Edexcel IGCSE生物 生态学 食物链 碳循环
生态学是 Edexcel IGCSE 生物 Paper 2 中分值最稳定的板块之一，几乎每个考季都会以食物链、能量传递、碳循环或人类活动影响的形式出现。很多同学能背出定义，却在"解释为什么"和"按步骤计算"的题目上丢分。本文按考纲顺序梳理六个核心考点：生态层级与影响因素、食物链与食物网、能量流动与生态金字塔、碳循环、氮循环、人类活动的影响，每个考点都配有计算示例与易错提醒。
Ecology is one of the most reliable scoring areas in Edexcel IGCSE Biology Paper 2. Questions on food chains, energy transfer, the carbon cycle and human impact appear in almost every session. Students often memorise the definitions but lose marks on "explain why" questions and on structured calculations. This article follows the specification order through six core topics: levels of organisation and factors, food chains and webs, energy transfer and ecological pyramids, the carbon cycle, the nitrogen cycle, and human influences. Every topic includes a worked example and a common pitfall.
一、生态层级与影响因素 / Levels of Organisation and Factors
考纲要求你能区分 individual（个体）、population（种群）、community（群落）和 ecosystem（生态系统）四个层级。种群是同一物种的全部个体，群落是同一区域内所有物种种群的总和，生态系统等于群落加上它所处的非生物环境。判断题目落在哪一层，是拿分的第一步。
影响生物分布的因素分为 biotic 与 abiotic 两类。Biotic factors 包括捕食、种间竞争、食物可得性与病原体；abiotic factors 包括温度、光照强度、水分、pH、氧气浓度与土壤矿物盐含量。考试常见的问法是：说明某因素如何影响某种生物的分布。这类题必须写出完整因果链，例如土壤含水量下降导致叶片气孔关闭、光合速率降低，最终生长减缓、生物量减少。
野外调查用 quadrat（样方）估计种群密度。典型计算：在 1 m x 1 m 的样方中平均记录到 12 株蒲公英，样地面积为 200 平方米，则估算种群数量为 12 x 200 = 2400 株。采样必须随机进行，若只在树荫或湿地取样，会系统性高估或低估结果。研究环境梯度（例如从林地到草地的变化）时改用 belt transect，沿样线等距放置样方并记录每个样方的物种。
Levels of organisation must be distinguished precisely: individual, population, community and ecosystem. A population is all the organisms of one species, a community is all the populations of different species living in the same area, and an ecosystem is the community plus the non-living environment. Biotic factors include predation, competition, food availability and pathogens. Abiotic factors include temperature, light intensity, water, pH, oxygen concentration and mineral content of the soil. Sampling with quadrats allows population density to be estimated: if the mean count in a 1 m x 1 m quadrat is 12 plants and the habitat covers 200 square metres, the estimated population is 12 x 200 = 2400 plants. Samples must be taken at random, and a belt transect is used when a gradient is being studied.
常见错误：把 population 与 community 混用；只写"温度影响生长"而不写因果链条。评分标准通常要求一个因素对应一个机制、一个结果，缺少中间环节就拿不到完整的解释分。
二、食物链、食物网与营养级 / Food Chains, Food Webs and Trophic Levels
食物链描述能量与生物量的传递路径，箭头必须指向能量流动的方向，也就是从被吃者指向取食者。第一条链上的生物是 producer（生产者），依靠光合作用把光能转化为化学能；随后依次是第一、第二、第三级消费者。多个食物链相互连接成为食物网，食物网中的同一物种可能同时占据两个营养级，例如以植物为食又捕食昆虫的鸟。
解释食物网题目的标准套路是三步：先写出受影响的两个或多个链条，再说明该物种数量的变化如何通过食物供给与捕食压力传递，最后指出其他食物来源会缓冲这种变化。例如初级消费者数量下降时，次级消费者可能转向替代猎物，因此数量变化幅度小于预期。
A food chain shows the transfer of energy and biomass, and the arrow always points in the direction of energy flow, from the organism being eaten to the organism that eats it. Producers convert light energy into chemical energy by photosynthesis, and consumers occupy the following trophic levels. Several interlinked food chains form a food web, and one species may occupy more than one trophic level. When answering food web questions, write out the affected chains first, then explain how the change passes through food supply and predation pressure, and finally note that alternative food sources reduce the size of the effect.
