📚 IGCSE CIE Science: Ecosystems Key Concepts Explained | IGCSE CIE 科学：生态系统 考点精讲
In IGCSE CIE Science, ecosystems form a fundamental part of the biology syllabus, covering the interactions between organisms and their environment. This article breaks down every essential concept you need to master, from food chains and energy transfer to the carbon cycle and human impacts, with clear bilingual explanations tailored for exam success.
在 IGCSE CIE 科学课程中，生态系统是生物学部分的基础，涵盖生物与其环境之间的相互作用。本文为你拆解每一个必须掌握的核心概念，从食物链和能量传递到碳循环及人类影响，用清晰的中英双语讲解，助你轻松应对考试。
1. What Is an Ecosystem? | 什么是生态系统？
An ecosystem is a natural unit consisting of all the living organisms (biotic components) in a particular area, interacting with each other and with their non-living (abiotic) environment. A pond, a forest, or even a single log can be an ecosystem.
生态系统是一个自然单位，由特定区域内的所有生物（生物组分）组成，它们彼此之间以及与非生物（非生物）环境相互作用。一个池塘、一片森林，甚至一根原木都可以是一个生态系统。
2. Biotic and Abiotic Factors | 生物因素与非生物因素
Biotic factors include all living things such as plants, animals, fungi, and bacteria. They affect each other through predation, competition, and symbiosis. Abiotic factors are non-living physical and chemical elements like sunlight, temperature, water, pH, and mineral salts. Both types determine which organisms can survive in a habitat.
生物因素包括所有生物，如植物、动物、真菌和细菌。它们通过捕食、竞争和共生相互影响。非生物因素是指非生命的物理和化学要素，如阳光、温度、水分、pH 值和矿物盐。两者共同决定了哪些生物能够在一个栖息地中生存。
3. Food Chains and Food Webs | 食物链与食物网
A food chain shows a single linear pathway of energy flow from one organism to another, starting with a producer. A food web is a network of interconnected food chains, representing the many feeding relationships in an ecosystem. Arrows point from the food to the feeder, showing the direction of energy transfer.
食物链展示能量从一种生物流向另一种生物的单一线性路径，起点是生产者。食物网是由多个食物链相互连接而成的网络，代表生态系统中众多的取食关系。箭头从食物指向取食者，表示能量传递的方向。
- Example food chain: Grass → Rabbit → Fox.
- 例子：草 → 兔 → 狐。
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 (plants/algae) occupy the first trophic level, primary consumers the second, secondary consumers the third, and so on. Only about 10% of energy is transferred from one trophic level to the next; the rest is lost mainly through metabolic heat, movement, and undigested materials.
食物链中的每一步称为营养级。生产者（植物/藻类）占据第一营养级，初级消费者占据第二级，次级消费者占据第三级，以此类推。只有大约 10% 的能量从一个营养级传递到下一个营养级；其余能量主要通过代谢产热、运动以及未消化的物质而散失。
5. Ecological Pyramids | 生态金字塔
Ecological pyramids are graphical representations of the structure of an ecosystem. There are three types: pyramids of numbers (count of organisms), pyramids of biomass (total dry mass), and pyramids of energy (energy content at each trophic level). Pyramids of energy are always upright, while pyramids of numbers and biomass can sometimes be inverted (e.g., a single tree supporting many insects).
生态金字塔是生态系统结构的图形化表示。共有三种类型：数量金字塔（生物体个数）、生物量金字塔（总干质量）和能量金字塔（每个营养级的能量含量）。能量金字塔总是正向的，而数量金字塔和生物量金字塔有时可能倒置（例如，一棵树支持许多昆虫）。
6. Reasons for Energy Losses | 能量损失的原因
At each trophic level, energy is lost because organisms use energy for respiration, growth, and reproduction. Not all parts of an organism are eaten, and some food is egested as faeces. Heat energy released during respiration is radiated away and cannot be reused by the ecosystem, making energy flow non-cyclical.
在每个营养级，能量都会损失，因为生物体需要能量用于呼吸、生长和繁殖。生物体的所有部分并非都被吃掉，部分食物以粪便形式被排出。呼吸过程中释放的热能会散失，无法被生态系统再次利用，这使得能量流动是非循环的。
7. The Carbon Cycle | 碳循环
Carbon is cycled through the ecosystem via processes that include photosynthesis, respiration, decomposition, and combustion. Carbon dioxide in the atmosphere is taken up by producers during photosynthesis to form organic compounds. These compounds are passed along food chains. Respiration by all organisms releases CO₂ back into the atmosphere. Decomposers break down dead matter, releasing CO₂, and the burning of fossil fuels adds extra CO₂.
