Ecosystem Structure and Function | 生态系统的结构与功能

📚 Ecosystem Structure and Function | 生态系统的结构与功能

An ecosystem is a dynamic complex of living organisms (biotic components) interacting with their non-living environment (abiotic components) within a defined area, linked through the continuous flow of energy and the cycling of matter. In CIE A-Level Biology, understanding ecosystem structure and function demands a rigorous grasp of trophic organisation, energy transfer efficiencies, nutrient cycling, and the dynamic nature of ecological succession.

生态系统是特定区域内生物有机体（生物成分）与其非生物环境（非生物成分）之间相互作用形成的动态复杂系统，通过持续的能量流动和物质循环紧密联系在一起。在 CIE A-Level 生物考试中，理解生态系统的结构与功能要求考生扎实掌握营养级组织、能量传递效率、养分循环以及生态演替的动态特性。

1. Ecosystem Definition and Components | 生态系统定义与组成部分

An ecosystem can be defined as a relatively self-contained unit consisting of a community of organisms and its physical environment, within which energy flows and nutrients cycle. The boundaries of an ecosystem are often arbitrary and are usually identified by the ecologist for convenience, such as a pond, a woodland, or a rocky shore. Every ecosystem comprises two fundamental components: the abiotic (non-living) component and the biotic (living) component.

生态系统可定义为一个相对自给自足的单位，由生物群落及其物理环境构成，在系统内部发生能量流动和养分循环。生态系统的边界通常具有人为任意性，生态学家常为研究便利而划定边界，例如池塘、林地或岩质海岸。每一个生态系统都由两大基本成分构成：非生物成分和生物成分。

The abiotic component includes physical and chemical factors such as light intensity, temperature, pH, water availability, mineral ion concentrations, and atmospheric gases. The biotic component is subdivided into producers (autotrophs), consumers (heterotrophs), and decomposers (saprotrophs). These components are not isolated; they interact continuously through feeding relationships, competition, and symbiotic associations.

非生物成分包括光照强度、温度、pH 值、水分可用性、矿质离子浓度和大气气体等物理与化学因子。生物成分又细分为生产者（自养生物）、消费者（异养生物）和分解者（腐生生物）。这些成分并非孤立存在，而是通过摄食关系、竞争和共生关联持续相互作用。

2. Abiotic Components and Their Ecological Roles | 非生物成分及其生态作用

Abiotic factors determine the distribution and abundance of species within an ecosystem. Light intensity affects rates of photosynthesis in plants and influences the vertical stratification of aquatic and terrestrial ecosystems. Temperature governs enzyme kinetics and metabolic rates; as a general approximation, metabolic rate increases with temperature until enzyme denaturation occurs.

非生物因子决定物种在生态系统中的分布与丰度。光照强度影响植物的光合速率，并决定水生态系统和陆地生态系统的垂直分层结构。温度通过影响酶动力学来控制代谢速率；一般而言，代谢速率随温度升高而加快，直至发生酶变性失活。

Water availability is a critical limiting factor in terrestrial ecosystems because turgor pressure, transport, and photosynthesis all require adequate hydration. Soil pH influences nutrient solubility; for instance, iron and aluminium become more soluble under acidic conditions, while many essential nutrients such as phosphorus become less available. The concentration of oxygen and carbon dioxide in aquatic ecosystems likewise constrains respiration and photosynthesis respectively.

水分可用性是陆地生态系统的关键限制因子，因为膨压维持、物质运输和光合作用均需充足的水分。土壤 pH 值影响养分的溶解度：例如铁和铝在酸性条件下溶解度升高，而磷等许多必需营养素在酸性和碱性条件下溶解度降低。水生态系统中溶解氧和二氧化碳的浓度分别对呼吸作用和光合作用产生限制作用。

Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) − Respiratory Loss (R)

Abiotic factors also interact with the biota to establish productivity ceilings. The efficiency with which solar radiation is converted into chemical energy sets an upper bound on biomass that an ecosystem can sustain, and this limit is realised through the combined action of light, temperature, water, and nutrient availability.

