Introduction to Energy Flow and Food Webs

Welcome to one of the most exciting parts of Environmental Management! Think of an ecosystem as a giant, living machine. For any machine to work, it needs energy. In nature, this energy starts with the Sun and flows through different organisms. In this chapter, we will look at how animals and plants are connected, how energy is lost along the way, and how we can draw diagrams to show these relationships.

Section 1: Who’s Who in the Ecosystem?

Before we look at how energy moves, we need to understand the "cast of characters" in an ecosystem. Here are the key terms you need to know:

  • Habitat: The specific place where an organism lives (e.g., a pond or a desert).
  • Species: A group of organisms that can reproduce to produce fertile offspring.
  • Population: All the members of the same species living in the same area at the same time.
  • Community: All the different populations of various species living and interacting in an area.
  • Niche: The "role" or "job" an organism has in its environment (e.g., what it eats, where it sleeps).

Biotic Interactions (Living Relationships)

Organisms don't live alone; they interact constantly. In this chapter, we focus on:

  • Predator: An animal that hunts, kills, and eats other animals.
  • Prey: An animal that is hunted and eaten by a predator.
  • Apex Predator: The "top dog" (or cat, or shark!) at the very end of a food chain that has no natural predators.
  • Competition: When organisms struggle for the same resources, like food, water, or space.

Note: For more details on the chemical side of things, like how plants make food, see our chapter on "Photosynthesis, respiration and the carbon cycle".

Section 2: Food Chains and Food Webs

Energy enters most ecosystems through sunlight. Plants (producers) capture this energy, and then it is passed on when animals eat them.

The Food Chain

A food chain is a simple diagram showing "who eats whom." Each stage in a food chain is called a trophic level.

Example: Grass \(\rightarrow\) Grasshopper \(\rightarrow\) Frog \(\rightarrow\) Snake

  • Trophic Level 1: Producers (Plants).
  • Trophic Level 2: Primary Consumers (Herbivores that eat plants).
  • Trophic Level 3: Secondary Consumers (Carnivores that eat herbivores).
  • Trophic Level 4: Tertiary Consumers (Carnivores that eat other carnivores).

Important Exam Tip: Always draw the arrows in the direction of energy flow. The arrow points from the "eaten" to the "eater." Think of it as the arrow saying "the energy goes into..."

Food Webs

In the real world, a snake doesn't just eat frogs; it might eat mice or small birds too. A food web is a network of many interconnected food chains. It is more realistic because it shows that most animals have more than one food source.

Section 3: Energy Flow and the \( 10\% \) Rule

Have you ever wondered why you don't see food chains with 20 different animals? It’s because energy is lost at every step. Not all the energy a cow gets from eating grass goes into making the cow bigger.

The \( 10\% \) Rule

On average, only about \( 10\% \) of the energy from one trophic level is passed on to the next. This means \( 90\% \) of the energy is "lost" to the environment.

Where does the other \( 90\% \) go?
  • Respiration: Energy used for movement, staying warm, and keeping the body running.
  • Heat: Energy lost to the surroundings as metabolic heat.
  • Waste: Energy trapped in undigested food (feces) or urine.
  • Uneaten parts: Parts of the organism that aren't eaten, like bones, roots, or fur.

Quick Math Example:
If a field of clover contains \( 1000 \text{ units} \) of energy:
- The rabbits eating the clover only receive \( 100 \text{ units} \) (\( 10\% \)).
- The foxes eating the rabbits only receive \( 10 \text{ units} \) (\( 10\% \) of \( 100 \)).
- The eagle eating the fox only receives \( 1 \text{ unit} \) (\( 10\% \) of \( 10 \)).
This is why top predators are usually quite rare!

Section 4: Ecological Pyramids

We use pyramids to represent the structure of an ecosystem visually. There are two main types you need to know for your exam:

1. Pyramid of Numbers

This shows the total number of individual organisms at each trophic level.
- Pros: Easy to count organisms.
- Cons: They aren't always "pyramid" shaped!
Example: One single large oak tree (Producer) can support thousands of caterpillars (Primary Consumers). In this case, the bottom of the pyramid would be a very thin bar, and the next level would be very wide. This is called an inverted pyramid.

2. Pyramid of Energy

This shows the total amount of energy available at each trophic level over a period of time.
- Rule: Pyramids of energy are ALWAYS pyramid-shaped (wider at the bottom, narrow at the top).
- This is because, as we learned with the \( 10\% \) rule, energy is always lost as you move up. You can never have more energy in the predators than you have in the producers!

Key Takeaways for Revision

  • Energy source: The Sun provides the initial energy for almost all life on Earth.
  • Arrows: In food chains/webs, arrows show the direction of energy flow (from prey to predator).
  • Efficiency: Only about \( 10\% \) of energy is passed to the next level; the rest is lost mainly as heat from respiration.
  • Trophic levels: Usually limited to 4 or 5 levels because there isn't enough energy left to support a higher level.
  • Pyramids: Energy pyramids are always true pyramids; number pyramids can be irregular shapes depending on the size of the organisms.

Common Mistakes to Avoid

  • Wrong Arrow Direction: Don't point the arrow to what is being eaten. Point it to the animal doing the eating!
  • Forgetting Heat Loss: In exam questions asking why food chains are short, always mention energy loss through respiration and heat.
  • Confusing Community and Population: Remember, Population = Particular species; Community = Collection of all species.