易错提醒：把箭头画反是失分最常见的原因；另外不要写"箭头代表被吃"，标准说法是箭头代表能量流动方向。书写食物网时请使用题目给出的物种名称，不要引入额外物种，否则逻辑链无法评分。
三、能量流动与生态金字塔 / Energy Transfer and Pyramids
生态系统中的能量起点是太阳，但只有很小一部分被生产者固定。绿色植物吸收的光能约有百分之一转化为生物量。在营养级之间，通常只有约百分之十的能量传递到下一级，其余能量通过呼吸作用以热量散失、随未消化物质排出，或存在于未被取食的躯体部分。因此食物链一般不超过四到五级，因为可传递的能量已经不足以支撑更多消费者。
典型计算：若生产者固定的能量为 20000 kJ，按百分之十的传递效率，第一级消费者可获得的能量约为 2000 kJ，第二级消费者约为 200 kJ，第三级消费者仅剩约 20 kJ。题目常要求计算传递效率，公式为：传递效率等于下一营养级能量除以本营养级能量再乘以百分之百。
生态金字塔有三种形式。数量金字塔按个体数量绘制，可能出现倒置，例如一棵树上的昆虫。生物量金字塔按干重绘制，海洋生态系统中浮游动物多于浮游植物时也会倒置。能量金字塔永远正置，因为它反映的是能量随营养级递减的真实规律，这也是它比数量金字塔更可靠的原因。
Energy enters an ecosystem as sunlight, and only a small fraction is fixed by producers, roughly one percent of the light energy absorbed. Between trophic levels about ten percent of the energy is transferred, while the rest is lost through respiration as heat, in undigested material, or in parts of the organism that are not eaten. This is why food chains rarely exceed four or five levels. A pyramid of numbers can be inverted, a pyramid of biomass can be inverted in some aquatic systems, but a pyramid of energy is always upright, which makes it the most reliable representation.
四、碳循环 / The Carbon Cycle
碳循环的核心是碳在生物体、大气和化石燃料之间的迁移。光合作用把大气中的二氧化碳固定为有机物；呼吸作用与分解作用把它们重新释放为二氧化碳。燃烧（combustion）化石燃料同样释放二氧化碳，燃烧与呼吸的区别是前者在短时间内释放长期储存的碳，因此会打破大气碳的收支平衡。
微生物作为 decomposer 在循环中不可替代。它们把死亡的生物体、落叶与排泄物中的有机物分解，使碳回到大气、矿物盐回到土壤。若缺少分解者，有机物中的碳与氮会被长期锁定，生产者将无法获得足够的矿物盐。
答题时用 flow 描述循环比较安全：从大气二氧化碳库出发，经光合作用进入植物，经取食进入消费者，经呼吸与分解回到大气，另一分支是动植物遗体经地质过程形成化石燃料，燃烧后再次成为大气二氧化碳。写出每个过程的名称比写一句笼统的"碳在生物之间循环"更容易拿满分。
The carbon cycle describes the movement of carbon between living organisms, the atmosphere and fossil fuels. Photosynthesis fixes carbon dioxide into organic compounds, respiration and decomposition release it back, and the combustion of fossil fuels releases carbon that was stored for millions of years. Decomposers are essential: they break down dead organisms, fallen leaves and waste, returning carbon to the atmosphere and mineral ions to the soil. When answering, describe the cycle as a flow with named processes, and keep the atmosphere, plants, consumers and fossil fuels as the four main carbon stores.
