Ecosystem | 生态系统

📚 Ecosystem | 生态系统

An ecosystem is a dynamic, interacting system of living organisms and their physical environment. In WJEC A‑Level Science, you explore how energy flows through food webs, how matter cycles between biotic and abiotic components, and how populations and communities change over time. This article will guide you through the core concepts, common exam questions, and key terminologies you need to master.

生态系统是生物有机体与其物理环境之间动态互作的系统。在 WJEC A‑Level 科学课程中，你会探索能量如何通过食物网流动、物质如何在生物与非生物组分之间循环，以及种群和群落如何随时间变化。本文将带你梳理核心概念、常见考题以及必须掌握的关键术语。

1. Defining an Ecosystem | 定义生态系统

An ecosystem comprises all the biotic (living) and abiotic (non‑living) factors in a defined area and their interactions. Biotic factors include producers, consumers, decomposers, while abiotic factors include temperature, light intensity, pH, water availability and mineral nutrients.

生态系统由特定区域内的所有生物（生物因素）和非生物（非生物因素）因子及其相互作用组成。生物因素包括生产者、消费者和分解者，而非生物因素则包括温度、光照强度、pH、水分可用性和矿质营养。

In WJEC exams, you must be able to distinguish between a habitat (the physical place where an organism lives), a population (all individuals of one species in a habitat), and a community (all populations of different species living and interacting in a habitat).

在 WJEC 考试中，你必须能够区分栖息地（生物体生活的物理地点）、种群（同一物种在某一栖息地的所有个体）和群落（生活在同一栖息地并相互作用的多个不同物种的种群）。

The ecosystem concept thus integrates community ecology with the physical environment. Energy flows and nutrient cycling are its two fundamental processes.

因此，生态系统的概念将群落生态学与物理环境整合在一起。能量流动和养分循环是其两个基本过程。

2. Energy Flow and Trophic Levels | 能量流动与营养级

Energy enters most ecosystems via sunlight captured by photosynthetic producers (photoautotrophs). This energy is transferred through trophic levels: producers → primary consumers → secondary consumers → tertiary consumers. At each transfer, a large proportion (typically around 90%) is lost as heat through respiration, movement and undigested matter.

能量通过光合生产者（光能自养生物）捕获的太阳光进入大多数生态系统。这些能量通过营养级传递：生产者 → 初级消费者 → 次级消费者 → 三级消费者。在每一次传递中，大部分能量（通常约90%）通过呼吸作用、运动以及未被消化的物质以热的形式散失。

Food chains show a single linear pathway, but in reality organisms feed from multiple sources forming food webs. Energy pyramids are always upright because energy decreases at higher trophic levels. Pyramids of biomass can be inverted (e.g. in aquatic ecosystems where phytoplankton biomass is low but turnover is rapid), but pyramids of energy are never inverted.

食物链显示单一的线性路径，但现实中生物体从多种来源取食，形成食物网。能量金字塔总是正立的，因为能量随着营养级上升而减少。生物量金字塔可能出现倒置（例如，在水生生态系统中浮游植物生物量低但周转迅速），但能量金字塔永远不会倒置。

Calculating the efficiency of energy transfer: Energy transfer efficiency = (Energy available after transfer / Energy available before transfer) × 100. Typical efficiencies range 10–20% between trophic levels. This limits the length of food chains.

计算能量传递效率：能量传递效率 = （传递后可用的能量 / 传递前可用的能量）× 100。营养级之间的典型效率在10–20%之间。这限制了食物链的长度。

3. Productivity | 生产力

Gross primary productivity (GPP) is the total energy fixed by photosynthesis in producers. Net primary productivity (NPP) is the energy remaining after plant respiration: NPP = GPP – R (where R is respiratory losses). NPP represents energy available to heterotrophs.

总初级生产力（GPP）是生产者通过光合作用固定的总能量。净初级生产力（NPP）是植物呼吸后剩余的能量：NPP = GPP – R（R为呼吸损失）。NPP代表可供异养生物利用的能量。

Similarly, secondary productivity refers to the rate at which consumers assimilate energy. Net secondary productivity = Gross secondary productivity – respiratory losses.

类似地，次级生产力指消费者同化能量的速率。净次级生产力 = 总次级生产力 – 呼吸损失。

You should be able to interpret productivity data in kJ m⁻² year⁻¹ and explain why NPP varies between biomes – temperature, light, water and nutrient availability all exert strong controls.

