📚 IGCSE AQA Science: Mastering Ecosystems | IGCSE AQA 科学：生态系统 考点精讲
Welcome to your focused revision guide on Ecosystems for IGCSE AQA Science. This topic explores how living organisms interact with each other and with their physical environment. We will break down the key concepts you need to know, from food chains and energy flow to nutrient cycles and human impacts. Each point is explained in English followed by Chinese, ensuring you grasp both the language of science and the essential facts.
欢迎来到 IGCSE AQA 科学生态系统专题复习指南。本主题探讨生物体如何相互作用以及与物理环境的关系。我们将分解你需要掌握的关键概念，从食物链和能量流动到营养循环以及人类的影响。每个要点先用英文解释，再用中文解释，确保你既能掌握科学语言，也能理解基本事实。
1. Defining an Ecosystem | 生态系统的定义
An ecosystem is a community of living organisms (biotic factors) interacting with the non-living (abiotic) parts of their environment, such as temperature, light, water and minerals. All ecosystems are made up of habitats and the populations of species living there, forming a self-supporting system.
生态系统是由生物群落（生物因素）与其环境中的非生物部分（如温度、光照、水和矿物质）相互作用而形成的一个体系。所有生态系统都由栖息地和生活在其中的物种种群组成，形成一个自我维持的系统。
2. Biotic and Abiotic Components | 生物与非生物组分
Biotic factors include producers (plants and algae), consumers (herbivores, carnivores, omnivores) and decomposers (bacteria and fungi). Abiotic factors are physical and chemical: light intensity, temperature, moisture, soil pH, wind speed, and concentration of oxygen and carbon dioxide. The distribution and abundance of organisms are controlled by these components.
生物因素包括生产者（植物和藻类）、消费者（食草动物、食肉动物、杂食动物）和分解者（细菌和真菌）。非生物因素是物理和化学因素：光照强度、温度、湿度、土壤 pH 值、风速以及氧气和二氧化碳的浓度。生物的分布和数量受这些组分的控制。
3. Food Chains and Food Webs | 食物链和食物网
A food chain shows the feeding relationship of organisms, with arrows indicating the direction of energy flow. It always starts with a producer. A food web is a network of interconnected food chains, showing that most organisms eat more than one type of food. Disruption in one part of a web can affect many other organisms.
食物链显示生物的取食关系，箭头表示能量流动的方向。它总是从生产者开始。食物网是由相互连接的食物链组成的网络，表明大多数生物不止吃一种食物。食物网中某一部分的中断会影响许多其他生物。
4. Trophic Levels and Energy Transfer | 营养级与能量传递
Each step in a food chain is called a trophic level. Producers occupy Level 1; primary consumers Level 2; secondary consumers Level 3, and so on. Only about 10% of the energy is transferred from one trophic level to the next; the rest is lost as heat through respiration, uneaten parts, and waste products.
食物链中的每一步都称为营养级。生产者占据第 1 级；初级消费者第 2 级；次级消费者第 3 级，依此类推。只有约 10% 的能量从一个营养级传递到下一个营养级；其余的能量通过呼吸作用以热的形式散失，或通过未被吃掉的部分和废物损失。
5. Efficiency of Energy Transfer | 能量传递效率
The efficiency of energy transfer between trophic levels can be calculated using the formula:
Efficiency = (Energy in biomass at higher level ÷ Energy in biomass at lower level) × 100%
能量传递效率可以用公式计算：
效率 = (较高营养级生物量中的能量 ÷ 较低营养级生物量中的能量) × 100%
For example, if a caterpillar consumes 100 kJ of energy from a plant but only stores 10 kJ in its body, the efficiency is (10 ÷ 100) × 100% = 10%. This low efficiency explains why food chains rarely have more than five trophic levels.
例如，如果一只毛毛虫从植物中摄取了 100 kJ 能量，但在体内只储存了 10 kJ，那么效率为 (10 ÷ 100) × 100% = 10%。这种低效率解释了为什么食物链很少超过五个营养级。
6. Pyramids of Number, Biomass and Energy | 数量、生物量和能量金字塔
A pyramid of number shows the count of organisms at each trophic level, but can produce inverted shapes (e.g., one tree supporting many insects). A pyramid of biomass shows the total dry mass, usually pyramid-shaped. A pyramid of energy shows energy content (kJ/m²/year), always a true pyramid because of energy losses at each level.
