FROM FOOD CHAINS TO FOOD WEBS: WHEN ECOLOGY BECOMES COMPLICATED
A simple food chain is one of the first ecological ideas that students meet in Biology:
Grass → Rabbit → Fox
It is neat, logical and easy to understand. The grass captures energy from sunlight, the rabbit eats the grass, and the fox eats the rabbit.
Unfortunately, nature is rarely that tidy.
A rabbit does not eat only one type of grass. A fox does not survive entirely on rabbits. Grass is eaten by many different animals, while rabbits may be hunted by foxes, birds of prey and even domestic animals.
The moment we begin to add these extra feeding relationships, the simple chain becomes a food web.
This is where school Biology starts to move from an idealised model towards what actually happens in an ecosystem. It also reveals one of the most important lessons in ecology: changing one part of an ecosystem can produce consequences in places we did not expect.
A broken link does not necessarily affect only the species immediately before and after it. The effects can spread throughout the entire food web.
WHY DO WE TEACH FOOD CHAINS FIRST?
Food chains are useful because they simplify a complicated idea.
They allow students to identify:
• producers;
• primary consumers;
• secondary consumers;
• tertiary consumers;
• predators;
• prey;
• herbivores;
• carnivores;
• omnivores.
A food chain also shows the direction in which biomass and chemical energy are transferred.
For example:
Oak leaves → Caterpillar → Blue tit → Sparrowhawk
The arrow points towards the organism receiving the biomass and energy. The caterpillar receives energy by eating the oak leaf. The blue tit receives energy by eating the caterpillar.
This direction sometimes causes confusion. Students may assume that the arrow means “is eaten by” or that it points towards the animal doing the eating. It is more helpful to think of it as showing the direction of energy transfer.
Food chains therefore provide a useful starting point.
However, they are only models. They deliberately leave out most of the complexity.
THE DIFFERENCE BETWEEN A FOOD CHAIN AND A FOOD WEB
A food chain shows one possible feeding pathway.
A food web shows many interconnected feeding pathways within the same habitat.
Consider a simplified woodland food web.
Oak trees provide leaves, fruit and seeds.
The leaves may be eaten by caterpillars.
The acorns may be eaten by mice, squirrels and jays.
The caterpillars may be eaten by blue tits, spiders and beetles.
The mice may be eaten by owls, foxes and weasels.
The blue tits may be eaten by sparrowhawks.
The fox may also eat insects, fruit, small birds and carrion.
Even this is still a very simplified picture. In a real woodland, there may be hundreds or thousands of feeding relationships.
Food webs show us that organisms rarely depend upon one food source or interact with only one other species.
NATURE HAS ALTERNATIVE ROUTES — BUT NOT UNLIMITED ONES
One advantage of a food web is that it may provide some resilience.
Suppose the rabbit population decreases. A fox may still survive by eating mice, voles, birds, insects or carrion.
That alternative food supply may prevent an immediate collapse in the fox population.
However, alternative food sources are not unlimited.
If rabbit numbers fall, foxes may eat more mice. The mouse population may then decline. Owls and weasels, which also depend upon mice, may find less food available.
The original change involved rabbits, but the consequences may eventually affect owls and weasels.
These species may not appear directly connected when we look at a simple food chain. The food web reveals the hidden connection.
This is why ecological changes are difficult to predict. Organisms can adjust their behaviour, switch food sources, move into different areas and compete more intensely.
WHAT HAPPENS WHEN ONE LINK IS DAMAGED?
The effect depends upon which species is affected, how many other species depend upon it and whether alternatives are available.
Some species occupy especially important positions within a food web. Their loss may produce a disproportionately large effect.
A decline in a common plant, insect or predator can therefore trigger a series of population changes known as a trophic cascade.
The effect can move upwards through the food web, downwards through it or in several directions at once.
EXAMPLE ONE: REMOVING A TOP PREDATOR
Imagine a habitat containing plants, rabbits and foxes.
Plants → Rabbits → Foxes
If the fox population falls, we might initially expect this to be good news for the rabbits.
Rabbit numbers may increase because fewer are being eaten.
