Who eats whom and who needs whom: food webs and the interactions between populations
By the end of this lesson you will be able to
- Define producer, consumer, decomposer and trophic level, and identify each in a food web, including organisms that feed at more than one level.
- Construct food chains from a food web and outline why every chain must begin with a producer.
- Predict and explain, in a linked chain of reasoning, how a change in one population affects others in the web, including the role of a keystone species.
- Distinguish competition, predation and the three kinds of symbiosis, with an example of each, and interpret a predator–prey graph.
- Analyse data from a simulated predator–prey experiment and evaluate how interdependence limits both populations.
You met this before
- An ecosystem is a community of populations of different species together with the non-living (abiotic) factors of their environment — light, water, temperature, soil — all interacting (Year 3, Ecosystems).
- A population is all the organisms of one species living in one area at one time; a community is all the populations of different species living there.
- Green plants and algae make glucose by photosynthesis: carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll (Year 3, Photosynthesis and respiration).
- Every organism releases energy from food by respiration: glucose + oxygen → carbon dioxide + water, with energy transferred to the cell (Year 3).
- Predator and prey populations rise and fall in linked cycles, with the predator peak coming after the prey peak; species that need the same limited resource compete (Year 3, Lessons 3.4 and 3.5).
In the chain grass → zebra → lion, name the producer, and state where the energy in the grass came from in the first place.
Reveal
The producer is the grass. Its energy came from sunlight: chlorophyll in the grass absorbed light energy and photosynthesis stored it as chemical energy in glucose, which the plant built into starch, cellulose and protein. The zebra obtains that chemical energy by eating the grass; the lion by eating the zebra.
Revise it in full: Year 3 · Ecosystems · Lesson 1 — From one elephant to the biosphere: the levels of life · Year 3 · Ecosystems · Lesson 4 — Competition, habitat and niche · Year 3 · Ecosystems · Lesson 5 — Predators and prey: the cycle that never settles
The puzzle
In the 1700s and 1800s hunters killed sea otters along the Pacific coast of North America for their thick fur, until by the early 1900s they had vanished from most of it.
Nobody hunted kelp — the giant brown seaweed that grows in underwater forests that can grow more than 30 m tall. Yet where the otters disappeared, the kelp forests disappeared too, leaving bare rock carpeted with spiny sea urchins.
When otters were protected and returned to some islands in the twentieth century, the kelp grew back within a few years, and with it the fish that shelter in kelp.
How can killing a furry mammal destroy a forest of seaweed it never eats? The answer is in a diagram of who eats whom — and it is the same reasoning the eAssessment asks you to write down, link by link.
Sea otters eat sea urchins. Sea urchins graze on kelp. Predict what happens to the number of urchins, and then to the kelp, when the otters are removed — and suggest what finally stops the urchins increasing for ever.
Compare your answer
With no otters eating them, fewer urchins die, so the urchin population increases. More urchins graze more kelp, so the kelp is eaten faster than it can regrow and the kelp forest shrinks to bare rock (an “urchin barren”).
The urchins cannot increase for ever: once the kelp is gone, their food is limited, so they compete for what is left, starve, grow smaller and stop breeding. Food supply becomes the limiting factor. Two feeding links, one change, and the whole community is different.
The science — structured notes
1.1 Producers, consumers, decomposers and trophic levels
Every ecosystem runs on energy that enters it as light. Producers — green plants, algae and some bacteria (cyanobacteria) — are autotrophs: they make their own organic food from carbon dioxide and water by photosynthesis, storing light energy as chemical energy.
Everything else is a heterotroph and must obtain ready-made organic food. Consumers eat other organisms: a herbivore eats plants, a carnivore eats animals, and an omnivore eats both. Decomposers — bacteria and fungi — feed on dead remains and waste by secreting enzymes onto them and absorbing the products; detritivores such as earthworms, termites and dung beetles eat dead material and speed the decomposers up (Lesson 3).
A trophic level is an organism’s feeding position in a food chain. Producers are trophic level 1; primary consumers (herbivores) are level 2; secondary consumers, which eat herbivores, are level 3; tertiary consumers are level 4. A predator at the top of a chain, with no natural predators of its own as an adult, is an apex predator.
