The Energy Flow: Food Chains and Food Webs
The Energy Flow: Food Chains and Food Webs
At the end of this lesson, you are expected to:
Describe the structure of an energy pyramid.
Explain the 10% energy transfer rule between trophic levels.
Illustrate how energy flows from producers to consumers in an ecosystem.
Imagine you are a tiny ant. Your goal is to reach a delicious crumb of bread on the other side of a big field. Along the way, you encounter a hungry bird that wants to eat you! If the bird eats you, it gets all the energy you stored from the food you ate. If you escape the bird and reach the bread, you get that energy.
Think about this:
What did you eat to get energy to start your journey?
What happens to your energy if the bird catches you?
What happens to the bird's energy if it eats you?
This little story shows how energy moves from one living thing to another. Today, we're going to learn about how this energy transfer works in nature using something called an "energy pyramid."
Welcome, young scientists! Today, we're going on an exciting journey to understand how energy moves through an ecosystem. Think of an ecosystem as a big neighborhood where plants, animals, and even tiny organisms live together and interact. Everything in this neighborhood needs energy to survive, grow, and do all the amazing things living things do!
What is Energy?
Before we dive into energy pyramids, let's quickly remember what energy is. Energy is the ability to do work. For living things, energy helps them move, think, grow, repair themselves, and even stay warm. Where do living things get their energy? Most of the energy on Earth starts with the sun!
The Sun: The Ultimate Energy Source
The sun is like a giant power plant in the sky, constantly sending out light and heat energy. Plants are super special because they can capture this sunlight and turn it into food through a process called photosynthesis. They use sunlight, water, and carbon dioxide from the air to make sugars, which are a form of stored energy. Because plants make their own food using sunlight, we call them producers.
Who Eats Whom? Trophic Levels
Now, imagine a hungry rabbit hopping through a field. What does the rabbit eat? It eats the grass, which is a producer! So, the rabbit gets energy by eating the plant. Animals that eat plants are called primary consumers or herbivores.
What happens if a fox comes along and is hungry? The fox might catch and eat the rabbit. The fox is getting energy from the rabbit. Animals that eat other animals are called consumers. Since the fox eats the rabbit (which ate the plant), the fox is a secondary consumer. If there was a bigger animal, like a lion, that ate the fox, the lion would be a tertiary consumer.
These different levels of eating are called trophic levels. Think of "trophic" as meaning "feeding."
Trophic Level 1: Producers (Plants, algae) - They make their own food using sunlight.
Trophic Level 2: Primary Consumers (Herbivores like rabbits, cows, grasshoppers) - They eat producers.
Trophic Level 3: Secondary Consumers (Carnivores or omnivores like foxes, snakes, birds that eat insects) - They eat primary consumers.
Trophic Level 4: Tertiary Consumers (Carnivores or omnivores like lions, eagles, sharks) - They eat secondary consumers.
And what happens when plants and animals die? There are special organisms called decomposers (like bacteria and fungi) that break down dead plants and animals. They get energy from this process, and in doing so, they return important nutrients back to the soil, which helps producers grow even more!
The Energy Pyramid: A Visual Story of Energy Flow
Now, let's imagine stacking these trophic levels on top of each other to form a pyramid. This is called an ecological pyramid, and specifically, we're focusing on the energy pyramid.
Base of the Pyramid: At the very bottom, forming the widest part of the pyramid, are the producers. They have the most energy because they capture it directly from the sun.
Second Level: Above the producers are the primary consumers (herbivores). They eat the producers.
Third Level: Above them are the secondary consumers (carnivores/omnivores). They eat the primary consumers.
Top Level: At the very top, forming the smallest part of the pyramid, are the tertiary consumers (top carnivores). They eat the secondary consumers.
The 10% Rule: Why the Pyramid Gets Smaller
Here's the most important part of the energy pyramid: The 10% Rule. When energy is transferred from one trophic level to the next, a lot of that energy is lost. Think about it: when a rabbit eats grass, it doesn't get all the energy the grass had.
Why is energy lost?
Metabolic Processes: Organisms use a lot of energy just to live! They use it for breathing, moving, digesting food, keeping their bodies warm, and all the other life processes. This energy is released as heat.
Not Eaten: Not all parts of an organism are eaten by the next level. For example, a rabbit might not eat the tough roots of the grass.
Indigestible Parts: Some parts of the food that are eaten might not be digestible and are passed out as waste.
Because of these losses, only about 10% of the energy from one trophic level is actually stored and available to the next trophic level. The other 90% is used up or lost as heat.
Let's look at an example:
Imagine a field of grass has 1,000,000 kilocalories (kcal) of energy.
Producers (Grass): 1,000,000 kcal
Primary Consumers (Rabbits eating grass): If rabbits eat the grass, they only get about 10% of the grass's energy. So, 10% of 1,000,000 kcal = 100,000 kcal.
Secondary Consumers (Foxes eating rabbits): If a fox eats a rabbit, it only gets about 10% of the rabbit's energy. So, 10% of 100,000 kcal = 10,000 kcal.
Tertiary Consumers (Eagle eating a fox): If an eagle eats a fox, it only gets about 10% of the fox's energy. So, 10% of 10,000 kcal = 1,000 kcal.
See how the amount of energy gets smaller and smaller as you go up the pyramid? This is why there are usually many more producers than primary consumers, and many more primary consumers than secondary consumers, and so on. There simply isn't enough energy to support a large number of organisms at the very top levels.