However, a larger rabbit population consumes more vegetation. Plant biomass may fall, particularly during winter or dry periods when plant growth is already limited.
As vegetation becomes scarce, the rabbits begin to compete more strongly with one another. Some may starve or become more vulnerable to disease.
Other herbivores may also suffer because the rabbits have consumed more of the available food.
The loss of a predator can therefore eventually damage the prey species that appeared to benefit from its disappearance.
Predators do not simply kill prey. They can help regulate prey populations and prevent overgrazing.
EXAMPLE TWO: THE DISAPPEARANCE OF INSECTS
Insects are often treated as if they are merely pests. In reality, they occupy crucial positions in many food webs.
They may be:
• herbivores;
• predators;
• pollinators;
• decomposers;
• parasites;
• prey for birds, bats, amphibians and fish.
Suppose an insecticide is used to control aphids on crops.
The chemical may reduce the aphid population, but it may also kill ladybirds, hoverflies and other non-target insects.
Fewer insects mean less food for birds and bats.
If pollinating insects are also affected, some plants may produce fewer seeds and fruits.
Those plants may then provide less food for mammals and birds later in the year.
A chemical intended to remove one agricultural pest can therefore influence pollination, seed production, bird populations and the availability of food across an entire habitat.
The unexpected consequences occur because the insect was part of many different relationships, not just one chain.
EXAMPLE THREE: POLLUTION IN A POND OR RIVER
Aquatic food webs provide excellent examples of indirect ecological damage.
A simplified pond food chain might be:
Algae → Water flea → Small fish → Pike
Now imagine that fertiliser runs from nearby land into the water.
The fertiliser contains nitrates and phosphates. These nutrients encourage rapid growth of algae.
At first, an increase in algae may appear beneficial because algae are producers. However, dense algal growth can block light from reaching plants below the water.
Those plants may die because they cannot photosynthesise effectively.
Microorganisms decompose the dead plant material. Their respiration uses dissolved oxygen from the water.
As the oxygen concentration falls, fish and aquatic invertebrates may suffocate.
Birds that feed on the fish may then lose an important food source.
A nutrient entering the water can therefore affect algae, submerged plants, microorganisms, invertebrates, fish and birds.
This process, known as eutrophication, demonstrates why ecological effects must be considered as a sequence rather than as a single event.
EXAMPLE FOUR: REMOVING HEDGEROWS
A hedgerow may appear to be nothing more than a line of bushes separating two fields.
Ecologically, it can be much more important.
A mature hedgerow may provide:
• nectar for pollinators;
• leaves for caterpillars;
• berries for birds and mammals;
• nesting sites for birds;
• shelter for insects;
• hunting routes for bats;
• cover for small mammals;
• habitat for spiders and beetles.
Removing the hedgerow does not affect only the plants that are cut down.
Insect numbers may fall because feeding and breeding sites have disappeared.
Birds may lose nesting sites and food.
Bats may lose both prey and a familiar navigation route.
Predators may then find fewer small mammals and birds.
The removal of one habitat feature can alter many parts of the surrounding food web.
THE IMPORTANCE OF PRODUCERS
Students sometimes concentrate on the predators because they appear more dramatic.
However, most food webs ultimately depend upon producers.
Plants and algae capture light energy through photosynthesis and convert it into chemical energy stored in biomass.
Without producers, there is no new biological energy entering the ecosystem.
A reduction in plant growth can therefore affect every trophic level above it.
Drought, disease, shading, pollution, overgrazing or habitat destruction may all reduce the amount of plant biomass available.
Herbivore populations may then fall. Predators may decline later as their prey becomes scarce.
The effect may not be immediate. This delay can make ecological changes difficult to recognise.
By the time predator numbers fall, the original reduction in plant growth may have occurred months earlier.
THE ROLE OF DECOMPOSERS
Food chain diagrams also tend to understate the importance of decomposers.
Dead organisms and waste materials contain nutrients. Bacteria and fungi break this material down and release mineral ions back into the environment.
Plants absorb these ions and use them to produce new biological material.
A more complete ecological model therefore includes the recycling of matter.