An animal is not fixed to one level: a baboon eating grass seeds is a primary consumer, and the same baboon eating a grasshopper is a secondary consumer.
| Term | Feeds on | Trophic level | Savanna example |
|---|---|---|---|
| Producer (autotroph) | Makes its own food by photosynthesis | 1 | Grass, acacia, mopane tree |
| Primary consumer (herbivore) | Producers | 2 | Zebra, impala, grasshopper, termite |
| Secondary consumer | Primary consumers | 3 | Cheetah, lizard, jackal |
| Tertiary consumer | Secondary consumers | 4 | Martial eagle eating a lizard; lion eating a baboon |
| Omnivore | Producers and consumers | 2 and 3 (or more) | Baboon, warthog, people |
| Decomposer | Dead organic matter and wastes, by external digestion | Not usually given a level; receives from every level | Soil bacteria, mushrooms and moulds |
“Decomposers are the last link in a food chain, so they are the tertiary or quaternary consumers.”
Reveal answer
False. Decomposers are not placed at the end of a chain. They feed on the dead remains and wastes of every trophic level — dead grass, zebra dung, a lion carcass — so they receive material from all levels, not from the one above them.
That is why a food web diagram usually shows decomposers in a separate box with arrows coming in from every organism, if it shows them at all.
1.2 Food chains and food webs
A food chain shows one feeding pathway: grass → zebra → lion. The arrows point from the organism that is eaten to the organism that eats it — the direction in which energy and matter flow. Reversing the arrows (“lion eats zebra”) is the most common error at this level and costs every mark that depends on it.
Real animals eat, and are eaten by, several species, so feeding relationships form a food web: many interconnected food chains. The diagram below is a simplified savanna web. Watch the gold dots: each one is a packet of energy moving along an arrow.
Three skills are examined again and again with a web like this. Identify an organism of a given type — name the actual organism shown (“grass”, not “plant”: the May 2021 markscheme refused “plant”). Construct a chain of a given length — start with a producer, follow the arrows, and check each organism really eats the one before it.
Classify an organism that appears at several levels — the baboon is both a primary and a secondary consumer; the lion is a secondary consumer when it eats a zebra and a tertiary consumer when it eats a baboon.
Use the savanna web to construct a food chain of four organisms that ends with the lion, and state the trophic level of each organism.
Start at the end and work backwards along arrows that point INTO each organism.
Lion ← baboon (the only secondary consumer the lion eats in this web).
Baboon ← grasshopper (the baboon also eats grass, but that would make a chain of only three).
Grasshopper ← grass (the producer).
Chain: grass → grasshopper → baboon → lion.
Levels: grass 1 (producer); grasshopper 2 (primary consumer); baboon 3 (secondary consumer); lion 4 (tertiary consumer).
Grass → zebra → lion is correct but has only three organisms; a question that asks for four gives no credit for three.
From the savanna web, identify: (i) a producer eaten by two herbivores; (ii) an organism that is a secondary consumer only; (iii) the number of different food chains that end with the martial eagle.
Reveal answer
(i) Acacia (eaten by impala and giraffe) — or grass (eaten by grasshopper, zebra, impala and baboon).
(ii) The lizard or the cheetah — each eats only a primary consumer (grasshopper or impala), so each feeds at trophic level 3 only. The baboon is not an answer: it also eats grass.
(iii) One: grass → grasshopper → lizard → martial eagle. Trace every path backwards from the eagle: it eats only the lizard, the lizard eats only the grasshopper, and the grasshopper eats only grass.
1.3 Interdependence: predicting what a change will do
Populations in a web are interdependent: the size of each depends on the sizes of others. A change in one population — disease, hunting, a new predator, a drought that kills the grass — spreads through the web along the feeding links. The eAssessment asks you to follow that spread, and it awards marks for each correctly linked step, not for the final answer alone.
The method: name the change → state what it does to the population directly linked (and why: more or less eaten, more or less food) → carry that effect one link further → state the final effect. Every step needs a “so” or “because”.
Rabies kills many of the baboons in the savanna web. Suggest two consequences for other organisms, and justify each.
Consequence 1: the grasshopper population increases.
