📚 IGCSE OCR Science: Ecosystems – Key Exam Points | IGCSE OCR 科学：生态系统 考点精讲
Ecosystems are dynamic interactions between living organisms and their environment. Understanding key ecological concepts is essential for IGCSE OCR Science.
生态系统是生物与其环境之间动态的相互作用。掌握关键生态概念对 IGCSE OCR 科学至关重要。
1. What is an Ecosystem? | 什么是生态系统？
An ecosystem includes all the living organisms (biotic factors) in a particular area, interacting with the non-living (abiotic) environment such as soil, water, and climate.
生态系统包括特定区域内的所有生物（生物因素）以及它们与非生物环境（如土壤、水和气候）之间的相互作用。
Ecosystems can be small, like a pond, or large, like a tropical rainforest. The boundaries of an ecosystem are not always fixed; they can overlap and change over time.
生态系统可小如池塘，大如热带雨林。生态系统的边界并非总是固定；它们可能重叠并随时间变化。
2. Components of an Ecosystem | 生态系统的组成
Biotic components are producers, consumers, and decomposers. Producers (e.g. plants, algae) make their own food via photosynthesis. Consumers feed on other organisms: primary consumers eat plants, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Decomposers (bacteria and fungi) break down dead organic matter, recycling nutrients back into the soil.
生物组成部分包括生产者、消费者和分解者。生产者（如植物、藻类）通过光合作用制造食物。消费者以其他生物为食：初级消费者吃植物，次级消费者吃初级消费者，三级消费者吃次级消费者。分解者（细菌和真菌）分解死有机物，将养分循环回土壤。
Abiotic factors include light intensity, temperature, water availability, pH, and soil mineral content. These factors determine which organisms can survive and how abundant they are in an ecosystem.
非生物因素包括光照强度、温度、水分可用性、pH 值和土壤矿物质含量。这些因素决定了哪些生物能够生存以及它们在生态系统中的丰度。
3. Energy Flow and Food Chains | 能量流动与食物链
Energy enters ecosystems through sunlight, captured by producers during photosynthesis. A food chain shows the one-way transfer of energy from one organism to another. The arrow always points from the organism being eaten to the organism that eats it.
能量通过阳光进入生态系统，被生产者在光合作用中捕获。食物链表示能量从一个生物到另一个生物的单向传递。箭头始终从被吃的生物指向吃它的生物。
Example: grass → rabbit → fox. At each step, energy is lost through respiration, movement, heat, and undigested materials. Only about 10% of the energy stored in one trophic level is passed on to the next. This limits the length of food chains.
例子：草 → 兔 → 狐狸。每一步，能量通过呼吸作用、运动、热以及未消化的物质而损失。一个营养级储存的能量仅约10%传递到下一级。这限制了食物链的长度。
4. Food Webs and Trophic Levels | 食物网与营养级
A food web is a network of interconnected food chains. It provides a more realistic picture of feeding relationships and shows that most organisms eat more than one type of food. Food webs increase ecosystem stability because if one species declines, predators can switch to other prey.
食物网是相互连接的食物链网络。它更真实地反映了摄食关系，并表明大多数生物不止吃一种食物。食物网增加了生态系统的稳定性，因为如果一种物种减少，捕食者可转向其他猎物。
Trophic levels: Level 1 = producers (autotrophs), Level 2 = primary consumers (herbivores), Level 3 = secondary consumers (carnivores), Level 4 = tertiary consumers (top carnivores). Decomposers feed at all levels, releasing nutrients.
营养级：第1级=生产者（自养生物），第2级=初级消费者（食草动物），第3级=次级消费者（食肉动物），第4级=三级消费者（顶级食肉动物）。分解者在各级取食，释放养料。
5. Carbon Cycle | 碳循环
Carbon is cycled between the atmosphere, organisms, and the Earth. Key processes: photosynthesis converts CO₂ into organic compounds in plants; respiration returns CO₂ to the atmosphere; combustion of fossil fuels releases stored carbon as CO₂; decomposition breaks down dead organisms, releasing CO₂. Oceans also act as a carbon sink, absorbing and releasing CO₂.
碳在大气、生物体和地球间循环。关键过程：光合作用将CO₂转化为植物中的有机化合物；呼吸作用将CO₂释放回大气；化石燃料的燃烧将储存的碳以CO₂形式释放；分解作用分解死生物体，释放CO₂。海洋也作为碳汇，吸收和释放CO₂。
Human activities such as deforestation and burning fossil fuels disrupt the carbon cycle, increasing atmospheric CO₂ and enhancing the greenhouse effect, which leads to global warming.
人类活动（如砍伐森林和燃烧化石燃料）扰乱碳循环，增加大气CO₂，加剧温室效应，导致全球变暖。
6. Nitrogen Cycle | 氮循环
Nitrogen is essential for making proteins and DNA. The nitrogen cycle involves several key processes: nitrogen fixation converts atmospheric N₂ into ammonia (NH₃) by nitrogen-fixing bacteria in soil or root nodules; nitrification converts ammonia into nitrites (NO₂⁻) then nitrates (NO₃⁻) by nitrifying bacteria; plants absorb nitrates (assimilation) to build proteins; when organisms die, decomposers carry out ammonification, converting organic nitrogen back to ammonia; denitrification converts nitrates back to N₂ gas, returning it to the atmosphere.
