Ecosystem
AdvancedAn ecosystem is a community plus its abiotic factors; energy flows one way through food chains, with only about ten per cent passing to each next level, so higher trophic levels get ever less, while carbon and nitrogen cycle endlessly, and predation, competition and symbiosis weave an interdependent food web.
- Levels of organisation: species → population → community → ecosystem → biome → biosphere; an ecosystem = community + abiotic factors.
- Each link in a food chain is a trophic level, arrows showing energy flow; interlinked chains weave a food web of interdependent species.
- Energy flows one way with about ten per cent per level: faecal loss plus heat lost in respiration narrow the pyramid, and heat cannot be recycled.
文體小知識：科學知識說明文——由組織層次到食物網，講解生態系的成分、能量單向遞減流動與碳氮循環，並分析捕食、競爭、共生等生物間關係與生態演替。
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1Ecology studies nature in levels of organisation: individuals of one species form a population; populations sharing an environment form a community; a community interacting with abiotic factors makes an ecosystem, and the largest level is the biosphere. Hong Kong's common ecosystems include freshwater streams, rocky shores, mangroves, grassland and woodland, each with its own assemblage of organisms and conditions — ideal for local field studies.
2An ecosystem has two classes of component. Abiotic factors such as light intensity, temperature, water, oxygen content and pH shape the distribution and abundance of organisms. The community, by feeding role, divides into producers (green plants and algae, converting light energy into chemical energy), consumers (animals feeding on other organisms) and decomposers (bacteria and fungi, breaking down dead bodies and wastes into inorganic matter returned to the environment) — none can be spared.
3Feeding relationships link into food chains, and food chains interweave into a food web. Each link is a trophic level: producers are the first, herbivores the second, carnivores above them. Energy flows one way along the chain, losing heavily at each step — only about ten per cent reaches the next level — so the energy pyramid narrows steadily upward and top predators are always few in both numbers and biomass; pyramids of numbers and of biomass often take the same shape.
4Energy is lost for three reasons: some organisms are never eaten, and some food is never digested, so energy leaves in faeces; much of what is ingested is burnt in respiration and lost as heat; only the remainder goes into growth, available to the next level. Heat, once lost, cannot be recycled, so an ecosystem needs a constant input of light energy to keep running. Matter behaves differently: carbon, nitrogen and other elements cycle endlessly between organisms and the environment through photosynthesis, respiration, decomposition and combustion.
5Species in a food web depend on one another: removing one species can set off a chain reaction — prey exploding, competitors expanding, or predators above going hungry. Ecological succession describes the directional change of communities over time: primary succession starts where no life has been before, secondary succession on disturbed ground where soil remains, ending in a relatively stable climax community. Grasping these rules is the foundation of conservation and environmental management.
1Summary 重點摘要
An ecosystem is a community plus its abiotic factors; energy flows one way through food chains, with only about ten per cent passing to each next level, so higher trophic levels get ever less, while carbon and nitrogen cycle endlessly, and predation, competition and symbiosis weave an interdependent food web.
2Vocabulary cards Vocabulary
All the individuals of one species within a given area and time.
All the populations sharing an environment, interacting through various relationships.
The system formed by a community interacting with its abiotic environment — the basic unit of ecological study.
The natural environment in which an organism or population lives, with its own abiotic conditions.
An autotroph such as a green plant or alga that converts light energy into chemical energy by photosynthesis — the start of every food chain.
A heterotroph that feeds on other organisms, covering herbivores and carnivores.
An organism — chiefly bacteria and fungi — that breaks dead bodies and wastes into inorganic matter, returning materials to the cycle.
The feeding position a link occupies in a food chain: producers first, herbivores second, then successive carnivore levels.
The network formed by interlinked food chains, showing how species depend on one another.
The directional change of a community over time, from pioneer species to a stable climax community.
3Step by step Steps
- Step 1: Levels of organisation and local ecosystemsEcology studies nature level by level: species → population → community → ecosystem → biome → biosphere. A population sums the individuals of one species; a community gathers populations; add the abiotic environment and you have an ecosystem. Hong Kong's five common ecosystems — freshwater streams, rocky shores, mangroves, grassland and woodland — each host their own signature species. For example, mangroves stand at river mouths where salinity swings and the mud is oxygen-poor; their buttress roots and salt-excreting adaptations show organisms fitting the abiotic factors.
- Step 2: Abiotic factors and the three biological rolesAbiotic factors — light intensity, temperature, water, oxygen content, pH and salinity — decide where organisms can live and how many thrive. The community divides by feeding role into three: producers make their own food, the gateway through which energy enters the ecosystem; consumers feed directly or indirectly on producers; decomposers break dead bodies and wastes into inorganic matter for producers to reuse. For example, the fast, oxygen-rich upper reaches of a stream suit insect larvae with high oxygen demands, while the slow, oxygen-poor lower reaches support a different cast — abiotic factors sculpting the community.
