📚 Edexcel IGCSE Science: Ecosystems Key Points | IGCSE Edexcel 科学：生态系统 考点精讲
Ecosystems are among the most exciting topics in Edexcel IGCSE Science, combining biology, chemistry, and geography. You will explore how living organisms interact with each other and their physical environment, how energy flows through feeding relationships, and how essential nutrients are recycled. This article breaks down every key concept you need to master, from food chains to the carbon and nitrogen cycles, alongside exam-focused tips.
生态系统是 Edexcel IGCSE 科学考试中最令人兴奋的主题之一，融合了生物学、化学和地理学的知识。你将探索生物如何彼此作用并与物理环境互动，能量如何通过摄食关系流动，以及必需的营养物质如何循环。本文分解了你必须掌握的每一个关键概念，从食物链到碳循环与氮循环，并提供应试技巧。
1. What is an Ecosystem? | 什么是生态系统？
An ecosystem is a natural unit consisting of all the living organisms (the community) in a particular area, interacting with the non-living (abiotic) components of their environment. The biotic factors include plants, animals, fungi, and microorganisms, while abiotic factors include sunlight, temperature, water, oxygen levels, and soil chemistry. Ecosystems can be as large as a rainforest or as small as a pond.
生态系统是一个自然单位，由特定区域内的所有生物（群落）与其环境中的非生物（非生命）成分相互作用而构成。生物因素包括植物、动物、真菌和微生物，而非生物因素包括光照、温度、水分、氧气含量和土壤化学。生态系统可以大如一片雨林，也可以小如一池塘。
2. Levels of Organisation | 组织层次
(all different populations living together) → ecosystem (community plus abiotic environment) → biome (large‑ phere (all ecosystems on Earth). In Edexcel questions, you must use these terms precisely.
要准确描述生态系统，你需要了解其层级结构：个体生物 → 种群（同一区域同一物种的群体） → 群落（所有不同种群的组合） → 生态系统（群落加非生物环境） → 生物群系（大尺度生态系统） → 生物圈（地球上所有生态系统）。在爱德思考试中，你必须精确使用这些术语。
3. Feeding Relationships: Food Chains and Food Webs | 营养关系：食物链与食物网
e.g., grass) and moving to primary, secondary, and tertiary consumers. The arrow (→) means 'is eaten by' or 'energy transferred to' ity and stability of an ecosystem: if one species declines, others have alternative food sources.
食物链展示了一条简单的线性摄食序列，从生产者（例如草）开始，再到初级消费者、次级消费者和三级消费者。箭头（→）表示“被……吃掉”或“能量传递给”。食物网是由多条相互连接的食物链组成的网络，表明大多数生物吃的食物不止一种。在食物网中，你可以看到生态系统的复杂性和稳定性：如果一个物种数量减少，其他物种还有替代的食物来源。
4. Energy Transfer and Efficiency | 能量传递与效率
Sunlight energy is captured by producers during photosynthesis. Only about 1–3% of solar energy is converted into chemical energy in plants. When a consumer eats a plant or another animal, only around 10% of the energy stored in the biomass of that trophic level is passed on to the next level. The rest is lost as heat through respiration, used for movement, excreted as waste, or remains uneaten. This poor efficiency explains why food chains rarely have more than five trophic levels.
太阳光能由生产者通过光合作用捕获。仅有约1–3%的太阳能被植物转化为化学能。当消费者吃掉植物或其他动物时，该营养级生物量中储存的能量只有约10%传递到下一级。其余的能量则通过呼吸作用以热量形式散失、用于运动、以废物形式排出或未被取食。这种低效率解释了为什么食物链很少超过五个营养级。
Energy transfer efficiency (%) = (energy in new biomass ÷ energy in consumed biomass) × 100
能量传递效率（%）=（新生物量中的能量 ÷ 被消耗生物量中的能量）× 100
5. Trophic Levels and Pyramids | 营养级与金字塔
Trophic level refers to the position an organism occupies in a food chain. Producers (trophic level 1) form the base; herbivores are primary consumers (level 2); carnivores that eat herbivores are secondary consumers (level 3), and so on. Ecological pyramids represent the structure of an ecosystem:
营养级是指生物在食物链中所处的位置。生产者（营养级1）构成基础；食草动物是初级消费者（2级）；以食草动物为食的食肉动物是次级消费者（3级），以此类推。生态金字塔代表生态系统的结构：
- Pyramid of numbers – simply counts organisms at each level. Can be inverted (e.g., one oak tree supports many caterpillars).