碳通过光合作用、呼吸作用、分解作用以及燃烧等过程在生态系统中循环。大气中的二氧化碳被生产者在光合作用中吸收，形成有机化合物。这些化合物沿食物链传递。所有生物的呼吸作用将 CO₂ 释放回大气。分解者分解死物，释放 CO₂，而化石燃料的燃烧则额外增加了 CO₂。
8. Populations and Communities | 种群与群落
A population is a group of organisms of the same species living in the same area at the same time. A community is made up of all the populations of different species living and interacting in an ecosystem. Population size can be affected by factors like food availability, predators, disease, and migration.
种群是在同一时间、同一地区生活的同种生物个体的组合。群落则由一个生态系统中所有不同物种的种群组成，它们共同生活并相互作用。种群大小会受到食物可得性、捕食者、疾病以及迁徙等因素的影响。
9. Predator-Prey Relationships | 捕食者-猎物关系
In a predator-prey cycle, the population sizes of predators and prey are interdependent. As prey numbers increase, predators have more food, so their population also rises. The increase in predators then reduces the prey population, which in turn causes a decline in predator numbers. This leads to a repeating cyclic pattern in population graphs.
在捕食者-猎物循环中，捕食者和猎物的种群数量相互依赖。当猎物数量增加时，捕食者食物增多，其种群也随之上升。捕食者数量的增加又会减少猎物数量，进而导致捕食者数量下降。这导致种群图中出现重复的周期性波动模式。
| Phase | Prey Population | Predator Population | 阶段 | 猎物数量 | 捕食者数量 |
| 1 | Increases | Starts low, then rises | 阶段 1 | 增加 | 开始低，随后上升 |
| 2 | Peaks, then declines | Peaks | 阶段 2 | 达到峰值，然后下降 | 达到峰值 |
| 3 | Low | Declines | 阶段 3 | 低 | 下降 |
| 4 | Starts to recover | Low | 阶段 4 | 开始恢复 | 低 |
10. Human Impacts on Ecosystems | 人类对生态系统的影响
Human activities such as deforestation, pollution, and overfishing can disrupt ecosystems. Deforestation reduces biodiversity, destroys habitats, and affects the carbon cycle. Water pollution from fertilisers causes eutrophication, leading to oxygen depletion and death of aquatic life. Conservation strategies, including setting up protected areas and sustainable resource use, are vital to preserving ecosystems.
人类活动，如砍伐森林、污染和过度捕捞，会扰乱生态系统。砍伐森林降低生物多样性，破坏栖息地，并影响碳循环。化肥造成的水污染导致富营养化，进而引起缺氧和水生生物死亡。保护策略，包括建立保护区和可持续资源利用，对维护生态系统至关重要。
11. Sampling Techniques in Ecosystems | 生态系统中的取样技术
To study the distribution and abundance of organisms, ecologists use sampling methods. Random sampling with quadrats is used for stationary organisms like plants, while transects are used to investigate changes across a habitat. A population estimate can be obtained using the capture-mark-recapture method for mobile animals.
为了研究生物的分布和数量，生态学家使用取样方法。对植物等静止生物使用样方随机取样，而样带则用于调查栖息地的变化。对于移动动物，可使用标记重捕法进行种群数量估算。
- Quadrats: square frames of known area placed randomly; count organisms inside.
- 样方：已知面积的正方形框架，随机放置；统计内部生物。
- Mark-recapture formula: Population size = (number caught first time × number caught second time) ÷ number of marked recaptured.
- 标记重捕公式：种群大小 = (第一次捕捉数 × 第二次捕捉数) ÷ 标记重捕数量。
12. Bioaccumulation and Biomagnification | 生物积累与生物放大
Bioaccumulation refers to the build-up of toxic substances like pesticides or heavy metals in an organism’s body over time, as the rate of intake exceeds the rate of elimination. Biomagnification is the increasing concentration of these toxins at successive trophic levels in a food chain, posing the greatest threat to top predators.
生物积累是指有毒物质（如农药或重金属）在生物体内随时间积累，因为摄入速率超过了排出速率。生物放大则是指这些毒素在食物链中依次递增的营养级上浓度逐渐增大的现象，对顶级捕食者构成的威胁最大。
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