Energy And Feeding Relationships

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Notes

Transfer of Energy

The **Sun** is the principal source of energy input to biological systems.
Energy flows through living organisms as **light energy** and **chemical energy**.
Energy is transferred from the Sun through food chains and eventually lost to the environment as **heat**.

Food Chains & Food Webs

A **food chain** shows the transfer of energy from one organism to the next, starting with a **producer**.
Arrows in a food chain represent the **transfer of energy** from one trophic level to the next.
Energy is transferred between organisms by **ingestion** (eating).
A **food web** is a network of interconnected food chains, showing more realistic feeding relationships.
Food webs illustrate **interdependence** – a change in one population affects others.
Most population changes result from **human impact** (e.g., overharvesting, introducing foreign species).

Trophic Levels (Extended)

A **trophic level** is the position of an organism in a food chain or web.
**Producers** form the first trophic level.
**Primary consumers** (herbivores) are the second trophic level.
**Secondary consumers** are the third, **tertiary consumers** the fourth, and **quaternary consumers** the fifth.
Food chains rarely exceed 5 trophic levels due to energy loss.

Efficiency of Energy Transfer (Extended)

Energy transfer between trophic levels is **not efficient**; much energy is lost to the environment.
Energy is lost through **movement**, **respiration** (heat), and **excretion** (e.g., urea).
Energy in **inedible parts** (bones, fur, roots) or **undigested waste** (faeces) does not pass to the next level.
Energy loss explains why food chains are rarely more than 5 organisms long.
It is more energy efficient for humans to eat **plants** directly than to eat herbivores.

The Carbon Cycle

Carbon is taken from the atmosphere as **CO₂** by plants during **photosynthesis**.
Carbon is passed to animals and decomposers through **feeding**.
Carbon is returned to the atmosphere as **CO₂** by **respiration** of plants, animals, and microorganisms.
Dead organisms can form **fossil fuels** over millions of years if decomposers are absent.
**Combustion** of fossil fuels releases CO₂ back into the atmosphere.
Human activities (burning fossil fuels, deforestation) increase atmospheric CO₂ levels.

Energy pyramid showing trophic levels and energy loss at each level.

Practice questions

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1.Define the term herbivore.
Easy
2.State the principal source of energy for food chains.
Easy
3.Complete the sentence about energy flow.
Easy
The arrows in a food chain show the transfer $of ____$ from one trophic level to the next.
4.Which of the following is a producer in the food chain: grass → rabbit → fox?
Easy
- Agrass
- Brabbit
- Cfox
- Dsun
5.A food web is a network of interconnected food chains.
Easy
True or false?
6.Name the trophic level of topi in the African grassland food web, given that topi eat star grass and are eaten by cheetahs.
Medium
7.State what the arrows in a food chain represent.
Medium
8.Match each process in the carbon cycle with its description.
Medium
- Photosynthesis
- Respiration
- Combustion
- Takes in carbon dioxide
- Releases carbon dioxide
- Burning of fossil fuels releases carbon dioxide

Slides

Energy in Ecosystems

The Sun is the principal source of energy input to biological systems.
Energy flows through living organisms as light energy from the Sun and chemical energy in organisms.
Energy is eventually transferred to the environment, e.g. as heat.

What is the principal source of energy for most ecosystems?

The Sun
Producers
Consumers
Decomposers

Food Chains

A food chain shows the transfer of energy from one organism to the next, starting with a producer.
Arrows in a food chain represent the transfer of energy from one trophic level to the next.
Energy is transferred from one organism to another by ingestion (eating).

In a food chain, $the ____$ show the transfer of energy from one organism to the next.

Food Webs

A food web is a network of interconnected food chains.
Food webs are more realistic as animals rarely eat just one type of food.
They show interdependence: how a change in one population can affect others.

Which of these are true about food webs?

A food web consists of a single food chain.
Food webs show interdependence between organisms.
A change in one population can affect others in the web.

Trophic Levels (Extended)

A trophic level is the position of an organism in a food chain or food web.
Producers are the first trophic level.
Primary consumers are the second trophic level, secondary consumers the third, and tertiary consumers the fourth.

Which trophic level do primary consumers occupy?

First
Second
Third
Fourth

Efficiency of Energy Transfer (Extended)

Energy transfer between trophic levels is not efficient.
Energy is lost through movement, respiration heat, excretion, and undigested parts.
This explains why food chains are rarely more than 5 organisms long.

Energy is lost at each trophic level through $____ (e.g., heat)$ and undigested waste.

The Carbon Cycle

Carbon is taken out of the atmosphere as CO₂ by photosynthesis in plants.
It is passed on to animals and decomposers by feeding.
It is returned to the atmosphere by respiration (plants, animals, decomposers) and combustion of fossil fuels.

Which process removes carbon dioxide from the atmosphere?

Respiration
Photosynthesis
Combustion
Decomposition

Human Impact on the Carbon Cycle

Burning fossil fuels (combustion) releases CO₂ into the atmosphere.
Deforestation reduces the number of producers that can absorb CO₂.
These activities increase atmospheric CO₂ levels, contributing to climate change.

Which of these human activities increase atmospheric CO₂?

Burning fossil fuels
Planting trees
Deforestation

Energy Loss and Food Chain Length

Only about 10% of energy is passed from one trophic level to the next.
The rest is lost as heat, movement, waste, etc.
This limits the number of trophic levels to usually 4 or 5.

Why are food chains rarely more than 5 organisms long?

There is not enough energy at higher trophic levels
Producers cannot support many consumers
Consumers eat too much
Decomposers break down all energy

Key Terms

Producer: an organism that makes its own food (e.g., plants via photosynthesis).
Herbivore: an animal that eats only plants.
Carnivore: an animal that eats other animals.
Omnivore: an animal that eats both plants and animals.

$A ____ is$ an organism that gets its energy from dead or waste organic material.

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