Justification: baboons eat grasshoppers; with fewer baboons, fewer grasshoppers are eaten, so more survive to reproduce.
Consequence 2: the lion eats more zebra and impala, so those populations decrease.
Justification: the lion has lost one of its prey species, so it must catch more of its other prey to get enough energy.
Chains of reasoning can branch: more grasshoppers could also mean more lizards (more food), and more grasshoppers eat more grass, leaving less for zebra and impala (more competition). The official scheme gave a mark for using terms such as predator, prey, population, trophic level and energy.
A keystone species is one whose effect on its community is far larger than its numbers would suggest, so that removing it changes the whole ecosystem. The sea otter is the classic example: by eating urchins it keeps the kelp forest standing, and hundreds of other species depend on the forest.
Elephants are keystone species on the savanna — they push over trees, which keeps woodland open as grassland, dig for water in dry riverbeds, and disperse the seeds of many trees in their dung. Flying foxes (fruit bats) are keystone pollinators and seed dispersers in parts of Australia and Asia.
Using the sea otter food chain kelp → sea urchin → sea otter, explain why protecting sea otters from hunting led to more fish around the islands where they returned.
Reveal answer
More sea otters eat more sea urchins, so the urchin population decreases [link 1]. Fewer urchins graze the kelp, so the kelp grows faster than it is eaten and the kelp forest regrows [link 2].
The kelp forest provides food (for grazers that fish eat), shelter from predators and places to breed for many fish species, so the fish populations increase [link 3]. The otter never eats kelp, yet it controls it through the urchins — an indirect effect that makes it a keystone species.
A student answers: “If the lions die out, the zebra will become extinct because the lions eat them. The grass will increase because the zebras are more.” Find three errors.
Reveal answer
(1) The direction is reversed: with no lions, fewer zebra are eaten, so the zebra population increases, not decreases.
(2) “Extinct” is wrong in both directions — examiners do not accept extinction as a population effect unless the whole species disappears; the markscheme for May 2021 Q2(b) said explicitly “do NOT accept extinction”.
(3) The last link is reversed and unexplained: more zebra eat more grass, so the grass decreases — and then the zebra population is limited by food, so it cannot rise for ever.
1.4 Interactions between populations: competition and predation
Feeding is only one way populations interact. Competition happens when organisms need the same resource and it is in limited supply — food, water, light, space, mates, nesting sites. Intraspecific competition is between members of one species (two impala for the same grass); interspecific competition is between different species (impala and zebra for the same grass; lions and hyenas for the same carcass).
Competition lowers survival and reproduction for all the competitors, so it limits population size. When two species need exactly the same resources in the same place, one usually outcompetes the other — the competitive exclusion principle — which is why species sharing a habitat differ in their niches (what, where and when they eat).
Predation is the killing and eating of one animal (the prey) by another (the predator). Grazing and browsing by herbivores is a related interaction — the plant is eaten but usually not killed. Predator and prey populations regulate each other through negative feedback.
More prey means more food, so more predators survive and breed. More predators eat more prey, so the prey decline. Fewer prey means predators starve or breed less, so they decline. Fewer predators let the prey recover. The graph below shows the resulting cycles; the time cursor moves across so you can compare the two populations at the same moment.
Notice three features you may be asked to describe. The two cycles have the same period (here 10 years). The predator peak lags behind the prey peak, because predators need time to breed after food becomes plentiful. There are far fewer predators than prey at every moment — note the two different scales — for reasons of energy you will meet in Lesson 2.
Real cycles are rarely this regular: weather, disease and other food sources disturb them, and many predators eat several prey species, which damps the swings.
In the graph, the prey population is at its lowest at 7.5 years. Describe what is happening to the predator population at that moment, and explain why the prey then start to increase even though predators are still fairly common.
Reveal answer
At 7.5 years the predators are falling steeply (from their peak of 60 at 5 years towards their low of 20 at 10 years). With prey scarce, predators cannot catch enough food; more starve and fewer young survive.
The prey start to increase because the predator population has already fallen enough that fewer prey are being killed than are born; the prey also have plenty of plant food, because there are so few prey competing for it. The prey recover first; the predators follow later — the lag.