Example: Grass -> Carabao -> Human
Let's use a Filipino example:
Producers (Trophic Level 1): Grass and other plants in the field. Let's say they have a total of 10,000 units of energy.
Primary Consumers (Trophic Level 2): Carabao (water buffalo) eat the grass. They get about 10% of the grass's energy. So, the carabao get about 1,000 units of energy.
Secondary Consumers (Trophic Level 3): Humans can eat carabao meat. If a human eats the carabao, they get about 10% of the carabao's energy. So, the human gets about 100 units of energy.
Notice how the energy decreases significantly at each step. This explains why you often see vast fields of grass supporting herds of carabao, but only a few humans who rely on them for food.
Why is this important?
Understanding energy pyramids helps us understand:
Food Chains and Food Webs: It shows how energy flows from one organism to another.
Population Sizes: It explains why there are fewer large predators than herbivores or producers.
Ecosystem Balance: It highlights the importance of producers as the foundation of most ecosystems.
4.a. Guided Practice: Build Your Own Energy Pyramid!
Let's practice with a simple food chain. Imagine this:
Sunlight
Algae (Producer)
Small Fish (Primary Consumer)
Larger Fish (Secondary Consumer)
Shark (Tertiary Consumer)
Let's pretend the Algae have 50,000 units of energy. Now, let's calculate how much energy is available at each level using the 10% rule.
Algae (Producers): 50,000 units of energy
Small Fish (Primary Consumers): How much energy do they get from the algae? (Calculate 10% of 50,000)
Answer: 5,000 units of energy
Larger Fish (Secondary Consumers): How much energy do they get from the small fish? (Calculate 10% of your answer from step 2)
Answer: 500 units of energy
Shark (Tertiary Consumers): How much energy does the shark get from the larger fish? (Calculate 10% of your answer from step 3)
Answer: 50 units of energy
Now, draw a pyramid shape. Write "Algae" at the bottom, then "Small Fish," then "Larger Fish," and "Shark" at the top. Write the amount of energy you calculated next to each level. Does your pyramid get smaller as you go up? Yes, it should!
Interactive Activity: Energy Flow Charades!
Let's play a game to act out energy flow!
Instructions:
Write down different organisms from various trophic levels on separate slips of paper (e.g., Grass, Rabbit, Fox, Sun, Bacteria, Deer, Wolf, Algae, Small Fish, Heron).
Divide yourselves into small groups.
One person from a group picks a slip of paper without showing others.
They then have to act out that organism or its role in getting energy (e.g., a rabbit nibbling, a fox stalking, the sun shining, bacteria decomposing).
The rest of the group guesses which organism or trophic level it is.
After guessing, discuss how energy would flow from the previous level to the guessed organism. For example, if they guessed "Rabbit," the group should say, "The rabbit gets energy from the grass (producer)." If they guessed "Fox," they'd say, "The fox gets energy from the rabbit (primary consumer)."
Independent Practice: Food Chain Detective
Look at the following food chains. For each one, identify the trophic level of each organism and draw a simple energy pyramid, showing the approximate energy transfer.
Food Chain 1:
Sun -> Flowers -> Butterfly -> Frog -> Snake
Food Chain 2:
Sun -> Phytoplankton -> Zooplankton -> Small Fish -> Tuna
Instructions:
For each food chain, list the organisms and their trophic levels (Producer, Primary Consumer, Secondary Consumer, etc.).
Draw a pyramid for each food chain.
Assign a starting energy value to the producer (e.g., 10,000 units).
Calculate the energy available at each subsequent level using the 10% rule.
Label each level of your pyramid with the organism and its energy value.
Have you ever wondered why farmers need large areas of land to raise animals like cattle or carabaos? It's all because of the energy pyramid!
Cattle are primary consumers; they eat a lot of grass (producers). To get enough energy to grow and produce milk or meat, a single cow needs to eat a huge amount of grass every day. If a farmer wants to raise 100 cows, they need enough grass to feed all of them. This means they need a very large field or many fields of grass.
Now, think about humans who eat beef. Humans are secondary consumers in this case. Since only about 10% of the energy from the grass is transferred to the cow, and then only about 10% of the cow's energy is transferred to the human, it takes a lot of grass (and therefore a lot of land) to produce a smaller amount of energy for humans. This is why vegetarian diets, which get energy directly from producers, are often more energy-efficient for supporting human populations.
Energy flows through ecosystems in steps called trophic levels.
Producers (like plants) are at the bottom and capture energy from the sun.
Consumers (herbivores, carnivores, omnivores) get energy by eating other organisms.
An energy pyramid visually shows how energy decreases at each higher trophic level.
The 10% Rule states that only about 10% of the energy from one trophic level is transferred to the next; the rest is lost as heat or used for life processes.
This energy loss explains why there are fewer organisms at higher trophic levels.
Observe Your Surroundings: Go outside or look out your window. Can you identify any producers (plants) and consumers (animals)? Try to think about what they might eat and what might eat them. Can you imagine a simple food chain and how energy might flow?
Create a Food Chain Diagram: Choose a local ecosystem (like a park, a pond, or even your backyard). Draw a food chain from that ecosystem, starting with a producer and showing at least three other organisms. Label each organism with its trophic level and draw a simple energy pyramid for that food chain, showing the 10% energy transfer.
Discuss with Family: Talk to your family about energy pyramids and the 10% rule. Explain why there are more insects than birds of prey in a field, or more small fish than sharks in the ocean.
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