Ecosystem Balance and Food Chains
FoundationAn ecosystem mixes living things and their physical setting; energy flows along food chains from producers through consumers to decomposers, woven into food webs. Each link's numbers restrain the others — touch one and all respond. Invasive species, overfishing or pesticide abuse upset the balance, and conservation must protect habitats and whole webs.
- An ecosystem binds producers, consumers, decomposers and the physical setting in mutual dependence.
- Food-chain arrows follow energy; energy shrinks each level, leaving top predators fewest.
- Complex food webs steady the system; every link is another's backup food.
文體小知識：生態科普說明文用食物鏈和食物網解釋生物之間的關係，並示範牽一髮動全身的生態平衡。
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1A woodland or a pond is an ecosystem: a whole where living things and their physical setting interact. The living part plays three roles. Producers — green plants and algae — turn sunlight into food by photosynthesis, the energy source of it all. Consumers, which cannot make their own food, include herbivores that eat plants and carnivores that eat animals. Decomposers — bacteria and fungi — break down remains and waste into nutrients, returning them to the soil for producers. Sunlight, water, air, soil and temperature decide what can live there.
2Energy flows along a food chain: in 'grass → rabbit → hawk' the arrow points the way energy moves — from the eaten to the eater. A chain rarely exceeds four or five links, for each level loses most of its energy as heat and activity, passing on only about a tenth: of ten thousand units of grass energy, about a thousand reach the rabbit and only about a hundred the hawk. Higher-level animals are therefore fewer. Chains weave into a food web: the hawk also eats snakes, and snakes eat mice too — the more complex the web, the more alternatives when one species vanishes, and the steadier the system.
3Balance is dynamic: predator and prey numbers restrain each other. The classic case is Canada's snowshoe hare and lynx, rising and falling in roughly ten-year cycles — more hares feed more lynx, more lynx cut the hares down, fewer hares starve the lynx, round and round. But when humans break the balance, results can be dire: the pesticide DDT, scarce in lake water, accumulates up the food chain (bioaccumulation) to millions of times the water concentration in fish-eating raptors, thinning eggshells and failing breeding; rabbits introduced to Australia, lacking predators, bred into plague and stripped the grasslands.
4Human activity presses hard on the balance: overfishing collapses fisheries, and reclaimed wetlands strand migrating birds. Hong Kong's Mai Po marshes, protected, keep their mangroves and mudflats and each winter host the endangered black-faced spoonbill — a success of protecting whole webs and habitats. Conservation wisdom thinks in wholes: closed seasons for fish to breed, sustainable seafood choices, risk assessment before introducing alien species — not patchwork fixes. An ecosystem is a precision machine, and every gear deserves respect.
1Summary 重點摘要
An ecosystem mixes living things and their physical setting; energy flows along food chains from producers through consumers to decomposers, woven into food webs. Each link's numbers restrain the others — touch one and all respond. Invasive species, overfishing or pesticide abuse upset the balance, and conservation must protect habitats and whole webs.
2Vocabulary cards Vocabulary
The whole of living things and their physical setting interacting in an area.
Living things that make their own food by photosynthesis, such as plants and algae.
Animals feeding on other living things, herbivore or carnivore.
Bacteria and fungi breaking remains and waste down into nutrients.
A chain showing energy passing from eaten to eater, as grass → rabbit → hawk.
Many food chains interwoven, steadying the system.
The process by which plants use sunlight, water and carbon dioxide to make food and release oxygen.
All the individuals of one species in an area.
Toxics gathering up the food chain until top predators hold extreme concentrations.
Species carried by humans to new settings that, lacking predators, multiply and wreck the ecology.
3Step by step Steps
- Step 1: Spot the three rolesEvery organism in an ecosystem plays a role: producers make their own food, consumers live on other organisms, and decomposers break down remains and dead leaves, returning nutrients to nature. Example: in a pond, algae are the producers; snails eating algae and kingfishers eating fish are consumers; bacteria breaking down leaves and bodies are decomposers. Non-living factors — sunlight, water, air and soil — likewise decide what can live there.
