain. They represent the energy flow through ecosystems, starting at level 1 with the producers and progressing up to level 4.
The four trophic levels are:
- Level 1: Producers - plants and algae that make their own food
- Level 2: Primary consumers - herbivores that eat plants and algae
- Level 3: Secondary consumers - carnivores that eat herbivores
- Level 4: Tertiary consumers - carnivores that eat other carnivores (apex predators with no predators)
Each trophic level depends on the one below it for energy. This creates a hierarchical structure where energy flows in one direction - from producers up through the various consumer levels.
Pyramids of biomass
Biomass means the mass of living material in an organism. Scientists often measure dry mass - this is the mass without any water, which gives a more accurate representation of the actual living tissue.
A pyramid of biomass shows how much biomass exists at each trophic level. Understanding why these pyramids get smaller at each level is crucial for comprehending energy flow in ecosystems.
Why pyramids of biomass get smaller at each level:
- Energy is used for respiration in all living organisms
- Not all parts of organisms can be digested and absorbed
- Energy is lost as heat and waste products like urine
Key facts about energy transfer:
- Only about 1% of light energy from the sun gets turned into food by plants through photosynthesis
- Only about 10% of biomass passes from one trophic level to the next
- The rest is lost through respiration, waste, and heat
Efficiency calculations
You can work out the efficiency of energy transfer between trophic levels using mathematical calculations. This helps scientists understand how efficiently energy moves through food chains.
Worked Example: Calculating Energy Transfer Efficiency
If 60g of caterpillars becomes 12g of thrush:
Step 1: Apply the efficiency formula
Step 2: Substitute the values
Step 3: Convert to percentage
So the efficiency is 20%.
Numbers at trophic levels
The number of organisms at each trophic level depends on two key factors that work together to determine population sizes within ecosystems.
The number of organisms at each trophic level depends on:
- The biomass of each individual organism
- The total biomass of that trophic level
For the same total biomass, there will be more small organisms than large organisms. This relationship explains the structure we observe in natural ecosystems.
Understanding organism numbers:
There might be thousands of tiny plants but only one large predator at the top. This happens because larger organisms require more energy to sustain themselves, so fewer can be supported at higher trophic levels.
Key Points to Remember:
- Trophic levels show feeding positions in food chains (producers → primary → secondary → tertiary consumers)
- Biomass decreases at each level due to respiration, waste, and heat loss
- Only 10% of biomass typically passes to the next trophic level
- Pyramids of biomass show the relative amounts at each level - they get smaller going up
- Numbers depend on size - more small organisms than large ones for the same biomass