Trophic levels in an ecosystem

Welcome to Ecology: Trophic Levels!

In this chapter, we are going to explore how energy and "stuff" (which biologists call biomass) move through an ecosystem. Think of it like a relay race where energy is the baton being passed from one runner to the next. However, unlike a real relay race, a lot of that energy gets dropped along the way! Don't worry if this seems a bit technical at first—we will break it down step-by-step.

What are Trophic Levels?

A trophic level is simply a fancy way of saying "feeding level." It describes the position an organism holds in a food chain. We number them starting from the bottom (the source of energy) and move upwards.

The Four Main Levels

Level 1: Producers
These are always at the start. They are plants and algae that make their own food through photosynthesis. They are the "kitchen" of the ecosystem.
Example: Grass, seaweed, or microscopic algae.

Level 2: Primary Consumers
These are herbivores. They get their energy by eating the producers.
Example: A rabbit eating grass or a snail eating a leaf.

Level 3: Secondary Consumers
These are carnivores that eat the herbivores.
Example: A fox eating a rabbit.

Level 4: Tertiary Consumers
These are carnivores that eat other carnivores. If they have no natural predators themselves, we call them apex predators.
Example: An eagle eating a fox or a lion.

The Recyclers: Decomposers

You won't usually see them as a "block" in a food chain, but decomposers (like bacteria and fungi) are vital. They break down dead plant and animal matter. They do this by secreting enzymes into the environment to digest the dead stuff. The small, soluble food molecules then diffuse back into the microorganism. They are the cleanup crew that returns nutrients to the soil!

Quick Review:
1. Level 1 = Producers (Plants)
2. Level 2 = Primary Consumers (Herbivores)
3. Level 3 = Secondary Consumers (Carnivores)
4. Level 4 = Tertiary Consumers (Top Carnivores/Apex Predators)

Key Takeaway: Trophic levels represent the "who-eats-whom" steps in an ecosystem, starting with plants at Level 1.

Pyramids of Biomass

Biomass is the total mass of living material in an organism. A pyramid of biomass is a diagram that shows how much "living stuff" is at each level of a food chain.

The Shape of the Pyramid

In a pyramid of biomass, Level 1 is always at the bottom. As you move up the levels, the bars always get narrower. This is because there is always less biomass at the top of a food chain than at the bottom. It will always look like a perfect pyramid shape!

Did you know?
Even if a tree is huge and there are thousands of tiny insects eating it, the biomass (the actual weight of the wood and leaves) of that one tree is usually much higher than the total weight of all those insects combined!

Key Takeaway: Pyramids of biomass always get thinner as you go up because biomass is lost at every stage of the food chain.

Transfer of Biomass: The 10% Rule

Biologists have noticed something strange: only about 10% of the biomass from one level is actually passed on to the next. This explains why food chains are rarely longer than four or five levels—eventually, there just isn't enough energy left to support another animal.

How is Biomass Lost?

If a cow eats 100kg of grass, it doesn't gain 100kg of body weight. Where does that 90% go? Think of it like a "leaky bucket":

1. Not everything is eaten: Predators rarely eat every single bit of their prey. Bones, fur, and teeth are often left behind.
2. Egestion (Pooping): Not everything eaten can be digested. Some material passes straight through as faeces.
3. Excretion (Peeing): Some absorbed material is lost as waste, such as urea in urine.
4. Respiration: This is the big one! Organisms use a huge amount of glucose in respiration to provide energy for movement and keeping warm. This releases carbon dioxide and water as waste products.

The Producer Starting Point:
Plants and algae are the starting point, but even they are only about 1% efficient. They only transfer about 1% of the light energy from the sun into chemical energy (biomass) through photosynthesis.

Calculating Efficiency

You might be asked to calculate how efficient a transfer is. Don't panic! The formula is quite simple:

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

Common Mistake to Avoid:
Do not confuse egestion with excretion. Egestion is getting rid of undigested food (faeces). Excretion is getting rid of waste products made by your cells (like urea in urine or CO2 from your lungs).

Memory Aid:
To remember why biomass is lost, remember "REAP":
R - Respiration (Energy used for heat/movement)
E - Excretion (Waste like urine)
A - All parts not eaten (Bones/roots)
P - Poo (Egestion of undigested food)

Key Takeaway: Biomass is lost through waste, respiration, and uneaten parts. Usually, only 10% makes it to the next level.

Quick Review Quiz

1. Which trophic level contains herbivores? (Answer: Level 2)
2. Why are pyramids of biomass always pyramid-shaped? (Answer: Because biomass is lost at each level; there is less at the top than the bottom).
3. What percentage of light energy do producers usually transfer? (Answer: About 1%).
4. How do decomposers eat? (Answer: They secrete enzymes to digest matter externally, then the nutrients diffuse in).

Keep practicing these definitions and the "REAP" mnemonic, and you'll be an Ecology expert in no time!