📚 Ecosystems: IGCSE WJEC Science Key Points | IGCSE WJEC 科学：生态系统考点精讲
Ecosystems are the foundation of life on our planet, connecting living organisms with their physical surroundings in a delicate balance of energy flow and nutrient cycling. For IGCSE WJEC Science, understanding ecosystems is not only about memorising definitions but also about grasping how food chains, carbon cycles, and human activities intertwine. This revision guide unpacks every key concept you need, from ecological terms to sampling techniques, all explained in clear, exam-focused detail.
生态系统是地球上生命的基础，它将生物与其物理环境联系起来，形成能量流动和养分循环的微妙平衡。对于 IGCSE WJEC 科学而言，理解生态系统不仅要记住定义，还要掌握食物链、碳循环以及人类活动如何相互交织。这份复习指南将逐一解析你需要掌握的每个核心概念，从生态学术语到采样技术，所有内容都以清晰、紧扣考点的细节进行讲解。
1. What is an Ecosystem? | 什么是生态系统？
An ecosystem is a natural unit that includes all the living organisms (biotic factors) and non-living physical components (abiotic factors) interacting in a specific area. A pond, for example, contains fish, plants, and microbes alongside water, sunlight, and dissolved oxygen, all functioning as one system.
生态系统是一个自然单位，包括特定区域内相互作用的所有生物（生物因素）和非生物物理成分（非生物因素）。例如，一个池塘里既有鱼类、植物和微生物，也有水、阳光和溶解氧，这些全部作为一个系统在运转。
Biotic factors comprise every living thing: plants, animals, fungi, and bacteria. Abiotic factors cover physical and chemical elements such as temperature, light intensity, pH, soil type, and water availability. Changes in any abiotic factor can dramatically affect the entire community.
生物因素包括所有生物：植物、动物、真菌和细菌。非生物因素涵盖物理和化学元素，如温度、光照强度、pH 值、土壤类型和含水量。任何非生物因素的改变都可能对整个群落产生巨大影响。
2. Key Ecological Terms | 关键生态学术语
Habitat: The specific place where an organism lives, providing the resources it needs to survive, such as food, shelter, and a breeding site. A log in a forest is a habitat for woodlice.
生境：生物体居住的具体地点，为其提供生存所需的资源，如食物、庇护所和繁殖场所。森林中的一根木头就是潮虫的生境。
Population: All the organisms of the same species living in the same habitat at the same time. For example, all the dandelion plants in one field make up a population.
种群：在同一时间生活在同一生境中的所有同种生物。例如，一片田野中所有蒲公英植株就是一个种群。
Community: All the populations of different species living and interacting together in a shared habitat. A woodland community includes bluebell populations, fox populations, earthworm populations, and many others.
群落：在同一个生境中共同生活、相互影响的所有不同物种种群。一个林地群落包括蓝铃花种群、狐狸种群、蚯蚓种群等等。
Niche: The role an organism plays within its ecosystem, including what it eats, where it lives, and how it interacts with other organisms and the environment. No two species can occupy exactly the same niche for long.
生态位：生物体在其生态系统中扮演的角色，包括它吃什么、住在哪里，以及它如何与其他生物和环境互动。长期来看，没有两个物种能占据完全相同的生态位。
3. Feeding Relationships: Food Chains and Webs | 摄食关系：食物链与食物网
A food chain is a linear sequence showing how energy and nutrients pass from one organism to the next. An arrow always points from the organism being eaten to the organism that eats it, indicating the direction of energy flow: grass → rabbit → fox.
食物链是一条线性序列，展示能量和营养物质如何从一个生物体传递到下一个。箭头始终从被吃的生物指向取食者，表示能量流动的方向：草 → 兔 → 狐狸。
Most organisms eat more than one type of food, so several food chains interconnect to form a food web. A food web gives a much more realistic picture of feeding relationships in an ecosystem and shows how energy can take many different routes.
大多数生物的食物不止一种，因此多条食物链相互连接，形成食物网。食物网能更真实地反映生态系统中的摄食关系，并展示能量可以通过许多不同路径流动。
When interpreting food webs, you should be able to identify producers, primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and tertiary consumers. Always remember that the arrow means ‘is eaten by’.
在解读食物网时，你需要能识别出生产者、初级消费者（食草动物）、次级消费者（吃食草动物的食肉动物）和三级消费者。始终记住箭头表示“被……吃掉”。
4. Producers, Consumers and Decomposers | 生产者、消费者与分解者
Producers are organisms that make their own food using sunlight energy through photosynthesis. Almost all food chains start with a producer, such as green plants or algae, which convert carbon dioxide and water into glucose and oxygen.
生产者是通过光合作用利用太阳能制造自身食物的生物。几乎所有的食物链都以生产者开始，例如绿色植物或藻类，它们将二氧化碳和水转化为葡萄糖和氧气。
Consumers cannot photosynthesise and must obtain energy by eating other organisms. Herbivores (primary consumers) eat plants, carnivores eat other animals, and omnivores consume both plants and animals. A squirrel, for instance, is an omnivore that feeds on acorns and bird eggs.
