19.2 Food Chains and Food Webs Revision Notes
1. Overview
This topic explores how energy flows through ecosystems, starting from the sun and moving through various organisms. Understanding food chains and webs allows us to predict how changes to one population—whether through human activity or natural shifts—can impact an entire biological community.
Key Definitions
- Producer: An organism that makes its own organic nutrients (usually glucose), usually using energy from sunlight through photosynthesis.
- Consumer: An organism that gets its energy by feeding on other organisms.
- Herbivore: An animal that gets its energy by eating plants.
- Carnivore: An animal that gets its energy by eating other animals.
- Decomposer: An organism that gets its energy from dead or waste organic material (e.g., bacteria and fungi).
- Food Chain: A chart showing the transfer of energy from one organism to the next, beginning with a producer.
- Food Web: A network of interconnected food chains showing the complex feeding relationships in an ecosystem.
- Trophic Level: The position of an organism in a food chain, food web, or ecological pyramid.
- Biomass: The total mass of living material in a specific area or trophic level.
Core Content
The Food Chain
A food chain is a linear sequence that shows how energy is transferred.
- Direction of Arrows: The arrows in a food chain represent the flow of energy. They point from the organism being eaten to the organism that eats it.
- Structure:
- Producer $\rightarrow$ Primary Consumer $\rightarrow$ Secondary Consumer $\rightarrow$ Tertiary Consumer $\rightarrow$ Quaternary Consumer.
- A linear sequence starting with Grass (Producer), followed by an arrow pointing to a Grasshopper (Primary Consumer), an arrow to a Frog (Secondary Consumer), and an arrow to a Hawk (Tertiary Consumer).
Producers and Consumers
- Producers are the foundation of almost all food chains. They convert light energy into chemical energy (carbohydrates) via photosynthesis.
- Consumers are categorized by what they eat and their position:
- Primary consumers are herbivores (eat producers).
- Secondary consumers are carnivores (eat primary consumers).
- Tertiary/Quaternary consumers are usually top predators.
Food Webs
In the wild, most animals eat more than one type of food. A food web is a more realistic model of an ecosystem.
- Interdependence: If one species is removed (e.g., through overhunting), it affects many others.
- Human Impact:
- Overharvesting: Removing too many of a species (e.g., overfishing) can cause their prey to overpopulate and their predators to starve.
- Invasive Species: Introducing a foreign species can disrupt the web as the new species may have no natural predators but may compete with native species for food.
Ecological Pyramids
- Pyramid of Numbers: Shows the number of individual organisms at each trophic level.
- Shape: Can be "inverted" or irregular. For example, one large oak tree (producer) can support thousands of insects (primary consumers).
- A pyramid with a very narrow base (one tree), a wide middle section (many insects), and a narrower top (few birds).
- Pyramid of Biomass: Shows the total mass of living material at each level.
- Shape: Almost always "pyramid" shaped (widest at the bottom).
- Advantages: More accurate than a pyramid of numbers because it accounts for the size of organisms.
Extended Content (Extended Curriculum Only)
Pyramid of Energy
This shows the total energy available at each trophic level over a specific period (e.g., per year).
- Shape: Always a true pyramid shape; it can never be inverted because energy is lost at every level.
- Advantages: The most accurate representation of energy flow; it accounts for the rate of production and seasonal changes.
Efficiency of Energy Transfer
Energy transfer between trophic levels is inefficient. Usually, only about 10% of the energy is passed to the next level.
- Why is energy lost?
- Respiration: Energy is used for movement and metabolism, eventually lost as heat to the surroundings.
- Excretion/Egestion: Energy remains in waste products (faeces/urine).
- Uneaten parts: Not all of an organism is eaten (e.g., bones, roots, fur).
Food Chain Length
Food chains rarely have more than five trophic levels. This is because there is insufficient energy remaining to support a viable population at a higher level after so much energy has been lost at each previous step.
Human Food Efficiency
It is more energy-efficient for humans to eat crop plants than livestock.
- Short Chain: Sunlight $\rightarrow$ Crop $\rightarrow$ Human (1 energy transfer).
- Long Chain: Sunlight $\rightarrow$ Crop $\rightarrow$ Livestock $\rightarrow$ Human (2 energy transfers).
- By eating plants directly, we eliminate a trophic level and avoid the 90% energy loss associated with raising livestock.
Key Equations
Efficiency of Energy Transfer (%) $$\text{Efficiency} = \left( \frac{\text{Energy available to the next level}}{\text{Energy received from the previous level}} \right) \times 100$$
- Units: Expressed as a percentage (%).
- Note: "Energy" can be substituted with "Biomass" in similar calculations.
Common Mistakes to Avoid
- ❌ Wrong: Drawing arrows pointing to the animal being eaten.
- ✅ Right: Arrows must point in the direction of energy flow (into the mouth of the consumer).
- ❌ Wrong: Thinking a pyramid of numbers must always be a perfect triangle.
- ✅ Right: A pyramid of numbers can be any shape (e.g., a "top-heavy" pyramid if the producer is a single large tree).
- ❌ Wrong: Assuming 100% of energy is transferred between levels.
- ✅ Right: Most energy (approx. 90%) is lost as heat, waste, or unused material.
Exam Tips
- Command Words: If asked to "Construct" a food chain, ensure you start with a producer and use arrows correctly. If asked to "Interpret" a food web, look for how a change in one population affects others "upstream" or "downstream."
- Trophic Levels: Remember that one organism can occupy different trophic levels in a food web depending on which food chain you are following.
- Calculation Questions: You are often asked to calculate the percentage efficiency. Always show your working and remember to multiply by 100 at the end.
- Real-world Context: Be prepared to discuss "Sustainability." Exams often ask why vegetarianism is more sustainable for a growing human population (refer back to the energy efficiency of short food chains).