你应能够解读以 kJ m⁻² year⁻¹ 表示的生产力数据，并解释为什么不同生物群系的 NPP 存在差异——温度、光照、水分和养分有效性都起着重要的控制作用。

4. Nutrient Cycles | 养分循环

Nutrients such as carbon, nitrogen and phosphorus are recycled within ecosystems via biogeochemical cycles. WJEC expects detailed knowledge of the carbon and nitrogen cycles including the roles of microorganisms.

碳、氮和磷等养分通过生物地球化学循环在生态系统中循环。WJEC 要求详细了解碳循环和氮循环，包括微生物的作用。

In the carbon cycle, key processes: photosynthesis (CO₂ fixation), respiration (release of CO₂), decomposition (release of CO₂ by saprobionts), combustion, and sequestration in fossil fuels and limestone. Human activities such as deforestation and fossil fuel burning disrupt the balance, increasing atmospheric CO₂.

在碳循环中，关键过程包括：光合作用（固定CO₂）、呼吸作用（释放CO₂）、分解作用（腐生生物释放CO₂）、燃烧，以及化石燃料和石灰岩中的固存。森林砍伐和化石燃料燃烧等人类活动破坏了平衡，增加了大气 CO₂ 浓度。

The nitrogen cycle involves nitrogen fixation (by free‑living bacteria e.g. Azotobacter and symbiotic Rhizobium in legume root nodules), ammonification (decomposers), nitrification (Nitrosomonas oxidising NH₃ to NO₂⁻, Nitrobacter oxidising NO₂⁻ to NO₃⁻), plant uptake, denitrification (anaerobic bacteria reducing NO₃⁻ back to N₂). Learn the specific bacteria names – they are frequent marks.

氮循环包括固氮作用（由自由生活的细菌如 Azotobacter 以及豆科植物根瘤中的共生根瘤菌进行）、氨化作用（分解者）、硝化作用（Nitrosomonas 将 NH₃ 氧化为 NO₂⁻，Nitrobacter 将 NO₂⁻ 氧化为 NO₃⁻）、植物吸收、反硝化作用（厌氧细菌将 NO₃⁻ 还原为 N₂）。务必记住具体的细菌名称——这是常见的得分点。

Phosphorus cycle lacks a gaseous phase; phosphorus is released by weathering of rocks and is often a limiting nutrient. Eutrophication arises when excess nitrate/phosphate enters water bodies, stimulating algal blooms, blocking light, causing oxygen depletion and biodiversity loss.

磷循环没有气态阶段；磷通过岩石风化释放，通常是限制性养分。当过量的硝酸盐/磷酸盐进入水体时会引起富营养化，刺激藻类大量繁殖，遮蔽光线，导致溶解氧耗竭和生物多样性丧失。

5. Feeding Relationships and Ecological Pyramids | 摄食关系与生态金字塔

Constructing food webs from provided data is a routine skill. You need to be able to identify producers, herbivores, carnivores, omnivores, and decomposers, and predict the impact of removing or adding a species. Trophic cascades demonstrate how changes at one level ripple through the web.

根据提供的数据构建食物网是一项常规技能。你需要能够识别生产者、食草动物、食肉动物、杂食动物和分解者，并预测移除或增加某物种的影响。营养级联效应显示了某一层级的变化如何在整个食物网中产生涟漪效应。

Pyramids of numbers can be deceptive – a single oak tree (producer) may support thousands of insects. Therefore, pyramids of biomass (dry mass per unit area g m⁻²) are more reliable. However, biomass pyramids may still be inverted if the producer level has a high turnover rate, which is why energy pyramids (kJ m⁻² year⁻¹) provide the most accurate representation of trophic structure.

数量金字塔可能具有欺骗性——一棵橡树（生产者）可能支撑成千上万只昆虫。因此，生物量金字塔（每单位面积干重 g m⁻²）更可靠。然而，如果生产者层级的周转率很高，生物量金字塔仍可能倒置，这就是为什么能量金字塔（kJ m⁻² year⁻¹）能最准确地反映营养结构。

6. Sampling Techniques | 取样技术

To investigate populations and communities, ecologists use a range of sampling methods. Quadrats (frame and point) estimate percentage cover, density and frequency of plant species. Random sampling avoids bias, while systematic sampling along a transect (line or belt) reveals zonation patterns correlated with environmental gradients.