1.5 Symbiosis: living closely together
Symbiosis is a close, long-term relationship between two species. There are three kinds, classified by who benefits (+), who is harmed (−) and who is unaffected (0).
| Relationship | Effect on each partner | Examples |
|---|---|---|
| Mutualism | + / + — both benefit | Bees and flowering plants (nectar for pollination); Rhizobium bacteria in the root nodules of beans and groundnuts (sugar for fixed nitrogen, Lesson 3); fungi and algae in a lichen; bacteria that digest cellulose in the gut of a zebra or a termite |
| Commensalism | + / 0 — one benefits, the other is unaffected | Cattle egrets catching insects disturbed by grazing buffalo or cattle; epiphytic orchids growing on tree branches for light |
| Parasitism | + / − — the parasite benefits, the host is harmed | Ticks and tapeworms on and in mammals; the malaria parasite Plasmodium in humans and mosquitoes; witchweed (Striga), which taps the roots of maize and sorghum |
Classify each relationship and give the sign pair: (i) Striga (witchweed) attaches to sorghum roots and takes water and sugars; (ii) nitrogen-fixing bacteria in groundnut root nodules receive sugars and supply ammonium ions; (iii) a hornbill follows a troop of banded mongooses and snatches insects they flush out; (iv) two male impala fight for a territory.
Reveal answer
(i) Parasitism (+ / −): the weed gains, the sorghum yield falls — Striga causes large crop losses across sub-Saharan Africa.
(ii) Mutualism (+ / +): the bacteria gain sugars and a protected home; the plant gains usable nitrogen for proteins.
(iii) Commensalism (+ / 0) if the mongooses are unaffected — in fact hornbills also warn of hawks, so it may be closer to mutualism, a reminder that real relationships are measured, not assumed.
(iv) Intraspecific competition (− / −): both males spend energy and risk injury for a limited resource.
1.6 Using data to show interdependence
The eAssessment presents food-web changes as data — two pyramids, a bar chart, a table of counts — and asks you to explain them. Use the numbers: state the direction and size of each change, then give the feeding link that explains it.
Mean counts in 1 m² quadrats: sea urchins fell from 48 to 6; kelp plants rose from 0.5 to 8.0. Calculate the percentage change in each and explain the pattern.
Percentage change = (new − original) ÷ original × 100
Urchins: (6 − 48) ÷ 48 × 100
Urchins: −42 ÷ 48 × 100 = −87.5 %, a decrease of about 88 %
Kelp: (8.0 − 0.5) ÷ 0.5 × 100
Kelp: 7.5 ÷ 0.5 × 100 = +1 500 %, a sixteen-fold increase
Explanation: the returning otters ate urchins, so urchin numbers fell by 88 %; with fewer urchins grazing, far more young kelp survived to grow. A small change at the top of the chain produced a very large change at the bottom — the signature of a keystone predator.
In a nature reserve, jackals were removed. Over five years, mean spring hares per hectare rose from 12 to 30, and the percentage of ground covered by grass fell from 64 % to 41 %. Calculate the percentage change in each, and write a two-link explanation.
Reveal answer
Percentage change = (new − original) ÷ original × 100
Spring hares: (30 − 12) ÷ 12 × 100
Spring hares: 18 ÷ 12 × 100 = +150 %
Grass cover: (41 − 64) ÷ 64 × 100
Grass cover: −23 ÷ 64 × 100 = −35.9 %, about −36 %
(Grass cover is already a percentage, so its change can also be quoted as a fall of 23 percentage points — say which you mean.)
With no jackals preying on them, fewer spring hares were killed, so their population rose by 150 %; more hares grazed more grass, so grass cover fell by about 36 %.
A food web has 8 species and 14 feeding links. “Connectance” is the fraction of all possible feeding links that actually exist; for S species, the possible links are S × (S − 1) ÷ 2. Calculate the connectance, and explain why a web with a higher connectance is usually more stable when one species is lost.
Reveal answer
Possible links = 8 × 7 ÷ 2 = 28
Connectance = 14 ÷ 28 = 0.50
In a highly connected web most consumers eat several species. If one prey species declines, its predators switch to alternative food, so their numbers change less and the effect does not spread as far through the web.