氮是制造蛋白质和 DNA 所必需的。氮循环包括几个关键过程：固氮作用将大气中的N₂转化为氨（NH₃），由土壤或根瘤中的固氮细菌完成；硝化作用由硝化细菌将氨转化为亚硝酸盐（NO₂⁻）再转化为硝酸盐（NO₃⁻）；植物吸收硝酸盐（同化作用）用于合成蛋白质；生物死亡后，分解者进行氨化作用，将有机氮转回氨；反硝化作用将硝酸盐转回N₂气体，返还大气。
7. Ecological Pyramids | 生态金字塔
Pyramids of numbers show the count of individual organisms at each trophic level. They can be upright, irregular, or even inverted (e.g., a tree supporting many insects). Pyramids of biomass show the total dry mass of living material; they are usually upright but can be inverted in aquatic ecosystems where plankton have a small biomass but reproduce rapidly. Pyramids of energy are always upright because energy decreases at each successive trophic level due to losses through respiration and waste.
数量金字塔显示各营养级的个体生物数量。它可能是正立、不规则甚至倒置的（如一棵树支持许多昆虫）。生物量金字塔显示生物的总干质量；通常正立，但在浮游植物生物量小但繁殖快的水生生态系统中可能倒置。能量金字塔总是正立的，因为每次营养级传递时能量因呼吸作用和废物而减少。
8. Population Dynamics and Predator-Prey Relationships | 种群动态与捕食者-猎物关系
Populations change due to births, deaths, immigration, and emigration. In a balanced ecosystem, populations tend to fluctuate around a carrying capacity—the maximum population size an environment can sustain.
种群因出生、死亡、迁入和迁出而变化。在平衡的生态系统中，种群往往围绕环境承载量（环境能维持的最大种群规模）波动。
Predator-prey cycles show a characteristic pattern: as prey numbers increase, predators have more food and their population rises. Increased predation then causes prey numbers to fall, which in turn reduces the predator population due to food shortage. The cycle then repeats. A classic example is the snowshoe hare and lynx.
捕食者-猎物周期呈现特征模式：猎物数量增加，捕食者食物充足，其种群上升。捕食增加导致猎物数量下降，进而因食物短缺使捕食者种群减少。循环随之重复。经典例子是雪鞋兔和猞猁。
9. Sampling Techniques | 采样方法
To estimate population size and distribution, ecologists use quadrats for plants and slow-moving organisms. A quadrat is a square frame placed randomly. By counting organisms in many quadrats, the total population can be estimated. A transect line and belt transect are used to study how distribution changes along an environmental gradient, such as from a shaded to an open area.
为估计种群大小和分布，生态学家使用样方调查植物和慢速移动生物。样方是一个随机放置的方形框。通过统计许多样方中的生物数量，可以估算总种群。样带和带状样带用于研究分布如何沿环境梯度（如从阴蔽处到开阔地）变化。
For mobile animals, the capture-mark-recapture method is used. A first sample is caught, marked harmlessly, and released. Later, a second sample is caught. The population estimate N is calculated using:
对于移动动物，使用捉放法。首次样本被捕获，无害标记后释放。之后捕获第二次样本。种群估计值N通过以下公式计算：
N = (M × C) / R
where M = number marked in first sample, C = total caught in second sample, R = number of marked individuals recaptured in the second sample. This method assumes no migration or death between samples and that marks are not lost.
其中M=首次标记数，C=第二次捕获总数，R=第二次捕获中已标记个体数。该方法假设样本间无迁移或死亡，且标记不会脱落。
10. Human Impact on Ecosystems | 人类对生态系统的影响
Human activities can disrupt ecosystems significantly. Deforestation destroys habitats, reduces biodiversity, and releases CO₂. Overfishing removes key species, altering food webs. Burning fossil fuels releases SO₂ and NOₓ, causing acid rain that damages plants and aquatic life. It also increases the greenhouse effect, leading to climate change. Eutrophication occurs when fertilisers run off into water bodies, stimulating algal blooms that block sunlight and deplete oxygen, causing fish death. Bioaccumulation of pesticides like DDT concentrates in top predators, causing reproductive failure. Conservation strategies include reforestation, sustainable fishing quotas, and captive breeding programmes.
人类活动可显著破坏生态系统。砍伐森林破坏栖息地、减少生物多样性并释放CO₂。过度捕捞移除关键物种，改变食物网。燃烧化石燃料释放SO₂和NOₓ，引起酸雨，损害植物和水生生物。这也加剧温室效应，导致气候变化。当肥料径流进入水体时发生富营养化，刺激藻类大量繁殖，遮蔽阳光并消耗氧气，导致鱼死亡。农药如DDT在顶级捕食者体内生物累积，导致繁殖失败。保护策略包括植树造林、可持续捕捞配额和人工繁殖计划。
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