- Step 2: Draw the arrows rightA food chain shows the order of eating and being eaten; the arrow shows the flow of energy and always runs from the eaten to the eater. Example: grass, rabbit and hawk link in turn — energy flows from grass to rabbit and on to hawk; reversing the order is wrong. The most common exam mistake is drawing the arrow backwards; remembering that the arrow means 'to whom the energy flows' keeps it safe.
- Step 3: Feel the energy shrink in numbersEnergy drains away at every level of a chain: only about a tenth passes on, the rest lost as heat or left unused. Example: of ten thousand units of grass energy, about a thousand reach the rabbit and a hundred the hawk — so top predators are always rare. This also explains why chains rarely run more than four or five levels; animals higher up need a larger range to feed them and are more easily threatened.
- Step 4: Weave chains into a webIn nature many chains cross and interlock into a food web, which makes the ecosystem more stable. Example: hawks eat rabbits and snakes; snakes eat mice and frogs; if frogs crash, snakes shift to mice — every branch is insurance for the system. The more branches, the smaller the shock when one species disappears; the more complex the web, the better the system weathers change.
- Step 5: Watch predator and prey swayPredator and prey numbers restrain each other, producing rhythmic rises and falls. Example: more hares feed more lynx; more lynx cut the hares; fewer hares starve some lynx — the two populations rise and fall in roughly ten-year cycles. Such waves are the very picture of dynamic balance; balance is not stillness but steadiness within the rise and fall.
- Step 6: Trace human damage's chain reactionToxins released by people bioaccumulate up the chain, magnifying to dangerous levels in top animals. Example: DDT at mere trace levels in water reaches millions of times that in fish-eating raptors, thinning eggshells until breeding fails. The warning is clear: pollution's worst effects land on the highest level; humans sit at the top of many chains and share the risk.
- Step 7: Practise whole-system careConservation must protect whole ecosystems and food webs, not single species alone. Example: Mai Po's protected mangroves and mudflats let black-faced spoonbills winter each year; choosing sustainable seafood and backing closed seasons guards the whole web. Once ecological balance breaks, recovery can take decades; closed seasons for fish to breed and risk checks on alien species both show whole-system thinking.
4Diagrams Diagrams
5Worked example Example
Example: a pond chain runs algae → daphnia → small fish → big fish. If pesticide-laced waste pours in, explain the effects on every level and why the big fish hold the highest poison concentration.
- Direct effect: the pesticide dissolves; trace poison is absorbed by algae and may kill the most sensitive algae and daphnia.
- Chain effect: fewer daphnia starve the small fish; fewer small fish starve the big fish — numbers fall at every level.
- Bioaccumulation: small fish eat many daphnia and keep the poison; big fish eat many small fish — at each level poison enters but never leaves.
- The numbers: suppose the water holds one unit; daphnia may hold ten, small fish over a hundred, big fish thousands — the top level is highest, and people eating them share the risk.
- Answer: The waste lowers every level, and the poison bioaccumulates up the chain: big fish, at the top level, eat many poisoned small fish and so hold the highest concentration — which is why water quality must be protected at the source.
6Common mistakes Mistakes
7Quick check Quick check
Q1What three roles make up the living part of an ecosystem?
Q2In 'grass → rabbit → hawk', what does the arrow mean?
Q3Why does a food chain rarely exceed four or five links?
Q4What is bioaccumulation? Give one consequence.
Q5Name two concrete actions protecting ecological balance.
8Recap Recap
- An ecosystem binds producers, consumers, decomposers and the physical setting in mutual dependence.
- Food-chain arrows follow energy; energy shrinks each level, leaving top predators fewest.
- Complex food webs steady the system; every link is another's backup food.
- Predator and prey numbers restrain each other in dynamic balance; bioaccumulation magnifies poisons at the top.
- Conservation thinks whole: habitats, closed seasons, sustainable seafood, and care against invasive species.