In a poorly connected web a consumer that depends on one food species crashes when that species crashes, and the effect cascades. (Ecologists debate the exact relationship — very highly connected webs can also spread disturbances — but losing a species that many others depend on, such as a keystone species, is always the greatest risk.)
Try it — how many predators can a herd support?
The lab releases a chosen number of predators into a fenced reserve holding 300 prey and counts the prey after 12 months. The model works month by month: prey breed (more slowly as they approach the number the reserve can feed), predators kill prey at a rate that depends on how often they meet, and predators breed in proportion to what they eat. It is interdependence you can measure.
A student’s means (three trials each, low birth rate, small reserve): 2 predators → 341 prey; 6 → 277; 10 → 226; 14 → 191; 18 → 161. Describe the relationship, quoting the successive falls, and explain its shape using interdependence.
Reveal answer
As the starting number of predators rises from 2 to 18, the prey remaining after 12 months falls from 341 to 161 — a negative relationship. It is not linear: each step of 4 predators removes fewer prey — falls of 64, 51, 35 and 30 animals — so the curve is steep at first and flattens.
Explanation: the rate at which predators kill depends on how often they meet prey. Early in the year, with 300 prey, each predator kills often; as the herd shrinks, encounters become rarer, so each predator kills fewer.
The surviving prey have more grass each, so they breed closer to their maximum rate, partly replacing losses. Predator success depends on prey numbers, and prey numbers depend on predators: the two populations limit each other.
Common mistakes to avoid
- Arrows the wrong way — the arrow means “is eaten by” and shows energy flow: grass → zebra, never zebra → grass.
- Naming a group, not the organism — “plant” or “tree” when the web shows “acacia” or “mango tree”: the markschemes refuse them.
- Starting a chain with the Sun or with a consumer — a food chain starts with a producer; the Sun is the energy source, not an organism.
- One-step answers to a three-mark question — each mark is a link: change → first population (with reason) → next population (with reason).
- “The population will become extinct” — a population increases or decreases; extinction is not accepted as the effect of one change in a web.
- Putting an omnivore at one level only — it occupies every level at which it feeds.
- Calling every symbiosis mutualism — parasitism and commensalism are symbiosis too; give the +/−/0 effect on both partners.
- Confusing competition with predation — competitors need the same resource; a predator eats its prey.
Study smart: how this is examined
Study routine. Today: redraw the savanna web from memory, then write every food chain it contains that ends with the lion (there are five).
In two days: pick any organism in the web, remove it, and write two three-link predictions with “so” and “because” — then check each link reads correctly. In a week: run the predator–prey lab as a full investigation, take the quiz, and describe the predator–prey graph in three sentences using the words period, lag and scale.
How we found out — from a diagram of Bear Island to the keystone
People have always known that wolves eat deer and deer eat grass. What changed in the twentieth century was the idea that feeding relationships form a system whose structure could be drawn, measured and predicted. Charles Darwin had pointed the way in 1859, closing On the Origin of Species with the image of a “tangled bank” of interdependent living things.
In 1921 a young Oxford zoologist, Charles Elton, joined an expedition to Bear Island and Spitsbergen in the Arctic, where so few species live that every feeding link could be recorded. With the botanist Victor Summerhayes he published, in 1923, one of the first food-web diagrams of a whole community.
In his book Animal Ecology (1927) Elton set out the ideas this lesson uses: food chains joined into a “food cycle” (our food web), the niche as an animal’s place in that web, and the pyramid of numbers — many small animals at the bottom supporting a few large ones at the top.
At almost the same time Alfred Lotka (1925) and Vito Volterra (1926) independently wrote the equations of predator–prey cycles that run inside this lesson’s lab.
Elton also drew on decades of fur-trading records of the Hudson’s Bay Company, which show roughly ten-year cycles in the numbers of lynx and snowshoe hare.
The keystone idea came from an experiment. From 1963 the American ecologist Robert Paine repeatedly removed every predatory starfish (Pisaster ochraceus) from a stretch of rocky shore at Mukkaw Bay, Washington, leaving a neighbouring stretch untouched as a control.