Energy and matter behave differently.
Energy flows through an ecosystem and is eventually dissipated to the surroundings, mainly as heat through respiration.
Matter is recycled.
Carbon, nitrogen, water and mineral ions pass repeatedly between living organisms and the non-living environment.
Without decomposers, nutrients would remain locked inside dead organisms and waste materials. Plant growth would eventually become limited, affecting the entire food web.
ENERGY IS LOST AT EVERY TROPHIC LEVEL
Food webs also help students understand why ecosystems usually contain fewer large predators than producers or herbivores.
Not all the biomass eaten by an organism becomes new biomass.
Some material:
• cannot be digested;
• is lost in faeces;
• is used in respiration;
• is used for movement;
• is used to maintain body temperature;
• is lost in waste products.
Only a proportion becomes biomass that can be transferred to the next trophic level.
This explains why food chains are usually relatively short. There is not enough usable energy to support an unlimited number of trophic levels.
At GCSE, students may be asked to calculate the efficiency of biomass transfer:
Efficiency = Biomass transferred to the next trophic level ÷ Biomass available at the previous trophic level × 100
At A level, students must examine productivity in more detail, including gross primary productivity, net primary productivity and the transfer of energy between trophic levels.
WHY SOME SPECIES MATTER MORE THAN THEIR NUMBERS SUGGEST
A species does not need to be the most numerous organism in a habitat to be ecologically important.
A predator may control the population of several herbivores.
A pollinator may support the reproduction of many plant species.
A decomposer may help release nutrients used by almost every producer.
A particular plant may provide food or shelter during a season when other resources are scarce.
These are sometimes described as keystone species or keystone resources because their ecological effect is much greater than their abundance might suggest.
Removing such a species can change the structure of an entire community.
This is one reason conservation cannot focus only on the largest, rarest or most attractive organisms.
COMPETITION CONNECTS SPECIES THAT DO NOT EAT EACH OTHER
Food webs are not shaped only by predation.
Two species may influence one another because they depend upon the same limited resource.
Owls and foxes may both eat mice.
The owl does not eat the fox, and the fox does not normally eat the owl. However, they are connected through competition.
If fox numbers increase and they consume more mice, less food may be available for the owls.
Similarly, different plant species may compete for:
• light;
• water;
• mineral ions;
• space.
Animals may compete for:
• food;
• territory;
• nesting sites;
• shelter;
• mates.
A change in one population can therefore affect another species without either organism eating the other.
FOOD WEBS AND BIOACCUMULATION
Pollutants can also move through food webs.
Some chemicals are not easily broken down or excreted. They accumulate inside organisms.
A small aquatic organism may absorb a tiny quantity of a pollutant from the water.
A fish eats many of these organisms.
A larger fish eats many smaller fish.
A bird of prey eats many larger fish.
At each trophic level, the pollutant may become more concentrated. This is known as biomagnification.
The top predator may therefore receive the highest concentration, even though it was never directly exposed to the original source of pollution.
This is another powerful example of unexpected damage travelling through a food web.
A PRACTICAL CLASSROOM MODEL
One effective way to teach food webs is to give each student the name of an organism from the same habitat.
One student might represent grass.
Others could represent:
• grasshoppers;
• rabbits;
• mice;
• frogs;
• small birds;
• snakes;
• foxes;
• hawks;
• fungi.
A ball of string can be passed between organisms with feeding relationships.
The result is a physical web stretching across the classroom.
Then one organism can be removed.
For example, the grasshopper population might be destroyed by pesticide use. Every student connected to the grasshopper lowers or releases their section of string.
Other connections become loose. Predators may need to depend more heavily upon alternative prey. That places additional pressure on another part of the web.
The activity makes an important point visible: no species exists in isolation.
INVESTIGATING FOOD WEBS OUTDOORS
Students can also investigate real feeding relationships.
Useful approaches include:
• using quadrats to estimate plant abundance;
• carrying out transects across changing habitats;
• examining leaves for signs of herbivory;
• observing pollinators visiting flowers;
• pond dipping to identify aquatic organisms;
• using camera traps to record mammals;
• examining owl pellets to identify prey remains;
• recording birds feeding in a garden;
• comparing insect numbers in mown and unmown areas.