- 金字塔–数字金字塔——仅计算各层次的生物数量。可能呈倒置状（例如一棵橡树供养许多毛虫）。
- Pyramid of biomass – measures the dry mass of living material. Almost always pyramid-shaped, but can be slightly inverted in aquatic systems over short periods.
- 生物量金字塔——测量生命物质的干重。几乎总是呈金字塔形，但在短时间内的水生系统中可能略微倒置。
- Pyramid of energy – shows energy content. This is always upright because energy is lost at every transfer.
- 能量金字塔——显示能量含量。由于每次传递都会损失能量，因此它总是正立的。
6. Decomposers and Nutrient Cycling | 分解者与物质循环
Decomposers, mainly bacteria and fungi, break down dead organic matter and waste products. This process releases enzymes that digest complex molecules into simpler inorganic substances like nitrates, phosphates, and carbon dioxide. Without decomposers, nutrients would remain locked in dead biomass and life could not continue. Decomposition is faster in warm, moist, oxygen‑rich conditions.
分解者主要是细菌和真菌，它们分解死去的有机物质和排泄物。这一过程释放酶，将复杂的分子消化为更简单的无机物，如硝酸盐、磷酸盐和二氧化碳。如果没有分解者，营养物将被锁在死亡的生物量中，生命将无法继续。分解在温暖、潮湿、富氧的条件下更快进行。
7. The Carbon Cycle | 碳循环
Carbon is the backbone of all organic molecules. In the carbon cycle, carbon dioxide (CO₂) in the atmosphere is absorbed by plants during photosynthesis (carbon fixation). It is passed to animals through feeding. Respiration in all living organisms returns CO₂ to the air. Decomposers respire as they break down dead matter, releasing CO₂. Combustion of fossil fuels and forests adds extra CO₂, unbalancing the cycle. In the oceans, CO₂ dissolves forming carbonic acid, and marine organisms use carbonate ions to build shells, which eventually form limestone rock – a long‑term carbon store.
碳是所有有机分子的骨架。在碳循环中，大气中的二氧化碳（CO₂）被植物通过光合作用吸收（碳固定）。它通过摄食传递给动物。所有生物的呼吸作用都将CO₂释放回空气中。分解者在分解死物时进行呼吸，释放CO₂。化石燃料和森林的燃烧额外增加了CO₂，使循环失去平衡。在海洋中，CO₂溶解形成碳酸，海洋生物利用碳酸根离子建造外壳，最终形成石灰岩——是一个长期的碳储存库。
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
8. The Nitrogen Cycle | 氮循环
Nitrogen is essential for making proteins and DNA. The atmosphere is 78% nitrogen gas (N₂), but most organisms cannot use it directly. The nitrogen cycle involves four key processes:
氮对于制造蛋白质和DNA至关重要。大气中78%是氮气（N₂），但大多数生物无法直接利用。氮循环涉及四个关键过程：
- Nitrogen fixation – N₂ is converted to ammonia (NH₃) or ammonium ions (NH₄⁺) by nitrogen‑fixing bacteria in root nodules of legumes (e.g., Rhizobium) or free‑living soil bacteria. Lightning can also fix nitrogen.
- 固氮——通过豆科植物根瘤中的固氮菌（如根瘤菌）或游离的土壤细菌，将N₂转化为氨（NH₃）或铵离子（NH₄⁺）。闪电也能固氮。
- Nitrification – ammonia is oxidized first to nitrite ions (NO₂⁻) by Nitrosomonas bacteria, then to nitrate ions (NO₃⁻) by Nitrobacter bacteria. Nitrates are absorbed by plant roots.
- 硝化作用——首先由亚硝化单胞菌将氨氧化为亚硝酸根离子（NO₂⁻），然后由硝化杆菌氧化为硝酸根离子（NO₃⁻）。硝酸盐被植物根吸收。
- Assimilation – plants use nitrates to build amino acids and proteins; animals obtain nitrogen by eating plants.
- 同化作用——植物利用硝酸盐合成氨基酸和蛋白质；动物通过食用植物获取氮。
- Denitrification – in anaerobic conditions (waterlogged soil), denitrifying bacteria convert nitrates back into N₂ gas, releasing it into the atmosphere.