易错点：把"呼吸作用"与"燃烧"混为一谈。二者的产物相同，但相对速率与碳的来源不同；论述温室效应时，必须强调化石燃料的燃烧是把长期封存的碳在极短时间内释放出来。
五、氮循环 / The Nitrogen Cycle
大气中含有约百分之七十八的氮气，但绝大多数生物无法直接利用。氮循环的关键是把稳定的氮气转化为可吸收的含氮化合物。氮固定菌（nitrogen fixing bacteria）把大气中的氮气转化为氨，其中根瘤菌与豆科植物共生，是教材中最常考的例子。
氨随后被硝化细菌（nitrifying bacteria）氧化为亚硝酸盐，再转化为硝酸盐，供植物吸收并用于合成蛋白质与核酸。动物通过取食获得含氮化合物，死亡后排出的含氮废物和遗体由分解者转化为氨，重新进入循环。反硝化细菌（denitrifying bacteria）则把土壤中的硝酸盐还原为氮气返回大气，完成闭环。
考卷中的高频对比是：为什么豆科植物常被轮作使用，因为与其共生的根瘤菌能固定氮气，提高土壤含氮化合物的含量，从而减少对化肥的依赖。另一个常见问法是解释为何长期淹水的土壤不利于植物吸收硝酸盐，因为缺氧环境使反硝化细菌活跃，硝酸盐被还原为氮气散失。
Atmospheric nitrogen makes up about seventy eight percent of the air but cannot be used directly by most organisms. Nitrogen fixing bacteria convert nitrogen gas into ammonia, and rhizobium living in the root nodules of legumes is the standard example. Nitrifying bacteria oxidise ammonia to nitrites and then to nitrates, which plants absorb to make proteins and nucleic acids. Decomposers return nitrogen from dead organisms and waste as ammonia, while denitrifying bacteria convert soil nitrates back into nitrogen gas. A common exam question asks why legumes are used in crop rotation: their symbiotic bacteria fix nitrogen and raise soil nitrate levels, reducing the need for fertiliser.
六、人类活动的影响与保护 / Human Influences and Conservation
森林砍伐减少光合作用固定的碳，同时使土壤失去养分与保水能力，导致水土流失、洪水风险上升与生物多样性下降。过度使用化肥引发水体富营养化：硝酸盐进入湖泊后促使藻类暴发生长，藻类死亡被分解时消耗大量溶解氧，鱼类因缺氧死亡。这一因果链是高频考点，答题时必须按顺序写出富营养化的每一步。
温室气体包括二氧化碳与甲烷，前者主要来自化石燃料燃烧与森林砍伐，后者主要来自牲畜反刍、水稻田与垃圾填埋场。气候变暖导致冰川融化、海平面上升与物种分布改变。工业排放的二氧化硫与氮氧化物形成酸雨，使土壤与湖泊酸化，直接损害植物组织与水生动物的鳃。
保护措施通常要求从两个层面回答：就地保护（建立自然保护区、保护栖息地、设立禁渔期与配额）与物种层面保护（人工繁育计划、再引入野化、种子库与基因库）。评估题目往往要求写出取舍，例如人工繁育成本高、再引入需要合适的栖息地，因此就地保护通常是首选策略。
Deforestation reduces photosynthesis and the carbon stored in wood, and it also removes nutrients and water from the soil, raising the risk of soil erosion and flooding. Excess fertiliser can leach into lakes and cause eutrophication: nitrates cause rapid algal growth, and when the algae die and decompose, dissolved oxygen falls and fish suffocate. Greenhouse gases include carbon dioxide from burning fossil fuels and methane from livestock, rice fields and landfill sites. Acid rain forms when sulfur dioxide and nitrogen oxides dissolve in rain, damaging plant tissue and the gills of aquatic animals. Conservation works at two levels: in situ protection of habitats with nature reserves, fishing quotas and protected areas, and ex situ measures such as breeding programmes, reintroduction and seed banks.
学习建议 / Study Tips
第一，把六个考点整理成三条主线：能量、碳、氮。每条线上写出物质从哪里来、经过哪些过程、回到哪里去，用自己的话复述一遍比反复背书更有效。第二，所有"解释"类题目坚持写因果链，每写一句就问自己是否有中间环节缺失。第三，把计算题（种群估算、能量传递效率）单独抄成一页，抄写时带上单位与公式，避免在单位换算上丢分。
第四，食物网题目先在草稿纸上画出受影响的链条，再动笔写解释，这样能避免遗漏。第五，高频易错词包括 population 与 community、pyramid of numbers 与 pyramid of energy、硝化细菌与反硝化细菌，建议用一个对照表把两两之间的差别写清楚。第六，用历年真题检验自己：先不看答案写出解释段落，再与评分方案逐点对照，重点记录自己漏掉的中间环节，这类错题本在冲刺阶段比新题更有价值。
Start by grouping the six topics into three themes: energy, carbon and nitrogen. For each theme, write the stores, the named processes and the direction of flow in your own words. For every explain question, demand a full causal chain from yourself, checking that no intermediate step is missing. Keep calculations on one page together with their units and formulas. In food web questions, sketch the affected chains before writing prose. Finally, build a comparison table for commonly confused pairs such as population and community, pyramid of numbers and pyramid of energy, nitrifying and denitrifying bacteria.
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