Within a few years mussels and barnacles, freed from their predator, had crowded out the other species; the number of species on the experimental shore fell from fifteen to eight, while the control shore was unchanged. Paine published the result in 1966 and coined the term “keystone species” in 1969.
In 1974 James Estes and John Palmisano compared Aleutian islands with and without sea otters and found kelp forests on one and urchin barrens on the other — the story of this lesson’s puzzle. Evidence that changed minds was not a model but a controlled removal: take one species out, keep a comparison, and count.
Explain why Paine’s untouched stretch of shore was essential to his conclusion, and state what type of variable the presence of starfish was.
Reveal answer
The presence or absence of starfish was the independent variable. The untouched shore was a control experiment: it experienced the same weather, tides, season and larval supply, so any change seen on both shores could be put down to those factors.
Only the difference between the two shores — diversity falling from fifteen to eight species where starfish were removed while staying the same on the control — could be attributed to removing the predator.
Where it is used now
- Vultures and the savanna clean-up. Vultures remove carcasses in hours, limiting the spread of anthrax and rabies-carrying feral dogs. Surveys published in 2016 found that most African vulture species had declined steeply over about three decades, much of it from poisoned carcasses left by poachers. Several southern African countries now run vulture-safe zones and poison-response training.
- Mopane worms and the web. The caterpillar of the mopane moth, harvested and eaten across Botswana, Zimbabwe, South Africa and Namibia, is a primary consumer of mopane trees and food for birds. Over-harvesting removes food from birds and reduces next year’s moths — a food chain with people in it.
- Biological control. Farmers use predators and parasites instead of pesticides: small wasps that parasitise stem-borer caterpillars in maize, and weevils that eat water hyacinth on African lakes. Each is interdependence engineered on purpose, and each needs checking that the control agent does not attack other species.
- Wolves in Yellowstone. In 1995 and 1996 grey wolves from Canada were released into Yellowstone National Park in the United States, where wolves had been absent for about 70 years. Elk numbers fell and some streamside willows recovered. Ecologists still argue how much of the plant recovery was caused by the wolves — a live example of why webs are hard to predict.
- Pollinators and food. Many fruit, vegetable and oil crops depend partly on insect pollinators; losses of wild bees from pesticide use and habitat loss are monitored worldwide because a break in that mutualism reaches the dinner plate.
Science now — and the scientist you need to be
What is happening in 2026
Drawing a food web used to mean years of watching animals eat. Now much of it is done with DNA. In DNA metabarcoding, fragments of DNA from a droppings sample are sequenced and matched against reference libraries, so one sample of bat or leopard dung lists dozens of prey species, including ones too small or too digested to identify by eye.
Combined with stable-isotope analysis (the ratio of heavy to light nitrogen atoms in tissue rises by a fairly predictable step at each trophic level), ecologists can now place an animal on a trophic level from a hair or a feather.
Camera traps and AI are the other revolution. Networks of motion-triggered cameras produce millions of images; since the Snapshot Serengeti project (cameras from 2010, online volunteers from 2012) enlisted the public to label them, machine-learning models have been trained to identify the species and count the animals in most images, leaving the difficult ones for people.
Those images reveal who visits which carcass, who avoids whom, and when predators and prey share a water hole. Over the next decade expect webs built automatically from DNA, sound recorders and cameras, updated as the seasons change — and expect to be asked what a web built by a machine has missed.
Careers here are called field ecologist, wildlife data scientist, conservation geneticist and biological control specialist; all of them start with reading a web correctly.
The mindshift
What you need to be able to do
Read any food web you have never seen; name organisms by type; construct chains; predict effects of a change in linked steps; classify interactions with their signs; and describe and explain a predator–prey graph. The reports list food chains among the strengths of candidates — the marks are lost in the unlinked explanation.
See an example
Asked for a farm-dam food web in southern Africa, one assistant gave “algae → tadpoles → dragonfly nymphs → catfish → fish eagle” and “grass → cattle → people” — sensible chains — but wrote “fish eagle → fish” (arrow reversed) in one line, and put “decomposers” at the top of a chain above the fish eagle. Both are the errors this lesson warns about.
