A food web is a set of connected feeding relationships. To answer food web questions, first decide what each arrow means, then follow only the links actually supplied. A convincing explanation connects organisms through food or predation and says what remains uncertain. It does not simply announce that every population must rise or fall.
This lesson uses two original, simplified ecosystems and constructed energy values. They are learning models, not field measurements. Species names stand for the organisms described in each record; the web is not a complete account of everything those organisms might eat in nature.
Start with one arrow
In this lesson, an arrow points from the food to the consumer. Grass → rabbit means that a rabbit eats grass and obtains chemical energy from it. The arrow does not point toward the organism doing the eating in a written sentence such as “the hawk eats the rabbit” until that sentence has been translated into rabbit → hawk.
A producer makes organic food from inorganic materials using an energy source. The grass in our model captures light energy through photosynthesis. A primary consumer eats a producer; a secondary consumer eats a primary consumer. A food chain is one continuous route through a web, whereas a web joins several routes. Count the feeding steps along the selected route before assigning a consumer level.
Worked example 1: turn a feeding record into a web
A constructed grassland record contains eight organisms: grass, grasshopper, field mouse, rabbit, frog, small bird, snake and hawk. Grasshoppers and rabbits feed on grass. Field mice feed on grass seeds and grasshoppers. Frogs and small birds eat grasshoppers. Snakes eat field mice and frogs. Hawks eat rabbits, field mice, small birds and snakes. Grass seeds are part of the grass producer, not a ninth organism.
Complete text alternative: Grass → grasshopper, field mouse and rabbit. Grasshopper → field mouse, frog and small bird. Field mouse → snake and hawk. Rabbit → hawk. Frog → snake. Small bird → hawk. Snake → hawk. Those twelve arrows are the entire supplied web.
Check the diagram against the record one consumer at a time. The field mouse needs two incoming food arrows. The hawk needs four. The frog has an incoming arrow from grasshopper and an outgoing arrow to snake. Do not add frog → hawk merely because some hawks eat frogs elsewhere; this question is about the stated record.
The grass has no incoming feeding arrow because it is the producer in this grazing web. That does not mean grass needs no resources. It requires light and suitable water, carbon dioxide and mineral supplies, none of which is a prey organism in this diagram.
Worked example 2: one organism, different chain positions
Write two chains ending in the hawk, one with the hawk as a secondary consumer and one with it as a fourth-order consumer.
Short route: grass → rabbit → hawk. The rabbit is the primary consumer, so the hawk is the secondary consumer. Long route: grass → grasshopper → frog → snake → hawk. Here the grasshopper is primary, frog secondary, snake tertiary and hawk fourth-order.
The hawk has not become a different species. Its position is being described relative to a particular energy pathway. Similarly, the field mouse is a primary consumer along grass → field mouse but a secondary consumer along grass → grasshopper → field mouse. This is why a single fixed trophic label can be misleading for an omnivore.
A valid chain must follow arrow direction continuously. Grass → rabbit → snake → hawk is invalid in this supplied model because rabbit → snake is absent. A chain can be ecologically imaginable and still fail to represent the information given.
Worked example 3: competition and a disturbance
Identify two organisms that could compete for grasshoppers, then predict one possible consequence of a large fall in frog numbers.
The field mouse, frog and small bird all consume grasshoppers. Any pair could compete for that shared food if it is limiting. Sharing a resource indicates potential competition; it does not prove strong competition when the resource is abundant. A predator and its prey are not automatically competitors for the prey itself.
If frog numbers fall, fewer grasshoppers may be eaten by frogs. Grasshopper numbers may initially rise if other influences remain similar. More grasshoppers could increase grazing on grass and provide extra food for small birds or field mice. This is a plausible indirect pathway, not a guaranteed sequence.
There is also a competing pathway. Snakes lose one food source when frogs decline and might eat more field mice. If mouse numbers fall, their consumption of grasshoppers may decrease. Alternatively, snakes may decline themselves or move away. The web alone gives feeding connections, not feeding rates, reproductive rates or the ability to switch prey.
A strong answer therefore names a direct change, gives its mechanism and adds a relevant condition. For example: “Grasshoppers may increase because frog predation decreases, provided extra predation by birds or mice does not offset the release.” “The whole ecosystem collapses” goes beyond the evidence.
