📚 A-Level Science: Ecosystems Key Revision Notes | A-Level 科学：生态系统考点精讲
Ecosystems are dynamic, interconnected systems that form the core of ecology in A-Level Science. Grasping how energy flows and matter cycles within them is essential for tackling exam questions on productivity, succession, and human impacts. This article breaks down every key concept into clear, bilingual explanations.
生态系统是 A-Level 科学中生态学的核心，是动态且相互关联的系统。掌握其中的能量流动与物质循环，对于解答有关生产力、演替和人类影响的考题至关重要。本文以清晰的双语对照方式，逐一拆解每个核心概念。
1. Definition of an Ecosystem | 生态系统定义
An ecosystem is a functional unit consisting of all the living organisms (biotic community) in a defined area, interacting with each other and with their non-living (abiotic) physical environment. Energy flows through the system while chemical elements are recycled.
生态系统是由特定区域内所有生物（生物群落）与其非生物（非生命）物理环境相互作用而形成的功能单位。能量在系统中流动，化学元素则被循环利用。
The boundaries of an ecosystem can be natural, like a pond or a forest, or artificially defined for study. Regardless of size, the principles of energy transfer and nutrient cycling remain the same.
生态系统的边界可以是自然的，比如一个池塘或一片森林，也可以是为研究而人为划定的。不论大小，能量传递和养分循环的原理均相同。
2. Biotic and Abiotic Factors | 生物与非生物因素
Biotic factors include all living components: producers, consumers (herbivores, carnivores, omnivores), and decomposers. These organisms influence each other through predation, competition, and symbiosis.
生物因素包括所有生物成分：生产者、消费者（食草动物、食肉动物、杂食动物）和分解者。这些生物通过捕食、竞争和共生相互影响。
Abiotic factors are non-living chemical and physical elements such as temperature, light intensity, water availability, soil pH, and salinity. They determine which species can survive and the rate of photosynthesis and decomposition.
非生物因素指非生命的化学与物理因素，如温度、光照强度、水分可用性、土壤酸碱度和盐度。它们决定了哪些物种能够生存，并影响光合作用与分解的速度。
3. Trophic Levels and Food Chains | 营养级与食物链
A trophic level represents a feeding stage in a food chain. Producers (first trophic level) convert light energy into chemical energy via photosynthesis. Primary consumers (herbivores) occupy the second level, and secondary and tertiary consumers follow.
营养级代表食物链中的一个取食层级。生产者（第一营养级）通过光合作用将光能转化为化学能。初级消费者（食草动物）占据第二营养级，之后是次级和三级消费者。
A food chain is a linear sequence showing ‘who eats whom’, from producers to apex predators. In reality, feeding relationships are rarely simple chains; most organisms belong to complex food webs.
食物链是一条线性的顺序，展示 “谁吃谁”，从生产者直到顶级捕食者。实际上，取食关系很少是简单的链状，大多数生物属于复杂的食物网。
4. Food Webs and Energy Flow | 食物网与能量流动
A food web is a network of interconnected food chains, illustrating multiple feeding pathways. It demonstrates how energy moves through an ecosystem from producers to various consumers, offering greater ecosystem stability.
食物网是由相互连接的食物链组成的网络，显示了多条取食路径。它体现了能量如何从生产者流向不同消费者，使生态系统具有更强的稳定性。
Energy enters ecosystems primarily via sunlight captured by producers. This energy then flows unidirectionally—from producers to consumers and finally to decomposers—with a large proportion lost as heat at each level due to respiration.
能量主要通过生产者捕获的阳光进入生态系统。随后能量单向流动，从生产者到消费者，再到分解者，在每一级都有大量能量因呼吸作用以热的形式散失。
5. Ecological Pyramids | 生态金字塔
Ecological pyramids provide graphical models of trophic structure. There are three main types: pyramid of numbers, pyramid of biomass, and pyramid of energy. Each reveals different limitations and ecological truths.