Key terms
| Producer (autotroph) | An organism that makes its own organic food from inorganic substances, usually by photosynthesis. |
| Consumer (heterotroph) | An organism that obtains energy and matter by eating other organisms. |
| Decomposer | A bacterium or fungus that feeds on dead organic matter by secreting enzymes onto it and absorbing the products. |
| Detritivore | An animal that eats dead organic matter (earthworm, termite, dung beetle). |
| Trophic level | The feeding position of an organism in a food chain (producer = 1). |
| Food chain / food web | A single feeding pathway / a network of interconnected food chains; arrows show the direction of energy flow. |
| Interdependence | The way the populations of a community depend on one another for food, pollination, shelter and other needs. |
| Keystone species | A species whose effect on its community is far larger than its abundance would suggest. |
| Competition | An interaction in which organisms need the same limited resource; intraspecific (same species) or interspecific (different species). |
| Predation | The killing and eating of one animal (prey) by another (predator). |
| Symbiosis | A close, long-term relationship between two species: mutualism (+/+), commensalism (+/0) or parasitism (+/−). |
| Apex predator | A predator at the top of a food chain with no natural predators as an adult. |
Think like a scientist
Criterion A A floodplain food web from a seasonal wetland is described in words. Water lilies and floodplain grasses are the producers. Tilapia eat water lilies; red lechwe (antelope) graze the grasses; tiger fish eat tilapia; the African fish eagle eats tiger fish and tilapia; crocodiles eat tiger fish, tilapia and red lechwe; lions eat red lechwe. Mosquito larvae feed on algae (also producers); tilapia eat mosquito larvae.
(a) Identify two primary consumers and the organism that feeds at the most trophic levels. [3]
(b) Construct the longest food chain in this web and state the trophic level of its last organism. [2]
(c) Outline why every chain in this web must begin with the water lilies, grasses or algae. [2]
(d) A dam upstream stops the annual flood, so the floodplain grasses die back. Predict and explain the effect on the lions and on the crocodiles. [4]
(e) Tiger fish and fish eagles both eat tilapia. Name this interaction and explain its effect on each species. [2]
(f) Crocodiles are sometimes described as a keystone species in wetlands. Suggest one way in which removing crocodiles could affect the red lechwe and then the grasses. [2]
Model answer
(a) Primary consumers: tilapia (when eating water lilies), red lechwe, mosquito larvae — any two [1 each].
Most trophic levels: the crocodile or the fish eagle, each at levels 3, 4 and 5 — for example the crocodile is level 3 eating lechwe, level 4 eating tilapia that ate mosquito larvae, and level 5 eating tiger fish that ate tilapia that ate mosquito larvae [1]. (Tilapia, at levels 2 and 3, is not the best answer.)
(b) Algae → mosquito larvae → tilapia → tiger fish → crocodile (or → fish eagle) [1]; the last organism is at trophic level 5, a quaternary consumer [1].
(c) Only producers can capture light energy by photosynthesis and convert it into chemical energy in glucose [1]; every consumer in the web depends on that stored chemical energy, passed along by feeding [1].
(d) Without the flood the grasses die back, so the red lechwe have less food [1]; the lechwe population decreases (starvation, fewer young survive, or animals leave) [1].
Lions depend on lechwe in this web, so the lion population decreases [1]. Crocodiles have other food — tilapia and tiger fish — so they are affected less, although fish may also decline if lily habitat shrinks without the flood; the crocodile population is likely to decrease less than the lions or stay similar [1].
(e) Interspecific competition [1]: tilapia are a limited shared food, so each predator catches fewer than it would alone, limiting the population of both [1].
(f) Without crocodiles, fewer red lechwe are killed, so the lechwe population increases [1]; more lechwe graze more grass, so the grasses decrease — overgrazing — until lechwe food is limiting [1].
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Define the terms producer and trophic level.
Reveal answer
Producer: an organism that makes its own organic food from inorganic substances (carbon dioxide and water), usually by photosynthesis using light energy [1]. Trophic level: the feeding position of an organism in a food chain, counting the producer as level 1 [1].
In the chain acacia → giraffe → lion, the lion population falls because of disease. Describe the effect on the acacia trees.
