Ecosystems and interdependence · System
Disturbing a food web
Pull one thread out of a web and the whole thing moves. The species that end up in trouble are usually not the ones anybody was thinking about.
Start here
Kill every ladybird in the wood. What happens to the oak tree?
A ladybird has never touched an oak leaf, and an oak tree has no opinion about ladybirds. There are three organisms in between them. The question is whether an effect can travel that far.
What is your prediction?
It reaches the oak. No ladybirds means aphids multiply; aphids drain sap from the young shoots and the tree grows less. Along the way the blue tits get an easy meal of aphids and neglect the caterpillars, so caterpillar numbers rise as well — a second attack on the same tree, arriving from a different direction. Effects in a web do not stop at the organisms you would have drawn a line to.
At the bench · one oak wood, every link in it
Take one out and follow it
nothing removed yet
Who eats whom
- Caterpillars and aphids feed on oak leaves and sap
- Ladybirds eat aphids, and nothing else
- Blue tits eat caterpillars and aphids
- Mice eat acorns and seeds from the wildflowers
- Owls eat mice
- Sparrowhawks eat blue tits, and mice when they must
- Bees pollinate the wildflowers
- Fungi and bacteria decompose everything that dies
Remove
Ladybirds — still in the wood
On a simple diagram they have exactly one line coming out of them.
- Straight awayNothing is eating aphids except the blue tits, and aphids breed extremely fast. Aphid numbers climb steeply within a season.
- A season or two laterBlue tits switch towards the easy aphid supply and take fewer caterpillars, so caterpillar numbers rise as well. The oak is now losing sap to aphids and leaves to caterpillars at the same time.
- Further outA tree under that much pressure grows less and produces fewer acorns. Fewer acorns means fewer mice, and fewer mice means the owls have a bad year — four steps from an insect that never went near an owl.
Seven of the eleven organisms in the wood changed, and the two that started the story — ladybirds and aphids — are the least interesting part of it.
Blue tits — still in the wood
They sit in the middle of this web, eating two things and being eaten by one.
- Straight awayBoth caterpillars and aphids increase, because their main bird predator has gone. The oak comes under pressure from two directions at once.
- A season or two laterLadybirds do well on the aphid glut, but they cannot eat caterpillars and cannot make up the difference. Sparrowhawks have lost their main prey and hunt mice instead.
- Further outMice are now hunted by both owls and sparrowhawks, so mouse numbers fall, and the owls — who eat nothing else — are squeezed out by a competitor that arrived because of a change three species away.
Every organism in the wood except the bees, the wildflowers and the decomposers changed, and the owls suffered most from losing a bird they never competed with.
Owls — still in the wood
A top predator. Nothing eats an owl, so it looks like the safest species to remove.
- Straight awayMouse numbers rise, held back only by the sparrowhawks, who take mice occasionally rather than as a main food.
- A season or two laterMore mice means more acorns and wildflower seeds eaten. Fewer seeds germinate, and the flowers begin to thin out.
- Further outFewer wildflowers means less for the bees, whose numbers drop — and the flowers that remain are pollinated less, so they set fewer seeds again. Removing the top of the web has damaged the bottom of it.
The effect ran from a predator down to the plants and then to an insect that has nothing to do with either. Top predators are usually the species whose removal travels furthest.
Caterpillars — still in the wood
A pest, from the tree’s point of view. This is the removal a gardener would choose.
- Straight awayThe oak loses fewer leaves and grows better for a season. So far, exactly what was wanted.
- A season or two laterBlue tits lose the food they raise their chicks on — a pair needs hundreds of caterpillars a day in spring — and far fewer chicks survive. Blue tit numbers fall sharply.
- Further outWith fewer blue tits, aphids are barely predated and increase; the oak is attacked by sap-suckers instead of leaf-eaters. Sparrowhawks, short of blue tits, turn to mice, and owl numbers fall.
The tree gained one good season and then lost the birds that were protecting it from something else. Removing a pest is not the same as solving a problem.
Bees — still in the wood
They eat nothing in this web and nothing in it eats them. They are here for a different reason.
- Straight awayThe wildflowers are not pollinated, so they set almost no seed. Nothing looks wrong for a year, because the plants are already grown.
- A season or two laterThe next generation of wildflowers barely appears. Ground cover thins, and the seeds mice depend on become scarce.
- Further outMouse numbers fall, and the owls with them. An insect connected to this web only by a service, not by a feeding line, turns out to hold up two vertebrates — which is the subject of Pollinators and food security.
The bees are in no food chain here and their removal still empties the web. Feeding is not the only kind of dependence.
The oak — still in the wood
The producer. Everything else here ultimately runs on it.
