Producers and Consumers in Food Chain
Definition
Food chain
A food chain is a linear sequence that shows how organic matter and energy flow from one organism to another.
- It begins with primary producers, which create their own carbon compounds through photosynthesis, and continues through consumers that obtain carbon compounds by consuming producers or other consumers.
- In each step of the food chain, energy is transferred, but it is lost as heat due to the inefficiency of energy transfer.
Producers
Definition
Producers
Producers, also known as autotrophs, are organisms that can produce their own food using photosynthesis (or chemosynthesis in some cases).
- All food chains begin with producers.
- Common producers include green plants, algae, and cyanobacteria, which convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆) and oxygen (O₂) using sunlight as the energy source.
- These producers form the first trophic level, supporting all other organisms in an ecosystem by providing the initial source of chemical energy.
Example
- Plants: Trees, grass, and other green plants absorb sunlight to produce glucose.
- Algae: Marine and freshwater algae perform photosynthesis and provide the primary energy source for many aquatic ecosystems.
Consumers
Definition
Consumer
Consumers are organisms that obtain energy by feeding on other organisms.
1. Primary Consumers (Herbivores)
- Primary consumers feed directly on producers.
- These organisms obtain their carbon compounds by consuming plants or algae.
- Primary consumers are the second trophic level in a food chain.
Example
- Cows and sheep (herbivores) consume grass.
- Zooplankton consume phytoplankton in aquatic ecosystems.
2. Secondary Consumers (Carnivores)
- Secondary consumers are carnivores or omnivores that obtain their carbon compounds by consuming other animals.
- Secondary consumers are the third trophic level in a food chain.
Example
- Lions (carnivores) that prey on herbivores like zebras.
- Frogs that eat insects (primary consumers).
3. Tertiary Consumers
- Tertiary consumers are predators that feed on secondary consumers.
- They are typically top predators in an ecosystem, and they often have few or no natural predators themselves
- Tertiary consumers occupy the fourth trophic level and are often the top of the food chain.
Example
- Eagles that prey on secondary consumers like fish or small mammals.
- Orcas (killer whales) hunt seals and other marine mammals.
Note
- Primary consumers (herbivores) feed on producers.
- Secondary consumers (carnivores/omnivores) feed on primary consumers.
- Tertiary consumers feed on secondary consumers.
- Apex predators sit at the top with no natural predators
Carbon Compounds Through the Chain
- Organic matter (carbon-based compounds) flows from producers to consumers.
- At each level, carbon compounds are:
- Consumed for energy and growth.
- Respired as carbon dioxide through cellular respiration.
- Transferred to higher levels through feeding.
- This flow maintains the carbon cycle and supports energy transfer through the ecosystem.
Trophic Levels in a Food Chain
Definition
Trophic level
A trophic level is the feeding position of an organism within a food chain or web, based on how it obtains its energy and nutrients.
- A trophic level is the position an organism occupies in a food chain based on its feeding role.
- Each level represents a step in the flow of energy and matter through an ecosystem.
- The main trophic levels are:
- Producers (autotrophs) are the first trophic level, converting solar energy into chemical energy via photosynthesis.
- Primary consumers (herbivores) are the second trophic level, feeding on producers.
- Secondary consumers (carnivores) are the third trophic level, feeding on primary consumers.
- Tertiary consumers (top predators) are at higher trophic levels and feed on secondary consumers.
Note
As energy and carbon compounds move through these trophic levels, energy is transferred, but some is lost at each stage, primarily as heat.
Exam technique
The November 2021 Question 1(a)(i) tested understanding of energy flow in a food chain.
Follow the path of energy:
- Grass (producer) → Zebra (primary consumer) → Lion (secondary consumer).
- The zebra is at the second trophic level because it gets energy directly from producers.
Tip: Trophic level ≠ food chain position by order. It’s the energy transfer position starting from the producer.
Decomposers in Energy Flow
Definition
Decomposers
Decomposers, such as fungi and bacteria, are organisms that break down organic matter into simpler substances.