为了调查种群和群落，生态学家使用一系列取样方法。样方（框式和点式）用于估算植物物种的盖度百分比、密度和频度。随机取样可避免偏差，而沿样带（线状或带状）进行的系统取样则可以揭示与环境梯度相关的带状分布格局。

For mobile animals, mark–release–recapture estimates population size using the Lincoln index: Estimated population size = (Number in first sample × Number in second sample) ÷ Number of marked individuals recaptured. Assumptions: no migration, births or deaths between samples; marking does not affect survival; marks are not lost; marked individuals mix randomly.

对于可移动的动物，标记-释放-重捕法利用林肯指数估算种群大小：估算种群大小 = （第一次样本数 × 第二次样本数） ÷ 重捕的标记个体数。假设条件包括：取样期间无迁入迁出、无出生死亡；标记不影响存活；标记不会脱落；标记个体随机混合。

Kick sampling and Surber samplers are used in freshwater streams to collect aquatic invertebrates for pollution indices. You must be able to evaluate the limitations of each technique.

踢样法和瑟伯取样器用于淡水溪流中采集水生无脊椎动物，以评估污染指数。你必须能够评价每种技术的局限性。

7. Estimating Biodiversity | 估算生物多样性

Species richness is simply the number of different species in a sample. Species evenness quantifies how equally individuals are distributed among species. Simpson’s index of diversity (D) combines both: a high value indicates high diversity. For WJEC, the formula provided is usually:

物种丰富度仅仅是样本中不同物种的数量。物种均匀度衡量个体在物种间的分布是否均等。辛普森多样性指数（D）综合了两者：数值高表示多样性高。WJEC 通常提供的计算公式如下：

D = 1 – Σ(n/N)²

where n = number of individuals of each species, N = total number of individuals. You should be able to calculate D, compare habitats, and interpret the impact of agriculture or pollution on diversity.

其中 n = 每个物种的个体数，N = 总个体数。你应该能够计算 D 值，比较不同生境，并解释农业或污染对多样性的影响。

Genetic diversity within a species, measured by heterozygosity or allele frequencies, is also part of the specification. Conservation efforts often aim to preserve both species and genetic diversity.

物种内的遗传多样性（通过杂合度或等位基因频率衡量）也是考纲的一部分。保护工作通常旨在同时保护物种多样性和遗传多样性。

8. Succession | 演替

Succession is the directional change in community structure over time. Primary succession occurs on bare rock (e.g. volcanic lava) with no soil. Pioneer species like lichens and mosses colonise first, followed by grasses, shrubs, and ultimately climax woodland. The process changes the abiotic environment (soil depth, organic matter, pH), making it more suitable for later species.

演替是群落结构随时间发生定向变化的过程。原生演替发生在裸露的岩石上（如火山熔岩），起初没有土壤。地衣和苔藓等先锋物种首先定居，接着是草本植物、灌木，最终形成顶极林地。这一过程改变了非生物环境（土壤深度、有机质、pH），使其更适宜后续物种的生存。

Secondary succession occurs where an existing community has been disturbed (e.g. after fire, logging) but soil remains. It proceeds much faster. Deflected succession describes when grazing, mowing or other activities prevent progression to climax, maintaining a plagioclimax (e.g. chalk grassland maintained by sheep grazing).

次生演替发生在现存群落受到干扰（如火灾、伐木后）但土壤依然存在的地方。其进展速度要快得多。偏途演替是指放牧、割草或其他活动阻止了向顶极群落的发展，维持了偏途顶极（例如，由绵羊放牧维持的白垩草原）。

You must be able to interpret transect data that show changes in species composition and relate these to soil development, microclimate amelioration, and competition.

你必须能够解读显示物种组成变化的样带数据，并将其与土壤发育、微气候改善和竞争联系起来。

9. Population Ecology | 种群生态学

Population size is regulated by a combination of density‑dependent factors (competition for food, disease, predation) and density‑independent factors (natural disasters, extreme weather). The carrying capacity (K) is the maximum population size an environment can sustain.

种群大小受密度制约因素（食物竞争、疾病、捕食）和非密度制约因素（自然灾害、极端天气）的共同调节。环境容纳量（K）是环境可持续维持的最大种群规模。

Population growth curves: exponential growth (J‑shaped curve) occurs when resources are unlimited; logistic growth (S‑shaped curve) incorporates carrying capacity and levels off. The lag phase, exponential phase, and stationary phase are typical for microorganisms in closed culture, mirroring larger population patterns.