Reveal answer
Fewer lions kill fewer giraffes, so the giraffe population increases [1]; more giraffes browse more acacia leaves and shoots [1]; so the acacia trees are damaged more, grow less and fewer young trees survive — the acacia population decreases [1]. (The third mark only with a link to more giraffes.)
State the type of symbiosis in each case: (i) a tapeworm in the gut of a dog; (ii) fungi that grow around plant roots, receiving sugars and supplying phosphate ions.
Reveal answer
(i) Parasitism — the tapeworm benefits, the dog is harmed [1]. (ii) Mutualism — both partners benefit (these are mycorrhizal fungi) [1].
Distinguish between intraspecific and interspecific competition, and explain why intraspecific competition is usually more intense.
Reveal answer
Intraspecific competition is between members of the same species; interspecific is between members of different species [1]. Members of one species have identical needs — the same food, the same nesting sites, the same space, and mates [1] — so they overlap completely, while different species usually differ in at least part of their niche, and only intraspecific competitors compete for mates [1].
Describe three features of a predator–prey graph of two populations over time.
Reveal answer
Both populations rise and fall in cycles with the same period [1]; the predator peaks come after the prey peaks (a lag) [1]; the predator population is much smaller than the prey population at all times [1].
Suggest two consequences for other organisms in the savanna web of this lesson if a disease killed most of the grasshoppers, and justify each.
Reveal answer
Consequence 1: the lizard population decreases [1] — lizards eat only grasshoppers in this web, so they have less food [1]. Consequence 2: the martial eagle population decreases [1] — it feeds only on lizards, which have declined [1]. (Also acceptable: more grass is available, so zebra and impala increase; baboons eat more grass or are eaten more by lions.)
7. Cattle egrets feeding beside buffalo catch about three times as many insects per minute as egrets feeding alone, and the buffalo’s feeding rate is the same with or without egrets. Some egrets also pick ticks off the buffalo. Evaluate the claim that the relationship is commensalism, using the data.
Reveal answer
For the claim: the egret clearly benefits (a threefold increase in feeding rate) [1] and the buffalo’s feeding rate is unaffected, which fits +/0 [1].
Against: feeding rate is only one measure of benefit or harm; if egrets remove ticks, the buffalo loses less blood and fewer tick-borne diseases — a benefit, making the relationship closer to mutualism [1].
Judgement: on these data, commensalism for most interactions, grading into mutualism where egrets remove ticks; a test would compare tick loads and health of buffalo with and without egrets [1].
8. Two species of grain-eating beetle were kept separately and together in flour at 29 °C. Alone, species A reached 320 beetles and species B 280. Together, after 20 weeks, species A numbered 330 and species B 0. At 24 °C together, A reached 30 and B 250. Explain these results using the principle of competitive exclusion and state what they show about the role of abiotic factors.
Reveal answer
Alone, each species grows to a population limited by its own food and space (intraspecific competition): 320 and 280 [1]. Together, they compete for the same food and space (interspecific competition) — the same niche [1].
At 29 °C species A outcompetes B (it reached 330 while B fell to 0): B is excluded, as the competitive exclusion principle predicts for two species with identical needs [1].
At 24 °C the outcome reverses — B dominates (250) and A is reduced to 30 [1] — so which species wins depends on an abiotic factor, temperature, which changes each species’ growth and survival; competition outcomes are not fixed properties of the species but of species in an environment [1]. (These results resemble Thomas Park’s flour-beetle experiments of the 1940s–1950s.)
Summary
- Producers capture light energy by photosynthesis; consumers (herbivores, carnivores, omnivores) eat other organisms; decomposers digest dead matter externally. Trophic levels count from the producer (1).
- Arrows in a food chain or web point from the food to the feeder — the direction of energy flow. Every chain starts with a producer; omnivores occupy more than one level.
- Populations are interdependent: explain the effect of a change link by link, with “so” and “because”. A keystone species (sea otter, starfish, elephant) has an effect far larger than its numbers.
- Competition (intra- and interspecific) limits populations; predator and prey regulate each other in cycles with the predator lagging. Symbiosis: mutualism +/+, commensalism +/0, parasitism +/−.
Next time → Energy through the ecosystem: why only about a tenth of the energy passes from one trophic level to the next, where the rest goes, and how pyramids of numbers, biomass and energy show it.