- 反硝化作用——在厌氧条件下（如涝渍土壤），反硝化细菌将硝酸盐转化回N₂气体，释放到大气中。
9. Population Dynamics and Competition | 种群动态与竞争
Population size depends on birth rate, death rate, immigration, and emigration. Limiting factors such as food, water, space, disease, and predation control population growth. A typical population growth curve shows a lag phase, exponential (log) phase, stationary phase, and sometimes death phase when resources run out. In an ecosystem, organisms compete for resources: plants for light, water, minerals, and space; animals for food, territory, and mates. Intraspecific competition (within the same species) is usually more intense because individuals have identical needs.
种群大小取决于出生率、死亡率、迁入和迁出。限制因素如食物、水、空间、疾病和捕食控制着种群增长。典型的种群增长曲线显示延迟期、指数（对数）期、稳定期，有时在资源耗尽时出现死亡期。在生态系统中，生物为资源而竞争：植物竞争光、水、矿物质和空间；动物竞争食物、领地和配偶。种内竞争（同一物种内）通常更为激烈，因为个体有着完全相同的需求。
10. Human Impact on Ecosystems | 人类对生态系统的影响
Human activities dramatically alter ecosystems:
人类活动极大地改变着生态系统：
- Deforestation – reduces biodiversity, destroys habitats, increases soil erosion, and contributes to CO₂ buildup.
- 森林砍伐——降低生物多样性，破坏栖息地，加剧水土流失，并导致CO₂积累。
- Agriculture – monocultures reduce genetic diversity; fertilisers can cause eutrophication (see below); pesticides harm non‑target species.
- 农业——单一种植降低遗传多样性；化肥可能导致富营养化（见下文）；农药危害非目标物种。
- Eutrophication – when nitrate and phosphate fertilisers run off into water, they cause rapid algal growth. Algae block sunlight, underwater plants die, and decomposers use up dissolved oxygen as they break down the dead algae, leading to the death of fish and other aerobic organisms.
- 富营养化——当硝酸盐和磷酸盐肥料流入水体时，引起藻类快速生长。藻类遮蔽阳光，水下植物死亡，分解者在分解死去藻类时耗尽溶解氧，导致鱼类和其他好氧生物死亡。
- Pollution – air pollution (SO₂, NOₓ) causes acid rain, damaging plants and aquatic life; plastic pollution harms marine animals.
- 污染——空气污染（SO₂、NOₓ）导致酸雨，损害植物和水生生物；塑料污染危害海洋动物。
11. Conservation and Sustainability | 保护与可持续发展
Conservation aims to maintain biodiversity and ecological balance. Strategies include establishing protected areas (national parks, marine reserves), captive breeding programmes for endangered species, reforestation, controlling pollution, and promoting sustainable farming. Sustainable development meets the needs of the present without compromising the ability of future generations to meet their own needs – for example, replanting trees after logging, using renewable energy, and reducing waste. In Edexcel exams you may be asked to evaluate different conservation methods using data.
保护旨在维持生物多样性和生态平衡。策略包括建立保护区（国家公园、海洋保护区）、针对濒危物种的圈养繁殖计划、重新造林、控制污染和推广可持续农业。可持续发展既能满足当代人的需求，又不损害子孙后代满足其自身需求的能力——例如，伐木后重新植树、使用可再生能源和减少废弃物。在爱德思考试中，你可能需要利用数据评估不同的保护方法。
12. Exam Tips for Ecosystem Questions | 生态系统试题应试技巧
Edexcel often sets structured questions where you label food webs, calculate energy transfer efficiency, or explain the effects of removing an organism. Always use precise terminology: say 'nitrifying bacteria' not just 'bacteria'. When describing carbon or nitrogen cycles, include specific chemical names and the role of decomposers. For 6‑mark evaluation questions, structure your answer: identify the issue, give scientific reasons, discuss pros and cons, and conclude. Practice drawing pyramids with labels and units. Remember: energy pyramids are always upright; numbers pyramids can be inverted.
爱德思考题常设置为结构化问题，要求你标注食物网、计算能量传递效率，或解释移除某种生物的影响。务必使用精确的术语：说“硝化细菌”而不只是“细菌”。在描述碳循环或氮循环时，要列出具体的化学名称和分解者的作用。对于6分的评估题，组织好答案结构：指出问题、给出科学理由、讨论优缺点并得出结论。练习绘制带标签和单位的金字塔。记住：能量金字塔总是正立的；数字金字塔可以是倒置的。
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