Welcome to Food Chains and Food Webs!
Hello future Biologists! This chapter is all about who eats whom and how energy flows through the natural world. Understanding food chains and webs is crucial because it shows us how all living things are connected. If one part of the system changes, the whole ecosystem feels the impact!
Don't worry if these terms seem confusing at first. We will break down these essential ecology concepts step-by-step.
1. Core Concepts: Chains, Webs, and Levels
1.1 What is a Food Chain? (Core)
A food chain shows the transfer of energy from one organism to the next, starting with the organism that produces its own food.
- It represents a straight line of feeding relationships.
- The arrow (→) means "is eaten by" or "energy is transferred to".
- Rule: The arrow always points from the organism that is eaten to the organism that eats it.
Example: Grass → Deer → Wolf
1.2 Trophic Levels: Position Matters (Core)
A trophic level is simply the position an organism holds in a food chain or food web. Think of it like a rank in the feeding hierarchy.
- Level 1: Producers (always the start)
- Level 2: Primary Consumers
- Level 3: Secondary Consumers
- Level 4: Tertiary Consumers
- Level 5: Quaternary Consumers
1.3 Food Webs: Interconnected Chains (Core)
Real ecosystems are never as simple as a single food chain. Most organisms eat, and are eaten by, multiple other organisms.
A food web is a network of interconnected food chains. It gives a much more accurate picture of the complex feeding relationships in a habitat.
Key Takeaway for Section 1
A food chain is a single path of energy transfer. A food web is many interconnected food chains. Trophic levels describe the organism's position in this transfer.
2. The Key Players in Energy Flow
2.1 Producers (Core)
A producer is an organism that makes its own organic nutrients (food), usually using energy from sunlight through the process of photosynthesis.
- They are the foundation of almost all food chains.
- Examples: Grass, trees, algae.
2.2 Consumers (Core)
A consumer is an organism that gets its energy by feeding on other organisms.
Consumers are categorized by what they eat:
-
Primary Consumers (Herbivores):
- They are at Trophic Level 2.
- A herbivore is an animal that gets its energy by eating only plants (producers).
- Examples: Rabbit, caterpillar, deer.
-
Secondary Consumers (Carnivores/Omnivores):
- They are at Trophic Level 3.
- They feed on primary consumers (herbivores).
- A carnivore is an animal that gets its energy by eating other animals.
- Examples: Fox eating a rabbit, spider eating an insect.
-
Tertiary Consumers:
- They are at Trophic Level 4.
- They feed on secondary consumers.
- Example: An eagle eating a snake (which ate a frog, which ate an insect).
Did you know? An organism like a human or a bear that eats both plants and animals is called an omnivore.
2.3 Decomposers (Core)
A decomposer is an organism that gets its energy from dead or waste organic material (like fallen leaves, dead bodies, or faeces).
- They play a crucial role by recycling nutrients back into the soil, which producers can then use.
- Examples: Bacteria and Fungi.
3. Visualising Ecosystems: Ecological Pyramids
We use diagrams called ecological pyramids to represent the amount of energy or material present at each trophic level.
3.1 Pyramids of Numbers (Core)
A pyramid of numbers shows the actual number of individual organisms found at each trophic level.
- The producer level is always at the bottom.
- Generally, the numbers decrease as you move up the chain.
⚠️ Common Issue: Irregular Shapes
Sometimes, a pyramid of numbers can look unusual (inverted or irregular). This happens when one large producer supports many small consumers.
Example: A single large oak tree (Producer) supports thousands of caterpillars (Primary Consumers).
3.2 Pyramids of Biomass (Core)
Biomass refers to the dry mass (total living material) of the organisms at a specific trophic level, measured in units like g m⁻² or kg m⁻².
A pyramid of biomass shows the biomass available at each trophic level.
- Advantage: This type of pyramid is usually a normal pyramid shape (wide at the bottom, narrow at the top), as it accounts for the actual size and mass of the organisms, not just how many there are.
- It gives a better representation of the energy available at each level compared to a pyramid of numbers.
Quick Review: Numbers vs. Biomass
If you see a wide top or thin bottom, it's probably a pyramid of numbers (e.g., one huge tree supporting many insects). Pyramids of biomass are generally always pyramid-shaped because the total mass must decrease going up the levels.
4. Energy Transfer (Supplement/Extended Content)
4.1 Pyramids of Energy (Supplement)
The most accurate way to represent energy flow is using a pyramid of energy. This shows the energy content (usually measured in kJ m⁻² year⁻¹) at each trophic level.
- Crucial Fact: Pyramids of energy must always be pyramid-shaped, as energy is always lost between levels and cannot be created.
- Advantage: This is the best way to represent energy flow because it shows how the total amount of energy available for the next level decreases consistently.
4.2 Inefficient Energy Transfer (Supplement)
Why do these pyramids get narrower? The transfer of energy from one trophic level to the next is highly inefficient (often only about 10% of the energy is transferred).
Where does the rest of the energy go? It is lost to the environment in several ways:
- Not consumed: The consumer doesn't eat the entire organism (e.g., bones, roots, fur).
- Faeces/Waste: Some energy is lost in undigested food (faeces) which is passed out.
- Respiration (Heat Loss): All organisms respire to release energy for life processes (like movement and growth). A large amount of this energy is lost to the environment as heat.
- Movement: Energy is used for finding food, escaping predators, or moving around.
Think of it like money: if you earn $100, you don't keep all $100. Some goes to taxes (respiration/heat), some to bills (movement), and some is wasted (uneaten parts). Only a small amount is left to pass on to the next generation!
4.3 Why Food Chains Are Short (Supplement)
Since so much energy is lost (90%) at each transfer, the amount of energy available drastically decreases after just a few trophic levels.
Food chains usually have fewer than five trophic levels because, by the fifth level (quaternary consumer), there is simply not enough energy left to support a large enough population to survive.
5. Humans and Environmental Impact
5.1 Human Impact on Food Chains (Core)
Humans often interfere with food webs, which can have significant negative consequences:
- Overharvesting/Overfishing: When we take too many organisms (like fish) from a trophic level, the population drops severely. This affects the organisms that rely on it for food (predators) and those it normally consumes (prey).
- Introducing Foreign Species: Bringing a new species into a habitat (an invasive species) can disrupt the existing food web, as the new species might have no natural predators, or it might consume native species too quickly.
5.2 Energy Efficiency in Food Production (Supplement)
This concept relates directly to energy loss. It explains why farming plants is generally a more efficient use of land and resources than farming livestock.
The Efficiency Argument (Why Eat Plants?):
- If humans eat crop plants (Producers/Trophic Level 1), they receive energy directly from the lowest level.
- If humans eat livestock (Primary Consumers/Trophic Level 2), that livestock first had to eat the crop plants. A massive amount of energy (about 90%) is lost as heat and waste during the transfer from the crop to the livestock.
Conclusion: It is more energy efficient for humans to feed directly on crop plants rather than eating the livestock that have been fed on those same crop plants, because fewer stages of energy loss occur.
Analogy: It is cheaper to buy ingredients and make a pizza (Trophic Level 1) than it is to buy a highly processed gourmet meal (Trophic Level 4) because fewer steps were needed.
Key Takeaway for Section 5
Ecosystems thrive on complex webs. Human interference (like overharvesting) can collapse these webs. To feed a growing population efficiently, we should focus on shorter food chains (eating lower trophic levels).