📚 IB WJEC Science: Ecosystems Key Points | IB WJEC 科学：生态系统考点精讲
Ecosystems form a central part of both IB Biology/ESS and WJEC Science specifications, requiring a deep understanding of energy flow, nutrient cycles, and interactions between organisms. This guide breaks down the essential concepts, paired with exam insights for top marks.
生态系统是IB生物学/ESS和WJEC科学课程核心内容，要求深入理解能量流动、物质循环和生物间相互作用。本指南拆解关键概念，并搭配高分考试技巧，助你稳拿考点。
1. Definition and Components of an Ecosystem | 生态系统的定义与组成
An ecosystem is a biological community of interacting organisms (biotic factors) and their physical environment (abiotic factors) in a defined area. Biotic components include producers, consumers, and decomposers, while abiotic factors encompass sunlight, temperature, water, pH, and soil minerals.
生态系统是在特定区域内由相互作用的生物群落（生物因子）及其物理环境（非生物因子）组成的整体。生物成分包括生产者、消费者和分解者，非生物因子则包括阳光、温度、水、pH值和土壤矿物质。
In both IB and WJEC exams, you must be able to distinguish between population (same species), community (all populations), and ecosystem (community + abiotic environment). The niche concept is also vital — it refers to an organism’s role, including its habitat, feeding relationships, and interactions.
在IB和WJEC考试中，你必须能区分种群（同一物种）、群落（所有种群）和生态系统（群落+非生物环境）。生态位概念同样关键——它指生物体的角色，包括其栖息地、摄食关系和相互作用。
2. Energy Flow and Trophic Levels | 能量流动与营养级
The sun is the ultimate source of energy for most ecosystems. Producers (autotrophs) convert light energy into chemical energy via photosynthesis. This energy is transferred through trophic levels: primary consumers, secondary consumers, tertiary consumers, and decomposers. However, only about 10% of the energy is passed on to the next level; the rest is lost as heat through respiration, movement, or remains uneaten.
太阳是大多数生态系统的终极能量来源。生产者（自养生物）通过光合作用将光能转化为化学能。该能量沿营养级传递：初级消费者、次级消费者、三级消费者和分解者。但只有约10%的能量传递到下一级；其余能量通过呼吸、运动或以未食用形式散失。
This inefficiency explains why food chains rarely exceed 4-5 trophic levels. IB students should be able to calculate energy transfer efficiency using the formula: Efficiency (%) = (energy assimilated at higher level / energy available at lower level) × 100. WJEC often asks similar quantitative questions in unit papers.
这种低效性解释了为什么食物链极少超过4-5个营养级。IB学生需能用公式计算能量传递效率：效率（%）=（上级同化能量/下级可用能量）×100。WJEC考试中单元试卷也常有类似定量问题。
3. Food Chains and Food Webs | 食物链与食物网
A food chain shows a single linear pathway of energy flow, for example: grass → grasshopper → frog → snake → hawk. A food web represents interlinked food chains within an ecosystem, demonstrating multiple feeding relationships and greater stability. Removal of one species has less impact in a complex web.
食物链展示能量流动的单一线性途径，例如：草→蚱蜢→蛙→蛇→鹰。食物网代表生态系统内相互连接的食物链，展示多重摄食关系且更具稳定性。在复杂食物网中，移除一个物种影响较小。
Both IB and WJEC expect you to interpret diagrams, identify trophic levels, producers, predators, and prey. WJEC may ask you to construct food webs from given data. The arrow represents energy transfer, pointing from the organism being eaten to the eater.
IB和WJEC都要求你解读图表，识别营养级、生产者、捕食者和猎物。WJEC可能会根据给定数据构建食物网。箭头代表能量传递，方向从被食者指向捕食者。
4. Pyramids of Numbers, Biomass and Energy | 数量、生物量和能量金字塔
Ecological pyramids visually represent trophic structure. The pyramid of numbers shows the count of organisms at each level. It can sometimes be inverted (e.g., one tree supporting many insects). The pyramid of biomass shows the total dry mass of living tissue, which is usually upright but can be inverted in aquatic ecosystems. The pyramid of energy is always upright and shows the most accurate energy flow pattern.
生态金字塔直观展示营养结构。数量金字塔显示各营养级生物个体数；有时会倒置（如一棵树支撑许多昆虫）。生物量金字塔显示活组织干重之和，通常正置，但水生生态系统可能倒置。能量金字塔始终正置，展示最准确的能量流动模式。
For WJEC, emphasis is often on pyramids of biomass and energy; IB ESS requires you to construct and compare all three, explaining why energy pyramids are never inverted due to the second law of thermodynamics. Practice drawing pyramids with accurate scaling.