The aim is not always to observe one organism eating another. Feeding relationships can also be inferred from evidence.
Chewed leaves indicate herbivory.
Seeds in droppings may show fruit consumption and seed dispersal.
Bones in owl pellets reveal prey species.
Changes in abundance between habitats can suggest dependence upon particular plants, shelter or environmental conditions.
FROM GCSE DESCRIPTION TO A-LEVEL ANALYSIS
At GCSE, students are often expected to:
• construct and interpret food chains;
• identify trophic levels;
• explain predator-prey relationships;
• calculate biomass transfer efficiency;
• interpret pyramids of biomass;
• explain the effects of environmental change.
At A level, the questions become more analytical.
Students may need to consider:
• net and gross productivity;
• energy transfer between trophic levels;
• nutrient cycles;
• population interactions;
• competition;
• succession;
• conservation;
• sampling reliability;
• statistical testing;
• the effect of abiotic and biotic factors;
• the limitations of ecological models.
A strong A-level answer should rarely describe only one direct effect.
For example, instead of writing:
“The number of foxes will fall because there are fewer rabbits,”
a stronger answer might explain:
“A reduction in rabbit numbers may reduce the food available to foxes. Foxes may initially switch to alternative prey, increasing predation pressure on mice or ground-nesting birds. Competition with other predators may increase, and fox reproductive success may eventually fall.”
That answer recognises that ecosystems contain alternatives, delays, competition and indirect effects.
ASK “WHAT HAPPENS NEXT?”
When answering ecological questions, students should repeatedly ask:
“What happens next?”
Suppose a plant species declines.
What happens to the herbivores that eat it?
Can they switch to another plant?
Will this increase competition?
What happens to the predators that eat those herbivores?
Could another species increase because competition has been reduced?
Will decomposition change?
Could soil nutrients or water quality be affected?
One ecological change may produce several possible outcomes. In examination questions, students should follow the evidence provided and avoid claiming that every possible effect will definitely happen.
Words such as “may”, “could”, “likely” and “depending upon” are often scientifically appropriate because ecosystems are complex.
A PERSONAL REFLECTION FROM TEACHING BIOLOGY
I often find that students are comfortable with a simple food chain but become less certain when several chains are joined together.
The diagram suddenly looks untidy.
There are arrows going in several directions, organisms appear more than once, and the simple rule of “one animal eats another” no longer seems sufficient.
However, that apparent untidiness is the most important part of the lesson.
The food web is not confusing because the Biology has been explained badly. It is complicated because the natural world is complicated.
In practical teaching, I try to move beyond the printed diagram. Pond samples, leaf damage, garden insects, bird observations and photographs of local habitats make the relationships more real.
A blue tit is no longer simply a “secondary consumer”. It becomes an animal depending upon a seasonal supply of caterpillars, nesting sites, suitable vegetation and a habitat capable of supporting all those things.
At Philip M Russell Ltd, practical Biology allows students to see that ecology is not simply a collection of definitions. It is the study of relationships, evidence and consequences.
CONCLUSION: NOTHING IN AN ECOSYSTEM EXISTS ALONE
Food chains are valuable because they introduce the transfer of energy and biomass in a clear, manageable way.
Food webs take the next step.
They show us that organisms are connected through feeding, competition, pollination, decomposition and habitat.
They also explain why environmental damage can spread far beyond its original source.
Removing a predator may damage vegetation.
Killing an insect may reduce bird populations.
Polluting a river may affect fish-eating birds.
Removing a hedgerow may alter an entire agricultural community.
The most important lesson is not simply that one organism eats another.
It is that every organism forms part of a network.
When one connection is weakened, the damage may appear somewhere completely unexpected.
Understanding food webs helps students answer examination questions, but it also helps us make better decisions about farming, conservation, pollution and the way we manage the natural environment.
Nature does not operate as a series of separate chains.
It operates as a web — and when we pull on one strand, the whole system may respond.
