Ecology and the Environment

Ecosystems and Food Webs — Revision Notes

Key Definitions and Terminology

• **Ecosystem**: A community of living organisms (biotic) interacting with each other and with their non-living (abiotic) environment in a particular area.
• **Community**: All the populations of different species living and interacting in the same area at the same time.
• **Population**: All the organisms of the same species living in a particular area at a particular time.
• **Habitat**: The place where an organism lives, which provides the conditions and resources it needs to survive.
• **Producer**: An organism that makes its own food by photosynthesis (e.g., green plants and algae); the starting point of all food chains.
• **Trophic level**: The position an organism occupies in a food chain or food web (e.g., producer = first trophic level, primary consumer = second trophic level).

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Main Concepts

1. Structure of Food Chains

• A **food chain** shows the transfer of energy from one organism to the next in a linear sequence.
• Arrows in a food chain point in the direction of **energy transfer** (i.e., from the organism being eaten to the organism doing the eating).
• Example: `Grass → Rabbit → Fox → Eagle`
• Each step along the chain represents a change in trophic level.

2. Consumer Levels

• **Primary consumers** (herbivores) feed directly on producers (e.g., grasshopper eating grass).
• **Secondary consumers** (carnivores or omnivores) feed on primary consumers (e.g., frog eating a grasshopper).
• **Tertiary consumers** feed on secondary consumers and are often **top predators** (e.g., hawk eating a frog).

3. Food Webs

• A **food web** is a network of interconnected food chains in an ecosystem, showing the complex feeding relationships between organisms.
• Food webs are more realistic than single food chains because most organisms feed on more than one species and are eaten by more than one predator.
• Removing or reducing one species in a food web can have **knock-on effects** on multiple other populations (see worked examples below).

4. Interdependence Within Ecosystems

• Organisms in a food web are **interdependent** — they rely on each other for food, and changes in one population affect others.
• A decrease in a prey species leads to increased competition among its predators, causing their numbers to fall.
• A decrease in a predator species leads to an increase in its prey, which may then cause overgrazing or increased competition at lower trophic levels.

5. Decomposers in Ecosystems

• **Decomposers** (bacteria and fungi) break down dead organisms and waste material, recycling nutrients back into the soil for producers to absorb.
• Although not always drawn into food chains, decomposers are essential for **nutrient cycling** within ecosystems.

6. The Role of Abiotic and Biotic Factors

• **Abiotic factors** (e.g., light intensity, temperature, water availability) affect the distribution and abundance of producers, which in turn affects the entire food web.
• **Biotic factors** (e.g., predation, competition, disease) directly influence population sizes within the food web.

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Worked Examples and Real-World Applications

Example 1: Predicting the Effects of Population Change in a Food Web

Consider this simplified food web from a grassland ecosystem:

```

Grass

/ \

Rabbit Grasshopper

| |

Fox Frog

\ /

Hawk

```

Question: What would happen if a disease killed most of the rabbits?

Answer:

• The **fox** population would **decrease** due to less food (less prey available).
• The **grass** population would **increase** because fewer rabbits are feeding on it.
• The **hawk** population may **decrease slightly** (less fox prey available) or it may feed **more on frogs**, causing the **frog population to decrease** and the **grasshopper population to increase** as a knock-on effect.

*This type of question requires you to trace through multiple pathways in the web.*

Example 2: A Marine Food Web

```

Phytoplankton → Zooplankton → Small fish → Mackerel → Tuna → Shark

\→ Seabird

```

• **Phytoplankton** are the producers (they photosynthesise in the ocean surface).
• If **overfishing** removes large numbers of mackerel, the small fish population may **increase** (less predation), while tuna and seabird populations may **decrease** (less food).
• This real-world example demonstrates why **sustainable fishing** is important for maintaining stable marine food webs.

Example 3: Importance of Producers

• In any ecosystem, **producers** support all other trophic levels. If light intensity decreases (e.g., due to heavy cloud cover or shading), producers photosynthesise less, grow less, and the entire food web is affected from the bottom up.

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Exam Technique Tips (Edexcel-Specific)

Tip 1: Arrow Direction Matters

> When drawing or interpreting food chains and food webs, always ensure arrows point from the food source to the feeder (i.e., in the direction of energy flow). A common error is reversing arrows. Edexcel mark schemes will not award marks for food chains with arrows pointing in the wrong direction, even if the organisms are listed correctly.

Tip 2: Use the Correct Cause-and-Effect Chain for "Explain" Questions

> When asked to explain the effect of a population change in a food web, Edexcel mark schemes require a logical chain of reasoning. Structure your answer as:

>

> 1. State the direct effect (e.g., "There is less prey available for the fox").

> 2. State the consequence (e.g., "So the fox population decreases due to increased competition for food / less food available").

> 3. Follow the knock-on effect to other organisms if asked.

>

> Simply stating "fox numbers go down" without explaining why will often lose you a mark. Always link cause → effect explicitly.

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*Remember: Food webs test your ability to think about multiple interacting relationships — practise tracing effects through several pathways, and always justify your reasoning with reference to feeding relationships and competition.*