消费者无法进行光合作用，必须通过取食其他生物来获取能量。食草动物（初级消费者）以植物为食，食肉动物捕食其他动物，杂食动物则既吃植物也吃动物。例如，松鼠是杂食动物，既吃橡果也吃鸟蛋。
Decomposers, mainly bacteria and fungi, break down dead organisms and waste materials, releasing minerals back into the soil. Without decomposers, nutrients would remain locked in dead matter and could not be recycled to support new plant growth.
分解者主要是细菌和真菌，它们分解死去的生物和废弃物，将矿物质释放回土壤。如果没有分解者，养分就会被锁在死物质中，无法循环利用以支持新植物的生长。
5. Trophic Levels and Energy Flow | 营养级与能量流动
A trophic level is the position an organism occupies in a food chain. Level 1 always contains producers (e.g. grass), level 2 includes primary consumers (e.g. grasshoppers), level 3 secondary consumers (e.g. frogs), and so on. Decomposers operate at every trophic level.
营养级是生物体在食物链中所处的位置。第1级总是生产者（如草），第2级是初级消费者（如蚱蜢），第3级是次级消费者（如青蛙），依此类推。分解者作用于每一个营养级。
Energy enters most ecosystems as sunlight, captured by producers during photosynthesis. This energy is then transferred to consumers when they eat plants or other animals. At each stage, a great deal of energy is lost as heat through respiration, movement, and undigested waste.
能量以阳光的形式进入大多数生态系统，被生产者通过光合作用固定。当消费者吃掉植物或其他动物时，这部分能量便被传递下去。在每个阶段，都会有大量能量通过呼吸作用、运动和未消化废物以热量的形式散失。
6. Energy Transfer Efficiency | 能量传递效率
When energy passes from one trophic level to the next, on average only about 10% is stored as new biomass. The remaining 90% is lost, largely as heat from respiration, in excretion, or in parts of the organism that are not eaten, such as bones or roots.
当能量从一个营养级传递到下一个时，平均只有大约 10% 被储存为新的生物量。其余约 90% 被损失掉了，主要是因为呼吸作用产生的热量、排泄物，或是未被取食的身体部分，如骨骼或根系。
This low efficiency explains why food chains rarely have more than four or five trophic levels. There simply is not enough energy left at the top to support a large population of predators. In WJEC questions, you may need to calculate energy transfer efficiency: efficiency (%) = (energy in new biomass ÷ energy consumed) × 100.
这种低效率解释了为什么食物链很少超过四到五个营养级。传递到顶层的能量根本不足以养活大量的捕食者种群。在 WJEC 考题中，你可能需要计算能量传递效率：效率 (%) = (新生物量中的能量 ÷ 摄入能量) × 100。
7. Pyramids of Numbers, Biomass and Energy | 数目、生物量与能量金字塔
Pyramid of numbers: Represents the count of organisms at each trophic level. It is often upright but can be inverted, for example when many aphids feed on a single large oak tree. The width of each bar reflects the number of individuals, not their size or mass.
数目金字塔：表示每个营养级的生物个体数量。通常是正立的，但也可能倒置，例如许多蚜虫以一棵大橡树为食时。每个横条的宽度反映个体数量，而非体型或质量。
Pyramid of biomass: Shows the total dry mass of living material at each level. Because biomass considers the actual mass of organisms, inverted pyramids are rare, though they can occasionally occur in aquatic ecosystems where phytoplankton reproduce very quickly.
生物量金字塔：显示每个营养级活生物质的干重总量。由于生物量考虑了生物的实际质量，倒置的金字塔很少见，但在浮游植物繁殖非常迅速的水生生态系统中偶尔会出现。
Pyramid of energy: Illustrates the total energy stored at each trophic level over a given time. This pyramid is always upright because energy is lost between levels. It is the most accurate representation of energy flow in a community.
能量金字塔：展示在一定时间内每个营养级储存的总能量。这个金字塔总是正立的，因为能量在各层级间会损失。它是群落中能量流动最准确的表征。
8. The Carbon Cycle | 碳循环
Carbon is a fundamental element in all living matter. It circulates between the atmosphere, organisms, oceans, and rocks. Photosynthesis removes carbon dioxide from the air and fixes it into carbohydrates in plants: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
碳是所有生命体的基本元素。它在大气、生物体、海洋和岩石之间循环。光合作用从空气中吸收二氧化碳，并将其固定到植物的碳水化合物中：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。
Respiration by plants, animals, and decomposers returns carbon dioxide to the atmosphere. When organisms die, decomposers break down their bodies, releasing CO₂. Combustion of fossil fuels and wood also adds carbon dioxide back into the air rapidly.