WJEC常侧重生物量和能量金字塔；IB ESS则要求构建并比较三种金字塔，解释为何能量金字塔因热力学第二定律永不倒置。练习按精确比例绘制金字塔。
5. Bioaccumulation and Biomagnification | 生物积累与生物放大
Bioaccumulation is the build-up of persistent chemicals, like DDT or heavy metals, in an organism’s body over time. Biomagnification is the increasing concentration of these substances at successive trophic levels. As each consumer eats multiple contaminated prey, the toxin concentration magnifies, often severely affecting top predators.
生物积累是持久性化学物质（如DDT或重金属）随时间在生物体内积累的过程。生物放大是指这些物质浓度沿营养级逐级增加的现象。由于每个消费者食用多个受污染猎物，毒素浓度被放大，常严重影响顶级捕食者。
IB and WJEC both use DDT as a classic case study. In WJEC GCSE, you may need to describe its effect on eggshell thinning in birds of prey. IB Biology links this to the carbon cycle and human health. Always explain using numbers or concentration factors (e.g., 0.01 ppm in water to 25 ppm in birds).
IB和WJEC都以DDT为经典案例。WJEC GCSE可能要求描述其对猛禽卵壳变薄的影响。IB生物学将此与碳循环和人类健康相联系。务必用数字或浓缩系数解释（如水0.01ppm至鸟类体内25ppm）。
6. Nutrient Cycles: Carbon Cycle | 物质循环：碳循环
The carbon cycle is a biogeochemical cycle that recycles carbon atoms through photosynthesis, respiration, feeding, decomposition, and combustion. Key processes: photosynthesis fixes CO₂ into organic compounds; respiration and combustion release CO₂ back; decomposers break down dead matter, returning carbon to soil and atmosphere.
碳循环是一种生物地球化学循环，通过光合作用、呼吸作用、摄食、分解和燃烧使碳原子循环。关键过程：光合作用将CO₂固定为有机物；呼吸和燃烧释放CO₂；分解者分解死物质，将碳归还土壤和大气。
IB requires you to draw and label a complete carbon cycle, including fossilization and limestone formation. WJEC focuses on the role of microorganisms in decomposition and the impact of deforestation and burning fossil fuels on atmospheric CO₂. Be prepared to discuss the greenhouse effect and climate change as a consequence.
IB要求绘制并标注完整碳循环图，包括化石化和石灰岩形成。WJEC关注微生物在分解中的作用以及滥伐森林和燃烧化石燃料对大气CO₂的影响。准备好讨论温室效应和气候变化作为后果。
7. Nutrient Cycles: Nitrogen Cycle | 物质循环：氮循环
Nitrogen is essential for proteins and nucleic acids, but most organisms cannot use atmospheric N₂ directly. The nitrogen cycle involves nitrogen fixation (by bacteria like Rhizobium or by lightning), nitrification (ammonium to nitrites to nitrates by nitrifying bacteria), assimilation by plants, and denitrification (nitrates back to N₂ by denitrifying bacteria in anaerobic conditions).
氮对蛋白质和核酸不可或缺，但多数生物无法直接利用大气N₂。氮循环包括固氮作用（由根瘤菌等细菌或闪电完成）、硝化作用（铵盐→亚硝酸盐→硝酸盐，由硝化细菌完成）、植物同化作用以及反硝化作用（硝酸盐→N₂，由反硝化细菌在厌氧条件下完成）。
WJEC typically requires naming the bacterial groups and their roles; IB Biology explores the process in more depth, including the role of mycorrhizae in plant nitrogen uptake. Both syllabi may ask you to sketch a labelled diagram. Remember: nitrates are the form absorbed by plants.
WJEC通常要求说出细菌群及其作用；IB生物学更深入探讨过程，包括菌根在植物氮吸收中的角色。两套大纲都可能让你绘制带标注的图表。记住：硝酸盐是植物吸收的形态。
8. Population Growth and Carrying Capacity | 种群增长与承载能力
Populations can grow exponentially under unlimited resources (J-shaped curve), but environmental resistance (food shortage, disease, predation) limits growth, leading to a logistic (S-shaped) curve. The maximum population size an environment can sustain is the carrying capacity (K).
在资源无限时种群呈指数增长（J型曲线），但环境阻力（食物短缺、疾病、捕食）限制增长，形成逻辑斯蒂（S型）曲线。环境能持续承载的最大种群规模即承载能力（K）。
IB students must explain the sigmoid growth curve phases: lag phase, exponential phase, transitional phase, and plateau phase. WJEC often uses yeast or bacteria experiments to illustrate population changes. Density-dependent factors (competition, parasites) and density-independent factors (natural disasters) are key.
