Ecosystems and Food Webs

# Ecosystems and Food Webs ## Learning Objectives By the end of this lesson, you will be able to: - Define ecosystems and identify their biotic and abiotic components - Understand the flow of energy through food chains and food webs - Construct and interpret food webs showing feeding relationships - Explain the roles of producers, consumers, and decomposers in ecosystems - Predict the effects of population changes on food webs ## Introduction Imagine walking through a forest. You notice birds singing in the trees, insects buzzing around flowers, and fungi growing on fallen logs. What you're observing is an **ecosystem** – a complex community of living organisms interacting with each other and their physical environment. Every organism, from the tiniest bacterium to the largest tree, plays a vital role in maintaining the balance of this natural system. Understanding how energy and nutrients flow through ecosystems is crucial for comprehending how life on Earth is sustained. In every ecosystem, organisms depend on each other for survival through feeding relationships. These relationships form intricate networks called food webs, which show us who eats whom and how energy transfers from one organism to another. In this lesson, we'll explore how ecosystems function, examine the different roles organisms play, and learn to construct and analyze food webs. This knowledge will help you understand the delicate balance of nature and why protecting biodiversity is so important for our planet's future. ## Key Concepts ### What is an Ecosystem? An **ecosystem** consists of all the living organisms (biotic factors) in a particular area, along with the non-living components (abiotic factors) with which they interact. Ecosystems can be as large as an ocean or as small as a pond. **Biotic factors** include: - Plants, animals, fungi, bacteria, and other microorganisms - All living things that interact within the environment **Abiotic factors** include: - Light intensity, temperature, water availability - Soil composition, pH levels, oxygen concentration - Wind, rainfall, and atmospheric conditions ### Producers, Consumers, and Decomposers Organisms in ecosystems are classified by how they obtain energy: **Producers (Autotrophs)** - Organisms that make their own food through photosynthesis - Examples: plants, algae, and some bacteria - Use sunlight energy to convert carbon dioxide and water into glucose - Form the base of all food chains **Consumers (Heterotrophs)** - Organisms that obtain energy by eating other organisms - **Primary consumers** (herbivores): eat plants (e.g., rabbits, caterpillars, deer) - **Secondary consumers** (carnivores): eat herbivores (e.g., foxes, small birds) - **Tertiary consumers** (top carnivores): eat other carnivores (e.g., hawks, lions) - **Omnivores**: eat both plants and animals (e.g., bears, humans) **Decomposers** - Break down dead organic matter and waste products - Examples: bacteria, fungi, earthworms - Return nutrients to the soil, completing the nutrient cycle - Essential for recycling materials in ecosystems ### Food Chains A **food chain** shows the linear transfer of energy from one organism to another. It starts with a producer and follows a single pathway. Example: Grass → Grasshopper → Frog → Snake → Hawk **Arrows** in food chains show the direction of energy flow (pointing from food source to consumer). Each stage in a food chain is called a **trophic level**. ### Food Webs A **food web** is a more realistic representation of feeding relationships, showing how multiple food chains interconnect. In nature, most organisms eat more than one type of food and are eaten by more than one predator. **Example pond food web:** - Water plants → Pond snails → Ducks - Water plants → Small fish → Large fish → Herons - Water plants → Insect larvae → Small fish → Large fish - Algae → Water fleas → Small fish Notice how organisms appear in multiple chains, creating a complex network of relationships. ### Energy Transfer Energy flows through ecosystems in one direction, from the Sun through producers to various levels of consumers. However, not all energy passes from one trophic level to the next: - Only about **10%** of energy transfers between trophic levels - The remaining 90% is lost as heat through respiration, movement, and waste - This explains why food chains rarely have more than 4-5 trophic levels - There isn't enough energy to support organisms higher up the chain ## Worked Examples ### Example 1: Constructing a Food Web **Question:** Using these organisms from a woodland ecosystem, construct a food web: Oak tree, caterpillar, blue tit (small bird), sparrowhawk, ladybird, aphid, fox, mouse, owl, grass **Solution:** **Step 1:** Identify the producers (plants that photosynthesize) - Oak tree, grass **Step 2:** Identify which animals eat the producers - Caterpillars eat oak tree leaves - Aphids eat oak tree sap - Mice eat grass seeds **Step 3:** Identify secondary consumers - Blue tits eat caterpillars, aphids, and ladybirds - Ladybirds eat aphids **Step 4:** Identify tertiary consumers (top predators) - Sparrowhawks eat blue tits - Owls eat mice - Foxes eat mice **Step 5:** Draw arrows from food source to consumer The completed food web would show these multiple interconnected pathways, demonstrating how organisms depend on each other for survival. ### Example 2: Predicting Population Changes **Question:** In a grassland ecosystem, a disease kills most of the rabbits. Predict what will happen to the populations of grass and foxes. **Solution:** **Step 1:** Identify the food chain - Grass → Rabbits → Foxes **Step 2:** Analyze the effect on grass - Rabbits eat grass (primary consumers) - Fewer rabbits means less grass is eaten - **Grass population will increase** **Step 3:** Analyze the effect on foxes - Foxes eat rabbits (secondary consumers) - Fewer rabbits means less food for foxes - **Fox population will decrease** due to starvation or migration **Step 4:** Long-term effects - Eventually, fewer foxes may allow surviving rabbits to recover, restoring some balance ### Example 3: Identifying Trophic Levels **Question:** Classify each organism in this food chain by its trophic level: Seaweed → Sea urchin → Sea otter → Shark **Solution:** - **Seaweed**: Producer (1st trophic level) - makes food through photosynthesis - **Sea urchin**: Primary consumer (2nd trophic level) - herbivore eating seaweed - **Sea otter**: Secondary consumer (3rd trophic level) - carnivore eating sea urchins - **Shark**: Tertiary consumer (4th trophic level) - top predator eating sea otters ## Practice Questions 1. Draw a food chain with four organisms from a garden ecosystem. Label each organism as producer, primary consumer, secondary consumer, or tertiary consumer. 2. Explain why food chains rarely have more than five trophic levels. 3. In a lake ecosystem, pollution kills most of the water plants. Predict and explain how this would affect the populations of: (a) small fish that eat plants, and (b) large fish that eat small fish. 4. Look at this food web: Grass → Rabbit → Fox; Grass → Mouse → Owl; Grass → Mouse → Fox What would happen if all the owls died? Explain your reasoning. 5. Explain the importance of decomposers in an ecosystem. What would happen if decomposers were removed? ## Summary - **Ecosystems** contain biotic (living) and abiotic (non-living) components that interact with each other - **Producers** make their own food through photosynthesis and form the base of food chains - **Consumers** obtain energy by eating other organisms (herbivores, carnivores, omnivores) - **Decomposers** break down dead matter and recycle nutrients back into the ecosystem - **Food chains** show linear energy transfer; **food webs** show interconnected feeding relationships - Only **10% of energy** transfers between trophic levels, limiting food chain length - Changes to one population affect other organisms in the food web - Understanding food webs helps us predict the effects of environmental changes on ecosystems ## Exam Tips - **Always draw arrows correctly in food chains and food webs** – arrows point FROM the food source TO the consumer (showing energy flow), not the direction the organism faces. This is a common mistake that loses marks. - **When asked to predict population changes**, work through the food web systematically. Consider both immediate effects (direct feeding relationships) and knock-on effects (indirect relationships). Use phrases like "increases because less are eaten" or "decreases due to reduced food supply" to show clear reasoning. - **Remember the 10% energy transfer rule** when explaining why food chains are limited in length. Examiners look for understanding that energy is lost as heat, through movement, and in waste products – not just that "energy is lost."