生态金字塔提供营养结构的图形模型。主要有三种类型：数量金字塔、生物量金字塔和能量金字塔。每种金字塔揭示不同的局限性和生态规律。
| Pyramid Type | Description | Shape Notes |
|---|---|---|
| Pyramid of Numbers | Number of organisms at each trophic level | Can be inverted (e.g., one tree supporting many insects) |
| Pyramid of Biomass | Dry mass of living material per unit area | Usually upright; may be inverted in aquatic ecosystems briefly |
| Pyramid of Energy | Energy content (kJ m⁻² yr⁻¹) at each level | Always upright; never inverted |
数量金字塔展示每个营养级的生物个体数量；生物量金字塔以单位面积干重表示；能量金字塔永远是正立的，因为它遵循热力学第二定律——每次能量传递都有损失。
The pyramid of energy is the most reliable for comparing ecosystems because it accounts for the rate of production and inevitable energy losses through respiration and excretion.
能量金字塔是进行生态系统比较时最可靠的方法，因为它考虑了生产速率以及通过呼吸和排泄不可避免的能量损失。
6. Productivity: GPP and NPP | 生产力：总初级生产力和净初级生产力
Gross primary productivity (GPP) is the total amount of chemical energy fixed by photosynthesis in a given area over time. Net primary productivity (NPP) is the energy left after subtracting the producer’s own respiratory losses (R).
总初级生产力（GPP）是给定时间内单位面积通过光合作用固定的化学能总量。净初级生产力（NPP）是扣除生产者自身呼吸损耗（R）后剩余的能量。
NPP = GPP – R
净初级生产率 = 总初级生产率 – 呼吸损耗
NPP represents the energy truly available to the next trophic level—herbivores and decomposers. Ecosystems with high NPP, such as tropical rainforests and estuaries, support greater biodiversity.
NPP 代表真正可供下一营养级（食草动物和分解者）使用的能量。具有高净初级生产力的生态系统，如热带雨林和河口，能支撑更大的生物多样性。
7. Energy Transfer Efficiency | 能量传递效率
On average, only about 10% of the energy stored in one trophic level is converted into biomass at the next level; the rest is lost through respiration, uneaten parts, egestion, and excretion. This is often called ‘the 10% rule’.
平均而言，一个营养级储存的能量只有大约 10% 转化为下一级的生物量；其余能量因呼吸、未被采食的部分、排遗和排泄而损失。这常被称为 “10% 法则”。
Efficiency = (Energy at higher level ÷ Energy at lower level) × 100%
效率 = (高营养级能量 ÷ 低营养级能量) × 100%
Low transfer efficiency explains why food chains rarely exceed four to five trophic levels—insufficient energy remains to support a viable population of apex predators.
低传递效率解释了为什么食物链很少超过四到五个营养级——剩余的能量不足以维持一个可存活的顶级捕食者种群。
8. The Carbon Cycle | 碳循环
Carbon is the backbone of organic molecules and cycles between the atmosphere, organisms, oceans, and rocks. Key processes include photosynthesis, respiration, decomposition, combustion, and sedimentation.
碳是有机分子的骨架，碳在大气、生物体、海洋和岩石之间循环。关键过程包括光合作用、呼吸作用、分解、燃烧和沉降。
Photosynthesis fixes atmospheric CO₂ into organic compounds. Respiration by plants, animals, and decomposers returns CO₂ to the atmosphere. Human activities, particularly burning fossil fuels and deforestation, have dramatically increased atmospheric CO₂ concentration.
光合作用将大气中的 CO₂ 固定为有机化合物。植物、动物和分解者的呼吸作用将 CO₂ 返回大气。人类活动，尤其是化石燃料燃烧和森林砍伐，已使大气 CO₂ 浓度急剧升高。
In aquatic systems, CO₂ dissolves to form carbonic acid, and marine organisms use carbonate ions to build shells, which can later form limestone. Geological processes store carbon over millions of years.
在水生系统中，CO₂ 溶解形成碳酸；海洋生物利用碳酸根离子构建贝壳，随后可形成石灰岩。地质过程将碳储存数百万年。
9. The Nitrogen Cycle | 氮循环
Nitrogen is essential for proteins and nucleic acids yet largely inaccessible in its atmospheric form (N₂). The nitrogen cycle involves nitrogen fixation, ammonification, nitrification, assimilation, and denitrification.