- Step 3: Food chains, trophic levels and the webA food chain joins feeding relationships with arrows, the arrowhead showing the direction of energy flow from the eaten to the eater. Each link is a trophic level: producers first, plant-eating herbivores second, carnivores that eat herbivores third, and so on; real chains rarely run past four or five links because energy cannot survive many cuts. Interwoven chains make a food web, which mirrors reality more closely. For example, a typical land chain runs grass → grasshopper → frog → snake → eagle; on the same meadow the grasshopper may fall to several kinds of bird, and stacking all such chains gives the web.
- Step 4: One-way energy flow, ten per cent per levelEnergy flows one way along a food chain, only about ten per cent reaching each next level, for three reasons: some organisms are never eaten and some food never digested, so energy drains away in faeces; much of what is eaten is burnt in respiration and lost as heat; only the remainder goes into growth, available to the next level. Lost heat cannot be recycled, so the ecosystem needs a steady input of light energy. The energy pyramid therefore narrows upward. For example, if grass fixes one hundred thousand kilojoules, the grasshopper level gets about ten thousand, the frog level about a thousand, and the eagle level only about a hundred — which is why top predators are rare.
- Step 5: The carbon and nitrogen cyclesUnlike energy, matter cycles endlessly. The carbon cycle: carbon dioxide is fixed into organic matter by photosynthesis and passed along food chains; respiration by organisms, decomposition by decomposers and combustion of fossil fuels all return carbon dioxide to the air. The nitrogen cycle in outline: atmospheric nitrogen is converted by nitrogen fixation into compounds plants can absorb; proteins in bodies and wastes are decomposed, returning nitrogen to the soil for reuse. For example, forest fires and the burning of coal and oil release vast amounts of carbon back to the atmosphere as carbon dioxide within a short time — clear evidence of human interference in the carbon cycle.
- Step 6: The relationships between organismsOrganisms in one habitat relate in many ways. Predation: one kills and eats the other. Competition: both vie for the same resources — food, space, light. Commensalism: one gains, the other is unharmed. Mutualism: both gain. Parasitism: the parasite lives on a host and draws nutrients, usually without killing it at once. Telling the types apart lets you predict population changes. For example, microbes in a cow's rumen digest cellulose while the cow provides board and lodging — mutualism; the dodder plant sucking sap through haustoria is parasitism; weeds fighting rice for light and fertiliser in a paddy field is competition.
- Step 7: Succession, stability and conservationEcological succession is the directional change of communities over time: primary succession starts on bare ground never before colonised, such as fresh lava; secondary succession starts on disturbed ground where soil remains, as after a wildfire; pioneer species give way step by step until a relatively stable climax community forms. Species in a food web depend on one another, so removing one can trigger a chain reaction. For example, a burnt hillside first grows grasses, then shrubs displace them, and within decades woodland may return; remove the snake from a meadow web and frog and rat numbers rise in the short term, pressing harder on the grass and crops.
4Diagrams Diagrams
5Worked example Example
In a grassland ecosystem the food chain runs grass → grasshopper → frog → snake, and the grass fixes 100000 kilojoules in total. Assuming an energy transfer efficiency of ten per cent between trophic levels, calculate the energy reaching the grasshopper, frog and snake levels in theory; explain two reasons why energy decreases level by level; then state what happens to the grasshopper population in the short term if the frogs are removed, and the effect on the grass.
- Step by step: grass 100000 kilojoules; grasshopper level 100000 × 110 = 10000 kilojoules; frog level 10000 × 110 = 1000 kilojoules; snake level 1000 × 110 = 100 kilojoules.
- Reason one: some organisms are never eaten and some food never digested, so energy is lost in faeces.
- Reason two: much of the ingested energy is lost as heat in respiration; heat cannot be recycled, so energy flow is one-way.
- With the frogs removed, grasshoppers lose their main predator and their numbers rise in the short term; they graze the grass harder, so its biomass falls, and the snakes in turn lose food.
- Answer: At ten per cent transfer: about 10000 kilojoules to the grasshopper, 1000 to the frog and 100 to the snake. Energy decreases because of faecal loss and heat lost in respiration; heat cannot be reused, so the flow is one-way. Removing frogs lets grasshoppers multiply at first, over-grazing the grass so its biomass drops, while the snakes suffer from less food.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the two classes of ecosystem component, and what three feeding roles divide the community?
Q2Why does energy decrease at each step of a food chain?
Q3In what form does carbon enter living things, and by which routes does it return to the air?
Q4How does predation differ from parasitism?
Q5What is a climax community?
8Recap Recap
- Levels of organisation: species → population → community → ecosystem → biome → biosphere; an ecosystem = community + abiotic factors.
- Each link in a food chain is a trophic level, arrows showing energy flow; interlinked chains weave a food web of interdependent species.
- Energy flows one way with about ten per cent per level: faecal loss plus heat lost in respiration narrow the pyramid, and heat cannot be recycled.
- Matter cycles endlessly: carbon is fixed by photosynthesis and released by respiration, decomposition and combustion; nitrogen enters life by fixation and returns to soil by decomposition.
- Relationships are varied — predation, competition, commensalism, mutualism, parasitism; succession ends at a climax community; removing one species from a web triggers chain reactions.