Worked example 4: calculate energy-transfer efficiency
The table gives constructed annual production values for successive trophic levels in a separate simplified grazing system. Each value is chemical energy incorporated into new biomass per square metre per year. The values refer to matching areas and periods and do not describe the individual eight-organism web above.
| Trophic level | Production (kJ m⁻² year⁻¹) |
|---|---|
| Producers | 15,000 |
| Primary consumers | 1,800 |
| Secondary consumers | 216 |
| Tertiary consumers | 27 |
Transfer efficiency = production at the receiving level ÷ production at the supplying level × 100%. Producer-to-primary efficiency = 1,800 ÷ 15,000 × 100 = 12%. Primary-to-secondary efficiency = 216 ÷ 1,800 × 100 = 12%. Secondary-to-tertiary efficiency = 27 ÷ 216 × 100 = 12.5%.
The overall producer-to-tertiary proportion is 27 ÷ 15,000 × 100 = 0.18%. It is not the sum of the three percentages. Using fractions gives the same check: 0.12 × 0.12 × 0.125 = 0.0018, equivalent to 0.18%.
Do not insert 10% unless a question supplies it as an assumption. Not all biomass is eaten, not all eaten material is assimilated, and organisms use energy in respiration, with energy ultimately dissipating as heat. Waste and dead material can enter detrital pathways, so energy absent from the next grazing level has not all instantly become heat at that particular transfer.
Know what the diagram cannot tell you
A food web without numbers cannot establish population size, total biomass or the exact energy transferred. One hawk symbol does not mean one hawk lives in the habitat. Likewise, more incoming arrows do not prove that an organism has more biomass or receives more energy than another.
Energy flow and nutrient cycling should be separated. Decomposers make nutrients available again as they break down organic material, but energy is not recycled indefinitely through the web. Decomposers receive material from many levels; they are not simply a final animal after the top predator. They are omitted from the small diagram to keep its commissioned feeding task readable.
Three practice tasks with solved answers
Use this different constructed pond record for the first two tasks: algae are eaten by zooplankton and snails; small fish eat zooplankton; ducks eat snails and small fish; herons eat small fish. The complete web is algae → zooplankton → small fish → heron, together with algae → snails → ducks and small fish → ducks.
Practice 1: follow two routes
Write a chain in which the duck is a secondary consumer and another in which it is a tertiary consumer.
Answer: Algae → snails → duck makes the duck secondary. Algae → zooplankton → small fish → duck makes it tertiary. Both routes use supplied links in the correct direction.
Practice 2: identify a competitor and qualify a prediction
Which organism shares small fish with the duck? What may happen to small fish if herons decline?
Answer: The heron is a potential competitor. Small fish may increase because heron predation decreases, provided duck predation, food availability, disease and other influences do not offset this effect. The web alone cannot give the size of the increase.
Practice 3: use the stated energy values
A supplied pond model gives primary-consumer production of 2,400 kJ m⁻² year⁻¹ and secondary-consumer production of 360 kJ m⁻² year⁻¹. Find the transfer efficiency and the difference between these values.
Answer: Efficiency = 360 ÷ 2,400 × 100 = 15%. Difference = 2,400 − 360 = 2,040 kJ m⁻² year⁻¹. This difference does not by itself separate uneaten material, waste and respiratory energy transfers.
Frequently asked questions
Which way should a food-web arrow point?
From the organism being eaten to the organism consuming it. The arrow represents transfer of chemical energy in food.
Can an organism have more than one trophic level?
Yes. Its level can differ between feeding routes. An omnivore may eat both producers and other consumers.
Does sharing food always mean strong competition?
No. It shows potential competition. Its strength depends partly on how limiting the shared resource is.
Will prey always increase when a predator declines?
No. That is one plausible effect, but other predators, food supply, disease and migration can change the outcome.
Is energy-transfer efficiency always ten percent?
No. Calculate it from the supplied compatible values. Ten percent is only a simplified assumption when a question specifies it.
Do decomposers recycle energy?
They help recycle nutrients. Energy passes through organisms and is eventually dissipated as heat; it is not recycled indefinitely.
Can I read population sizes from an unnumbered food web?
No. Organism symbols and arrow counts describe feeding connections, not the number or biomass of organisms.
Sources and further reading
These references support the scientific background. Examples, datasets, diagrams and practice answers on this page are original. Reference links are for background reading, not instructions to perform a practical activity.
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