- Decomposers (bacteria and fungi) break down dead organisms into simple inorganic substances, releasing nutrients back into the soil or water.
- Though not traditionally shown in linear food chains, they play a vital role in energy transformations and nutrient recycling.
- Decomposers feed at all trophic levels, ensuring continuous energy flow within the ecosystem.
Example
- Fungi decompose dead wood, releasing nitrogen and phosphorus for plants.
- Bacteria break down animal remains into ammonia, nitrates, and carbon dioxide.
Energy Flow and Matter Cycling in Food Webs
- Producers capture solar energy and convert it into chemical energy in the form of glucose.
- Consumers (herbivores, carnivores) consume producers or other consumers, transferring energy from one trophic level to the next.
- Decomposers break down dead organisms and waste, transforming the organic matter into simpler compounds, which can be used by producers to make new organic matter.
Note
- By recycling nutrients, decomposers contribute to the efficiency of the food web, ensuring the continued flow of energy and the recycling of carbon.
- This makes ecosystems more resilient and sustainable.
Energy and Matter Transfer in Food Chains
- In a food chain, energy and organic matter are transferred from one trophic level to the next.
- However, energy transfer is never 100% efficient, and there are significant losses at each step due to a variety of factors.
- These losses occur as energy and organic matter are transformed and processed by organisms at each level of the food chain.
The Inefficiency of Energy Transfer - The 10% Rule
- The transfer of energy between trophic levels is never 100% efficient.
- On average, only 10% of energy is passed on to the next trophic level.
- This is known as the 10% rule of ecological efficiency.
- The rest of the energy is lost at each step due to various biological processes.
Types of Energy Losses
- Unharvested Biomass: Not all available food is consumed (e.g., roots, bark, bones remain unused).
- Incomplete Consumption: Some parts of food (like cellulose or chitin) cannot be digested and are egested as waste.
- Incomplete Absorption: Not all digested nutrients are absorbed by the consumer’s body.
- Respiratory Losses: A significant portion of absorbed energy is used for cellular respiration, released as heat energy.
- Movement and Maintenance Costs: Energy is spent on locomotion, maintaining body temperature, and reproduction.
Example
A rabbit consumes 1000 kJ of energy in grass, but only about 100 kJ becomes available to the fox that eats it.
Ecological Efficiency and Limitations
- The inefficiency of energy transfer limits the number of trophic levels in most ecosystems (usually 4–5).
- Higher trophic levels require large areas and biomass to sustain their energy needs.
- Top predators are fewer because energy availability limits population size.
Example
A lion population is much smaller than antelope or zebra populations because of the low energy transfer efficiency from prey to predator.
The Second Law of Thermodynamics and Energy Loss
- According to the second law of thermodynamics, every energy transformation increases entropy (disorder), meaning some energy is always lost as heat.
- Ecosystems are open systems and require continuous input of solar energy to sustain themselves.
- Eventually, all energy leaves the system as heat, emphasizing that energy flows, but matter cycles.
Exam technique
In the November 2021 exam (Question 1a iii), the question asked how the second law of thermodynamics applies to a savanna food chain.
- When answering, don’t just write “energy is lost as heat.”
- The command term explain means you need to describe how the law applies.
Strong answer example:
As energy flows from grass to zebra to lion, some is converted to heat at each step. Because energy transformations are never 100% efficient, usable energy decreases at higher trophic levels.
Tip: Always connect the law to energy transfer and loss between trophic levels.
Theory of Knowledge
How might the inefficiency of energy transfer influence human decisions about agriculture and food production? Consider the ethical and environmental implications of these choices.
Self review
- Explain why producers are the foundation of all food chains.
- Define a trophic level and describe how energy passes through them.
- Describe the role of decomposers in energy flow within ecosystems.
- List four ways energy is lost at each trophic level.
- Why are food chains rarely longer than four or five trophic levels?
