Energy transfer between trophic levels

Introduction to Energy Transfer

In an ecosystem, energy is the fuel that keeps everything running. It all starts with the sun, which provides energy for plants (producers) to grow. But as we move from a plant to the rabbit that eats it, and then to the fox that eats the rabbit, something interesting happens: energy is lost at every single step. In this chapter, we will explore why this happens and how to calculate exactly how much energy actually makes it to the next level.

Note: This topic is part of the "Biology Only" (1BI0) curriculum, marked with a "B" in your syllabus. If you are studying Triple Science, this is essential for your Paper 2 exam!

What are Trophic Levels?

A trophic level is simply the position an organism holds in a food chain.
1. Trophic Level 1: Producers (like green plants or algae).
2. Trophic Level 2: Primary consumers (herbivores that eat plants).
3. Trophic Level 3: Secondary consumers (carnivores that eat herbivores).
4. Trophic Level 4: Tertiary consumers (carnivores that eat other carnivores).

Energy Loss Between Levels (Syllabus 9.7B)

Have you ever wondered why food chains are usually quite short? You rarely see a "sixth" or "seventh" consumer. This is because energy is transferred to less useful forms at each trophic level. By the time you get to the top of the chain, there just isn't enough energy left to support another group of animals.

Why is energy lost?

Only about \(10\%\) of the energy in one level is actually passed on to the next. The other \(90\%\) is "lost" to the environment in several ways:
1. Respiration: Organisms use energy to stay alive, move, and keep their body temperature stable (especially mammals and birds). This energy eventually ends up as heat which radiates into the surroundings.
2. Waste Products: Not everything an animal eats is digested. Some energy is lost in faeces (poo) and urea (urine).
3. Uneaten Parts: Predators rarely eat every single bit of their prey. Roots, bones, fur, and feathers contain energy, but if they aren't eaten, that energy doesn't move to the next trophic level. Instead, it is broken down by decomposers (which we cover in the "Cycles" chapter).

Quick Analogy: Imagine you have \(\$100\) (energy from the sun). If you have to pay a \(\$90\) tax at every shop you visit, you won't be able to visit many shops before your money runs out! This "tax" is the energy lost to the environment.

Biomass and Efficiency (Syllabus 9.8B)

Biomass is the total mass of living material in an organism or a particular area. Because energy is lost at each level, the amount of biomass usually decreases as you go up the food chain.

Calculating Efficiency

In your exam, you might be asked to calculate the efficiency of energy transfer. This tells us what percentage of energy actually made it from one level to the next. Don't worry if you find math tricky—just follow this simple formula:

Efficiency \(=\) \((\frac{\text{Energy transferred to next level}}{\text{Energy available at previous level}}) \times 100\)

Step-by-Step Example:

A field of grass contains \(4000 \text{ kJ}\) of energy. The sheep eating the grass only gain \(400 \text{ kJ}\) of energy. What is the efficiency?
1. Identify the energy at the start: \(4000 \text{ kJ}\)
2. Identify the energy transferred: \(400 \text{ kJ}\)
3. Divide them: \(400 \div 4000 = 0.1\)
4. Multiply by \(100\): \(0.1 \times 100 = 10\%\)

The efficiency is \(10\%\).

Did you know?

Because so much energy is lost through movement and heat, farmers sometimes keep animals in temperature-controlled sheds and restrict their movement. This reduces the energy the animals "waste," meaning more energy goes into growing meat or producing milk (increasing the efficiency of food production)!

Common Mistakes to Avoid

1. Mixing up the numbers: When calculating efficiency, always put the smaller number (the one being transferred) on top of the fraction.
2. Forgetting to multiply by 100: Efficiency is always a percentage. If your answer is \(0.1\), you haven't finished yet!
3. Thinking energy is "destroyed": Energy isn't destroyed; it is just converted into less useful forms like heat. Remember: energy can only be transferred, not created or destroyed.

Summary Checklist

Key Takeaways:
- Energy is lost at every trophic level through respiration (heat), movement, and waste.
- Only about \(10\%\) of energy is usually passed on to the next level.
- Efficiency calculations compare the energy in one level to the energy in the level before it.
- Food chains are short because eventually, there isn't enough energy left to support more organisms.

Quick Review Question

If a rabbit eats \(2000 \text{ J}\) of plant material but only \(180 \text{ J}\) is stored in its body tissues, what is the efficiency of this transfer?
(Answer: \((\frac{180}{2000}) \times 100 = 9\%\))