植物、动物和分解者的呼吸作用将二氧化碳送回大气。当生物死亡时，分解者将其尸体分解，释放出 CO₂。化石燃料和木材的燃烧也迅速向空气中重新排放二氧化碳。
Over millions of years, some dead organic matter becomes fossilised, forming coal, oil, and natural gas. These fossil fuels store vast amounts of carbon that had been removed from the atmosphere by ancient photosynthesis. Burning them releases this stored carbon, disrupting the natural balance.
在数百万年间，一些死有机质会变成化石，形成煤、石油和天然气。这些化石燃料储存了古代光合作用从大气中移除的大量碳。燃烧它们会释放这些储存的碳，打破自然平衡。
Oceans also act as major carbon sinks. Carbon dioxide dissolves in seawater, where it can be used by marine organisms to build shells (calcium carbonate, CaCO₃). Over time, these shells form limestone rock, locking carbon away for geological timescales.
海洋也是主要的碳汇。二氧化碳溶解在海水中，可被海洋生物用来制造外壳（碳酸钙，CaCO₃）。随着时间的推移，这些外壳形成石灰岩，将碳锁定在地质时间尺度上。
9. Measuring Populations: Sampling with Quadrats | 种群数量测量：样方法
To estimate the size of a plant or slow-moving animal population, ecologists often use a quadrat — a square frame of known area, typically 0.25 m². Quadrats are placed randomly in the study area to avoid bias, often with coordinates generated by a random number table.
为了估算植物或缓慢动物的种群大小，生态学家经常使用样方——一个已知面积（通常为 0.25 平方米）的方形框。样方被随机放置在研究区域内以避免偏差，通常使用随机数表生成坐标。
In each quadrat, the number of the target organism or the percentage cover is recorded. Several quadrats are sampled, and the average count or cover is calculated. The population estimate for the whole area can then be worked out using:
在每个样方中，记录目标生物的数量或覆盖百分比。采集多个样方后，计算平均数量或覆盖度。然后可以通过以下公式估算整个区域的种群数量：
Estimated population = average count per quadrat × (total area / quadrat area)
估算种群数 = 每个样方的平均数量 × (总面积 / 样方面积)
When sampling along an environmental gradient, a line called a transect is used. In a belt transect, quadrats are placed at regular intervals along the line, allowing you to investigate how the distribution of species changes with factors like light or moisture.
当沿着环境梯度采样时，会使用一条叫做样线的线。在样带调查中，样方沿样线按固定间隔放置，这使你能够探究物种分布如何随光照或水分等因素发生变化。
10. Human Impact on Ecosystems | 人类对生态系统的影响
Human population growth demands more food, energy, and land, leading to habitat destruction, pollution, and the overuse of natural resources. One major effect is deforestation — the large-scale removal of trees — which reduces biodiversity, destroys habitats, and releases stored carbon when trees are burned or decay.
人口增长需要更多的食物、能源和土地，从而导致栖息地破坏、污染和自然资源的过度使用。一个主要影响是森林砍伐——大规模清除树木——这会降低生物多样性，破坏栖息地，并在树木被烧毁或腐烂时释放储存在其中的碳。
Water pollution from sewage and agricultural fertilisers can cause eutrophication. Excess nitrate and phosphate lead to algal blooms that block light, causing underwater plants to die. Decomposers then use up oxygen, killing fish and other aquatic life.
来自污水和农业化肥的水污染会导致富营养化。过量的硝酸盐和磷酸盐引发藻华，遮挡光线，导致水下植物死亡。随后分解者消耗氧气，致使鱼类和其他水生生物死亡。
Air pollution includes sulphur dioxide (SO₂) and nitrogen oxides (NOₓ), produced by burning fossil fuels. These gases dissolve in rainwater to form acid rain (containing H₂SO₄ and HNO₃), which damages trees, leaches minerals from soil, and makes lakes too acidic for life.
空气污染包括燃烧化石燃料产生的二氧化硫 (SO₂) 和氮氧化物 (NOₓ)。这些气体溶解在雨水中形成酸雨（含有 H₂SO₄ 和 HNO₃），损害树木，淋溶土壤中的矿物质，并使湖泊酸性化，不适合生物生存。
Some pollutants accumulate along food chains in a process called bioaccumulation. A classic example is DDT, an insecticide that builds up in fatty tissues and becomes more concentrated at higher trophic levels, harming top predators such as birds of prey.
有些污染物会沿着食物链累积，这一过程称为生物积累。一个经典例子是 DDT（一种杀虫剂），它在脂肪组织中积累，并在较高营养级变得更加浓缩，对猛禽等顶级捕食者造成伤害。
To reduce these impacts, sustainable practices such as reforestation, using renewable energy, reducing fertiliser use, and establishing protected areas are essential. Understanding ecosystems helps society make informed decisions to protect our planet’s future.
为了减少这些影响，重新造林、使用可再生能源、减少化肥用量以及建立保护区等可持续做法至关重要。理解生态系统能帮助社会做出明智的决定，以保护我们星球的未来。
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