Food chains and food webs
Building on the concept of photosynthesis, energy enters an ecosystem from the Sun. This energy is captured by producers (autotrophs) and transferred through a series of organisms. The position of an organism in this sequence is called its trophic level.
A food chain shows the linear transfer of energy from one organism to the next. It always begins with a producer. Arrows point in the direction of energy flow (from the eaten to the eater).
Producers are organisms that make their own organic nutrients, usually using energy from sunlight through photosynthesis. They form the first trophic level.
Consumers are organisms that get their energy by feeding on other organisms. They are classified by their position in the food chain:
- Primary consumers eat producers (herbivores).
- Secondary consumers eat primary consumers (carnivores or omnivores).
- Tertiary consumers eat secondary consumers.
- Quaternary consumers eat tertiary consumers.
Herbivores are animals that get their energy by eating plants (producers). Carnivores are animals that get their energy by eating other animals. Decomposers (such as bacteria and fungi) get their energy from dead or waste organic material, recycling nutrients back into the ecosystem.
A food web is a network of interconnected food chains. It shows how different food chains in an ecosystem overlap and link together. Unlike a simple food chain, a food web illustrates that most organisms feed on more than one type of prey and are eaten by more than one type of predator.
Consider a grassland ecosystem. We can construct a food chain as follows:
- Producer: Grass (makes its own food via photosynthesis).
- Primary Consumer: Grasshopper (eats the grass). This is a herbivore.
- Secondary Consumer: Frog (eats the grasshopper). This is a carnivore.
- Tertiary Consumer: Snake (eats the frog). This is a carnivore.
Representation:
\text{Grass} \rightarrow \text{Grasshopper} \rightarrow \text{Frog} \rightarrow \text{Snake}
Note that the arrow points from the grass to the grasshopper because energy flows from the grass to the grasshopper when it is eaten.
Students often draw arrows pointing from the predator to the prey (e.g., Snake → Frog). This is incorrect. The arrow must point in the direction of energy transfer. Since the frog gains energy by eating the snake? No, the snake eats the frog. So the energy flows from Frog to Snake. Correct: Frog → Snake.
Mistake 2: Misidentifying Trophic Levels
Students sometimes forget that producers count as a trophic level. If asked for the trophic level of a primary consumer, do not say 'Level 1'. The producer is Level 1. The primary consumer is Level 2. Similarly, secondary consumers are Level 3.
Tip 1: Overharvesting
When describing overharvesting (e.g., overfishing), state that removing a species reduces its population. This affects organisms at adjacent trophic levels. For example, 'Overfishing of cod (a secondary consumer) reduces the food source for sharks (tertiary consumers), leading to a decline in the shark population.' Conversely, it may cause an increase in the prey population (e.g., smaller fish) due to reduced predation.
Tip 2: Introducing Foreign Species
When describing introduced species, explain that they often lack natural predators in the new habitat. For example, 'Introducing a foreign predator can lead to the decline of native prey species because the native prey has no evolved defenses against it.' Alternatively, if the foreign species is a competitor, state that it outcompetes native species for resources (food/space), leading to a decrease in biodiversity.
Why this works: Examiners look for specific causal links: Action → Population Change → Effect on Other Organisms. Vague statements like 'it harms the environment' are not accepted.
Learning Objectives Covered: LO11, LO12, LO15, LO16, LO17, LO18
Ecological pyramids (or biological pyramids) are graphical representations of the trophic levels in a food chain. There are three main types:
Pyramid of Numbers: Shows the number of organisms at each trophic level.
- Note: This can sometimes be inverted (e.g., one large tree supports many insects). It does not always reflect energy content accurately because it ignores organism size.
Pyramid of Biomass: Shows the total dry mass (biomass) of organisms at each trophic level, usually in g/m^2 or kg/m^2. Dry mass is used to exclude water weight.
- This pyramid is always upright because biomass decreases as you go up trophic levels. It is a more accurate representation of energy storage than numbers.
Pyramid of Energy (Supplement): Shows the energy content (usually in kJ/m^2/year) at each trophic level.
- This pyramid is always upright and is the most accurate representation of energy flow because it accounts for the rate of reproduction and metabolic losses.
Energy transfer between trophic levels is inefficient. Typically, only about 10% of energy is passed on to the next level. The remaining ~90% is lost.
Reasons for Energy Loss (LO17):
- Respiration: Energy is used for metabolic processes and lost as heat.
- Movement: Energy is used for muscle contraction and locomotion.
- Excretion/Egestion: Not all food is digested; energy is lost in waste products (urine/feces).
- Uneaten Parts: Predators do not eat every part of the prey (e.g., bones, fur, roots).
Why Food Chains are Short (LO18):
Because energy decreases significantly at each step, there is insufficient energy to support a large population at higher trophic levels. After 4-5 levels, the remaining energy is too small to sustain a viable population of top predators.
Consider a food chain: Oak Tree → Caterpillar → Robin.
- Pyramid of Numbers: The base (Oak Tree) has 1 organism. The next level (Caterpillars) has 1000 organisms. This pyramid is inverted at the bottom, which looks counter-intuitive for energy flow.
- Pyramid of Biomass: The Oak Tree has a huge dry mass (e.g., 1000 kg). The Caterpillars have less total mass (e.g., 50 kg). The Robin has even less (e.g., 2 kg). This pyramid is upright and correctly reflects that the tree contains more stored energy than the insects.
Advantage of Biomass over Numbers: Biomass accounts for the size of organisms. A single large producer can support many small consumers, so numbers alone are misleading.
Context: When asked to 'discuss the advantages' of one pyramid type over another, you must compare them directly.
Advantage of Pyramid of Energy over Biomass/Numbers (LO16):
- Time Factor: Pyramids of energy are measured over a period of time (e.g., per year). This accounts for organisms with different life spans and reproduction rates. For example, phytoplankton reproduce quickly; their standing biomass might be low at any one moment, but their annual energy production is huge. A pyramid of numbers or biomass taken at a single snapshot might misrepresent this.
- Accuracy: It directly measures the energy available to the next trophic level, making it the most accurate indicator of ecosystem efficiency.
Advantage of Pyramid of Biomass over Numbers (LO12):
- Size Variation: It accounts for the difference in size between organisms. One tree is not equivalent to one insect in terms of energy content. Biomass provides a standardized measure of organic matter.
- Energy is lost at each trophic level due to respiration, movement, and waste (egestion/excretion). 2. Eating the crop plant places humans at the primary consumer level (2nd trophic level), while eating the cow places them at the secondary consumer level (3rd trophic level). 3. Therefore, fewer energy transfers occur when eating plants, resulting in less total energy loss and more available energy for human consumption.
- Four horizontal bars stacked vertically. 2. The bottom bar (Phytoplankton) is the widest. Each subsequent bar (Zooplankton, Small Fish, Shark) must be progressively narrower, forming a pyramid shape. Labels must be correct.