- Straight awayCaterpillars and aphids have nothing to eat and collapse. Acorns stop, so the mice lose their main winter food.
- A season or two laterBlue tits and ladybirds lose their prey; owls lose the mice. Every consumer in the wood is now short of food, and there is no alternative producer at this scale.
- Further outThe decomposers have a brief glut of dead wood and leaf litter and then nothing arriving. Without a producer the web does not reorganise — it empties.
Every other removal redistributed the energy in the wood. Removing the producer removes the energy itself, which is the one loss a web cannot absorb.
Three things a web does
Connected does not mean fragile, and it does not mean safe.
Rule one
Effects travel sideways
Not only up and down the chain. The species that suffers most is often a competitor, or something that shared a predator, rather than anything the removed species touched.
Rule two
Alternative routes absorb shocks
A predator that eats six things barely notices losing one. A specialist that eats one thing does not survive losing it. Which is why webs with more connections are steadier.
Rule three
Effects are delayed
Populations take seasons or years to respond, so the damage from a removal often appears long after anyone is still watching for it.
Key fact
Removing one species changes the numbers of species it never touched, because every organism in a web is connected to every other through some route. A web with many alternative routes absorbs the change; a web with few does not.
Think again
“Removing a species only affects the things directly above and below it.”
That is what a food chain would predict, and it is why the chain is a teaching device rather than a description. In a web, an effect travels along every route out of the missing species and keeps going, and the routes that matter most are often sideways rather than up and down. Take the blue tits out of the wood: caterpillars increase, obviously — but so do aphids, because blue tits were eating those too, and the ladybirds now have more food than they can use while the sparrowhawks have lost their main prey and start taking mice instead, which affects the owls. Three or four species deep, and none of them shares a line with a blue tit on a simple diagram. The practical consequence is that you cannot predict the result of a removal by looking only at what the species ate and what ate it.
“If it goes wrong, you just put the species back.”
Sometimes you can, and it is expensive, slow and often only partly successful. Frequently you cannot, because while the species was absent the ecosystem rearranged itself: another organism moved into the space, the plants it depended on died out, or the population that would have been the source is extinct. Extinction has no undo — that is what the word means, and it is the subject of When the environment changes: extinction. Even where a return is possible, the wood you put the species back into is not the wood it left. This is the reason ecologists argue for caution rather than confidence: not because every disturbance is a disaster, but because the outcome genuinely cannot be predicted in advance, and the experiment cannot be run twice.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Follow one step
All the ladybirds in the wood are killed by a pesticide. What is the immediate effect?
Rung 2 · The one that catches people
Owls are removed from the wood. Which species is affected that most people would not predict?
Rung 3 · Trace a chain of effects
Blue tits disappear from the wood. Trace the effects through at least three further species, saying what causes each step, and finish by explaining why a food chain diagram would not have predicted the last one.
Rung 4 · Take it somewhere new
A farmer sprays an insecticide on a field bordering the wood. It kills aphids, and it also kills ladybirds and most other insects. Predict what happens over the following two years, and explain why the aphid problem often comes back worse than before.
Key note
Every organism in a food web is connected to every other by some route, so removing one changes the numbers of species it never touched. Effects travel sideways as well as up and down, and they take time to appear. A web with many alternative feeding routes absorbs a loss more easily than one with few, and a species that has gone cannot reliably be put back.
Going further
Wolves were hunted out of Yellowstone National Park by the 1920s and reintroduced in 1995, and what the park has shown since is how far past its own prey a top predator reaches. Elk numbers fell. Elk behaviour changed too — they stopped feeding for long spells in the open valley bottoms where they were easiest to ambush. Willow and aspen, the plants elk browse hardest, began growing back along those streams. Beavers need willow, and beavers returned to the stretches where it recovered; their dams then changed what could live in the water. Follow the links: a wolf never touches a willow, and never touches a beaver. It is joined to the willow through the elk and to the beaver through the willow, and that is enough. This is the difference between a chain and a web. Pull one strand hard and things you were not watching move — which is why remove the predator and there will simply be more prey is never the whole answer.
Before this lesson
Connects to
At GCSE this becomes
- Trophic cascades, keystone species, and the effect of biodiversity on the stability of an ecosystem.
Where to next
Ask Mr Badmus AI
Want to trace a removal through a web of your own?
The wood is a teaching web of eleven organisms, two of which — the wildflowers, and the fungi and bacteria — stand for whole groups rather than single species. Real woodland webs contain hundreds, the outcomes described are plausible directions rather than predictions, and effects that appear here in three rounds take years in a real ecosystem and do not always arrive.
Lesson content © MrBadmusAI.