种群增长曲线：当资源不受限制时出现指数增长（J 型曲线）；逻辑斯谛增长（S 型曲线）包含环境容纳量并趋于平稳。迟缓期、指数期和稳定期是封闭培养中微生物的典型阶段，反映了更大范围种群的模式。

Life tables and survivorship curves (Type I, II, III) describe mortality patterns. Type I (e.g. humans) have high survival until old age; Type II (e.g. birds) have constant mortality; Type III (e.g. trees, fish) produce large numbers of offspring with very low juvenile survival.

生命表和存活曲线（I 型、II 型、III 型）描述了死亡模式。I 型（如人类）在老年之前存活率高；II 型（如鸟类）死亡率恒定；III 型（如树木、鱼类）产生大量后代，幼体存活率极低。

10. Human Impacts and Conservation | 人类影响与保护

Habitat loss, overexploitation, invasive species, pollution and climate change are the major threats to biodiversity. WJEC expects you to apply ecological principles to explain the effects of these pressures and evaluate management strategies.

栖息地丧失、过度开发、入侵物种、污染和气候变化是对生物多样性的主要威胁。WJEC 希望你运用生态学原理来解释这些压力的影响，并评价管理策略。

Conservation can be in‑situ (within natural habitat, e.g. nature reserves, SSSIs) or ex‑situ (off‑site, e.g. seed banks, captive breeding, zoos). The criteria for SSSI selection include species rarity, habitat representativeness, and fragility. You should discuss the advantages and limitations of each approach, and how international cooperation (CITES, CBD) supports conservation.

保护可以是就地保护（在自然栖息地内，如自然保护区、SSSI）或迁地保护（场外，如种子库、圈养繁殖、动物园）。SSSI 的选择标准包括物种稀有性、栖息地代表性和脆弱性。你应该讨论每种方法的优缺点，以及国际合作（CITES、CBD）如何支持保护工作。

Monitoring changes in ecosystems uses indicator species (e.g. lichens for air SO₂, mayfly larvae for clean water). Biological oxygen demand (BOD) measures water pollution: high BOD indicates high organic pollution and low oxygen.

监测生态系统的变化利用指示物种（例如，地衣监测空气中的 SO₂，蜉蝣幼虫指示清洁水体）。生化需氧量（BOD）测量水体污染：高 BOD 表示有机污染严重，氧气含量低。

11. Practical Skills and Data Handling | 实验技能与数据处理

Exam questions will often provide raw ecological data for you to calculate percentages, mean, standard deviation, and to draw bar charts or line graphs. You need to apply statistical tests: Spearman’s rank correlation for associations between two variables, chi‑squared for associations between species distributions or frequencies, and Student’s t‑test for comparing two means (e.g. plant height in two habitats).

考题常常会提供生态学原始数据，要求你计算百分数、平均值、标准差，并绘制条形图或折线图。你需要运用统计检验：斯皮尔曼秩相关用于两个变量间的关联，卡方检验用于物种分布或频率之间的关联，学生t检验用于比较两个平均数（例如，两个生境中的植物高度）。

Always state null hypotheses, calculate degrees of freedom, compare calculated test statistics against critical values at p=0.05, and write a conclusion in biological context. These skills are consistently assessed and carry high marks.

务必写出零假设、计算自由度、将计算出的检验统计量与 p=0.05 的临界值比较，并在生物学背景下给出结论。这些技能是持续的考察重点，分值很高。

12. Common Exam Pitfalls and Summary | 常见考试陷阱与总结

Avoid confusing energy pyramids with biomass pyramids; remember energy pyramids are always upright. Do not forget the specific names of nitrifying and denitrifying bacteria – marks are often lost here. When calculating Simpson’s index, be careful with the order of operations (square n/N before summing and subtracting from 1). In succession, distinguish clearly between primary and secondary, and explain how abiotic factors change.

避免混淆能量金字塔和生物量金字塔；记住能量金字塔总是正立的。不要忘记硝化细菌和反硝化细菌的具体名称——常常在此丢分。计算辛普森指数时，注意运算顺序（先将 n/N 平方，求和后再从 1 中减去）。在演替部分，清晰区分原生和次生演替，并解释非生物因素如何变化。

Mastering ecosystem ecology means connecting concepts – from a single bacterium in the soil to global biogeochemical cycles. Use the terminology precisely, practise with sample data, and you will be well‑prepared for your WJEC examination.

掌握生态系统生态学意味着将概念相互联系——从土壤中的单个细菌到全球生物地球化学循环。精确使用术语，用样题数据进行练习，你将为自己 WJEC 考试做好充分准备。

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Ecosystem | 生态系统