IB学生须解释S型增长曲线各期：延滞期、指数期、过渡期和平稳期。WJEC常使用酵母或细菌实验说明种群变化。密度制约因素（竞争、寄生）和非密度制约因素（自然灾害）是关键。
9. Ecological Succession | 生态演替
Succession is the orderly change in community structure over time. Primary succession begins on barren land (no soil), such as bare rock after a volcanic eruption. Pioneer species like lichens and mosses colonize, breaking down rock and forming soil. Over time, grasses, shrubs, and finally climax community (e.g., forest) develop. Secondary succession occurs after a disturbance where soil remains.
演替是群落结构随时间有序变化的过程。原生演替从无土壤的贫瘠地开始，如火山喷发后的裸岩。地衣、苔藓等先锋物种定居，分解岩石形成土壤，随后依次出现草本、灌木，最终达到顶级群落（如森林）。次生演替发生在土壤保留的干扰后。
IB ESS and Biology highlight the changes in soil depth, biodiversity, and biomass. WJEC may test succession through a specific example like sand dune succession (psammosere). Key terms: sere, pioneer community, and climax community. Know how to read a profile diagram of succession.
IB ESS和生物学强调土壤深度、生物多样性和生物量的变化。WJEC可能通过沙丘演替（旱生演替）等具体实例考查。关键术语：演替系列、先锋群落和顶级群落。要会读演替剖面图。
10. Human Impact on Ecosystems | 人类对生态系统的影响
Human activities such as deforestation, agriculture, urbanization, and pollution disrupt ecosystems. Deforestation reduces biodiversity, accelerates soil erosion, and contributes to climate change by releasing stored carbon. Agricultural practices cause eutrophication via fertilizer runoff, leading to algal blooms and oxygen depletion in water bodies.
砍伐森林、农业、城市化和污染等人类活动破坏生态系统。森林砍伐降低生物多样性，加速土壤侵蚀，并通过释放储存的碳加剧气候变化。农业施肥导致径流引起富营养化，造成水体藻华和缺氧。
IB requires evaluation of environmental impact assessments (EIA) and mitigation strategies. WJEC asks students to describe the effects of human population growth on food chains and pollution. Case studies: tropical rainforest destruction in the Amazon or overfishing in the North Sea. Link to sustainability is essential.
IB要求评价环境影响评估（EIA）和缓解策略。WJEC要求学生描述人口增长对食物链和污染的影响。案例研究：亚马孙热带雨林破坏或北海过度捕捞。必须联系可持续发展。
11. Conservation and Sustainability | 保护与可持续发展
Conservation aims to protect biodiversity and natural resources through measures like habitat restoration, breeding programmes, legislation, and protected areas (e.g., national parks). Sustainability ensures resource use meets present needs without compromising future generations. The three pillars are environmental, social, and economic sustainability.
保护旨在通过栖息地恢复、繁殖计划、立法和保护区（如国家公园）等措施保护生物多样性和自然资源。可持续发展确保满足当代需求而不损害后代。三大支柱为环境、社会和经济可持续性。
IB ESS focuses heavily on environmental value systems (EVSs) and the tension between exploitation and preservation. WJEC might ask about reforestation or sustainable fishing quotas. Both syllabi value the ability to evaluate conservation success using indicator species or biodiversity indices like Simpson’s Diversity Index.
IB ESS高度重视环境价值体系（EVSs）以及开发与保存间的矛盾。WJEC可能涉及再造林或可持续捕鱼配额。两套体系都看重利用指示物种或辛普森多样性指数等评估保护成效的能力。
12. Exam Tips for IB and WJEC | IB与WJEC考试技巧
For IB: Data-based questions often show graphs on predator-prey cycles or population dynamics; practice describing trends using numerical comparisons (e.g., “decreased by 50%”). In extended response, structure your answer with definitions, explanations, and relevant examples. Use the command terms precisely (explain, discuss, evaluate). ESS requires explicit links to EVSs.
IB：基于数据的题目常给出捕食者-猎物周期或种群动态图；练习用数字对比描述趋势（如“减少了50%”）。在长篇回答中，用定义、解释和相关例子组织答案。精准使用指令术语（解释、讨论、评价）。ESS要求明确联系环境价值体系。
For WJEC: Exam papers often feature structured questions with labelled diagrams. Make sure you can interpret food webs, carbon/nitrogen cycle diagrams. Learn key definitions (e.g., community, niche, carrying capacity) as they are commonly tested. In practical-based questions, recall the methods for using quadrats and transects to estimate population size. Always show working for calculations.
WJEC：试卷常出现结构化问题，附带标注图表。确保能解读食物网、碳/氮循环图。牢记关键定义（如群落、生态位、承载能力），因为它们常被考查。在实验类题目中，回顾使用样方和样线估算种群大小的方法。计算题务必展示步骤。
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