氮对蛋白质和核酸至关重要，但大气形式的氮气（N₂）大多不可直接利用。氮循环包括固氮、氨化、硝化、同化和反硝化作用。
Nitrogen fixation converts N₂ into ammonia (NH₃) or ammonium ions (NH₄⁺), carried out by free-living bacteria (e.g., Azotobacter) and symbiotic bacteria (e.g., Rhizobium in legume root nodules). Lightning also fixes small amounts.
固氮作用将 N₂ 转化为氨（NH₃）或铵离子（NH₄⁺），由自由生活的细菌（如固氮菌）和共生细菌（如豆科植物根瘤中的根瘤菌）完成。闪电也能固定少量氮。
Nitrification is a two-step oxidation: NH₄⁺ → NO₂⁻ (by Nitrosomonas) → NO₃⁻ (by Nitrobacter). Plants absorb nitrate and ammonium and assimilate them into amino acids. Denitrification returns N₂ to the atmosphere under anaerobic conditions, closing the cycle.
硝化作用是两步氧化过程：NH₄⁺ → NO₂⁻（亚硝酸菌）→ NO₃⁻（硝酸菌）。植物吸收硝酸盐和铵盐，将其同化为氨基酸。反硝化作用在缺氧条件下将 N₂ 送回大气，完成循环。
10. Ecological Succession | 生态演替
Succession is the directional, predictable change in species composition over time in a given area. Primary succession occurs on bare, lifeless surfaces such as lava flows or exposed rock, starting with pioneer species like lichens and mosses.
演替是特定区域内物种组成随时间发生的定向、可预测的变化。初级演替发生在裸露无生命的表面上，如熔岩流或裸露岩石，由地衣、苔藓等先锋物种开始。
Pioneers weather the rock and add organic matter, forming a thin soil that supports grasses, then shrubs, and finally climax communities such as woodland. Secondary succession occurs where soil already exists, e.g., after a forest fire, and reaches climax far more quickly.
先锋物种风化岩石并增加有机质，形成薄薄的土壤，进而支持草本植物、灌木，最终形成如树林般的顶级群落。次级演替发生在土壤已存在的地方，例如森林火灾后，比初级演替快得多。
The climax community is a stable, self-perpetuating ecosystem that remains until a major disturbance resets the succession. In exam essays, be prepared to explain how abiotic factors change throughout the sere.
顶级群落是一个稳定、能自我延续的生态系统，直至重大干扰重置演替。在考试论文题中，要能解释在整个演替系列中非生物因素如何变化。
11. Human Impacts on Ecosystems | 人类对生态系统的影响
Deforestation, intensive agriculture, urbanisation, and pollution alter abiotic conditions and simplify food webs, greatly reducing biodiversity. Habitat fragmentation isolates populations, making them more vulnerable to extinction.
森林砍伐、集约化农业、城市化和污染改变了非生物条件并简化了食物网，极大地降低了生物多样性。栖息地碎片化使种群隔离，更容易灭绝。
Climate change, driven by elevated greenhouse gas emissions, shifts temperature and precipitation patterns, forcing species to migrate or face local extinction. Ocean acidification from extra CO₂ threatens marine calcifiers like corals and shellfish.
由温室气体排放增加引起的气候变化改变了温度和降水模式，迫使物种迁徙或面临局部灭绝。过量 CO₂ 导致的海洋酸化威胁着珊瑚和贝类等海洋钙化生物。
Conservation strategies include establishing protected areas, restoring habitats, enforcing fishing quotas, and controlling invasive species. Understanding ecosystem dynamics helps design sustainable management plans that balance human needs with ecological integrity.
保护策略包括建立自然保护区、恢复栖息地、实施捕捞配额和控制入侵物种。理解生态系统动态有助于设计可持续管理方案，在人类需求和生态完整性之间取得平衡。
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