数量金字塔显示每个营养级的生物数量，但可能产生倒置形状（例如一棵树支撑许多昆虫）。生物量金字塔显示总干重，通常呈金字塔形。能量金字塔显示能量含量（kJ/m²/年），由于每一级都有能量损失，因此始终是一个真正的金字塔。
7. The Carbon Cycle | 碳循环
Carbon is cycled between the atmosphere, organisms, and the Earth. Photosynthesis removes CO₂ from the air; respiration, combustion, and decay return it. Decomposers break down dead matter, releasing CO₂ or forming fossil fuels over millions of years. Human activities like burning fossil fuels and deforestation increase atmospheric carbon dioxide.
碳在大气、生物体和地球之间循环。光合作用从空气中吸收 CO₂；呼吸作用、燃烧和腐烂将其返回。分解者分解死去的物质，释放 CO₂ 或在数百万年内形成化石燃料。燃烧化石燃料和砍伐森林等人类活动增加了大气中的二氧化碳。
8. The Water Cycle | 水循环
Water evaporates from bodies of water and transpires from plants, condenses into clouds, and returns as precipitation. It then runs off into rivers or infiltrates soil to become groundwater, which eventually reaches the sea. Living organisms take in water and return it through excretion and transpiration.
水从水体蒸发，从植物蒸腾，凝结成云，并以降水形式返回。然后它流入河流或渗入土壤成为地下水，最终到达海洋。生物体吸收水并通过排泄和蒸腾将水返回。
9. The Nitrogen Cycle (Overview) | 氮循环（概述）
Nitrogen gas in the atmosphere is fixed by nitrogen-fixing bacteria into nitrates that plants can absorb. Plants convert nitrates into proteins. Animals obtain nitrogen by eating plants or other animals. Decomposers convert organic nitrogen back into ammonium, and nitrifying bacteria turn it into nitrates again. Denitrifying bacteria release nitrogen gas back into the air.
大气中的氮气由固氮菌固定为植物可吸收的硝酸盐。植物将硝酸盐转化为蛋白质。动物通过吃植物或其他动物获得氮。分解者将有机氮转化回铵，硝化细菌再将其转化为硝酸盐。反硝化细菌将氮气释放回空气中。
10. Population Dynamics and Competition | 种群动态与竞争
Populations within an ecosystem can grow or shrink depending on resources, predation, disease, and competition. Intraspecific competition occurs between members of the same species; interspecific competition between different species. A predator-prey cycle often shows linked oscillations: when prey increases, predators increase after a delay, then prey decreases, and so on.
生态系统中的种群数量会因资源、捕食、疾病和竞争而增长或缩减。种内竞争发生在同一物种的成员之间；种间竞争发生在不同物种之间。捕食者-猎物循环通常显示相关的振荡：当猎物增加时，捕食者在一段延迟后增加，然后猎物减少，如此反复。
11. Human Impact: Deforestation and Pollution | 人类影响：森林砍伐和污染
Deforestation reduces biodiversity, destroys habitats, and disrupts the carbon cycle by removing trees that absorb CO₂. Burning forests releases stored carbon dioxide directly. Water pollution from fertilisers causes eutrophication, leading to algal blooms that deplete oxygen and kill aquatic life. Air pollution from burning fossil fuels contributes to acid rain and global warming.
森林砍伐降低了生物多样性，破坏了栖息地，并通过移除吸收 CO₂ 的树木而扰乱碳循环。燃烧森林会直接释放储存的二氧化碳。来自化肥的水污染导致富营养化，引发藻类大量繁殖，耗尽氧气并杀死水生生物。燃烧化石燃料造成的空气污染导致酸雨和全球变暖。
12. Conservation and Sustainability | 保护与可持续性
Conservation involves protecting ecosystems and the species within them. Methods include setting up nature reserves, replanting forests (reforestation), breeding programmes for endangered species, and laws to control pollution and overfishing. Sustainability means meeting our needs without compromising the ability of future generations to meet theirs, which requires careful management of resources.
保护涉及保护生态系统及其内部的物种。方法包括建立自然保护区、重新造林、濒危物种繁殖计划，以及控制污染和过度捕捞的法律。可持续性意味着在不损害后代满足其需求的能力的前提下满足我们的需求，这需要对资源进行谨慎管理。
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