非生物因子还与生物群落相互作用，共同设定生产力的上限。太阳辐射转化为化学能的效率决定了生态系统可维持的最大生物量上限，而这一上限由光照、温度、水分和养分可用性的综合作用所实现。

3. Biotic Components: Producers, Consumers, and Decomposers | 生物成分：生产者、消费者与分解者

Producers, also called autotrophs, synthesise organic compounds from inorganic raw materials using an external energy source. Photoautotrophs, such as flowering plants, algae, and cyanobacteria, capture light energy through photosynthesis. Chemoautotrophs, such as certain sulfur-oxidising bacteria in deep-sea hydrothermal vents, obtain energy by oxidising inorganic compounds. In terrestrial ecosystems, green plants are the dominant producers; in aquatic ecosystems, phytoplankton play this role.

生产者又称自养生物，利用外部能量源将无机原料合成有机化合物。光能自养生物（如开花植物、藻类和蓝细菌）通过光合作用捕获光能；化能自养生物（如深海热泉中的某些硫氧化细菌）则通过氧化无机化合物获取能量。在陆地生态系统中，绿色植物是主要生产者；在水生生态系统中，浮游植物承担这一角色。

Consumers are heterotrophs that obtain energy by ingesting other organisms. Primary consumers (herbivores) feed directly on producers; secondary consumers (carnivores) feed on herbivores; tertiary consumers feed on other carnivores. Omnivores occupy multiple trophic levels because they consume both plant and animal matter. Some consumers are detritivores, feeding on dead organic matter and contributing to its physical fragmentation.

消费者是异养生物，通过摄食其他生物获取能量。初级消费者（草食动物）直接以生产者为食；次级消费者（肉食动物）以草食动物为食；三级消费者捕食其他肉食动物。杂食动物因同时取食植物和动物而跨越多个营养级。部分消费者属于碎屑食性动物，以死亡的有机物为食并促进其物理碎裂。

Decomposers, or saprotrophs, are mainly bacteria and fungi that secrete extracellular enzymes onto dead organic matter, absorbing the soluble breakdown products. Decomposition releases inorganic nutrients such as ammonium, phosphate, and carbon dioxide back into the abiotic pool, making them available for uptake by producers. Without decomposers, nutrients would remain locked in dead biomass and primary productivity would collapse.

分解者又称腐养生物，主要是细菌和真菌，它们向死亡的有机物分泌胞外酶，并吸收可溶性的分解产物。分解过程将铵、磷酸根和二氧化碳等无机养分释放回非生物库中，供生产者重新吸收利用。若无分解者，养分将封锁在死亡生物量中，初级生产力将崩溃。

4. Trophic Levels and Food Chains | 营养级与食物链

A trophic level is a feeding position in a food chain. The first trophic level always consists of producers, the second of primary consumers, the third of secondary consumers, and so on. Each transfer of energy from one trophic level to the next involves substantial losses, so food chains rarely extend beyond four or five trophic levels.

营养级是食物链中的一个摄食位置。第一营养级总是由生产者构成，第二营养级为初级消费者，第三营养级为次级消费者，依此类推。能量从某一营养级向下一营养级传递时涉及大量损耗，因此食物链很少超过四个或五个营养级。

A food chain is a linear sequence of organisms through which energy and matter are transferred. A typical terrestrial food chain might be:

食物链是一条线性的生物序列，能量和物质通过该序列进行传递。典型的陆地食物链可表示为：

Oak tree → Greenfly → Ladybird → Blackbird → Sparrowhawk

When representing a food chain, the arrow should be interpreted as “is eaten by”, so it indicates the direction of energy transfer from the consumed organism to the consumer. Ecologists conventionally start food chains with producers and never include decomposers in the main chain, even though decomposers process the remains of every trophic level.

在表示食物链时，箭头应理解为“被……所食”，指示能量从被食生物向捕食者传递的方向。生态学家通常以生产者作为食物链的起点，且不将分解者纳入主链中，尽管分解者处理着每个营养级的遗体残留。

5. Food Webs and Ecological Complexity | 食物网与生态复杂性

A food web is a network of interconnected food chains within an ecosystem, representing the multiple feeding relationships among organisms. Real ecosystems are rarely simple linear chains because most consumers feed on more than one species at different times, and many species are preyed upon by multiple predators.

食物网是生态系统内相互交织的食物链网络，代表生物之间多重的摄食关系。真实生态系统很少呈现简单的线性链式结构，因为大多数消费者在不同时期取食多个物种，而许多物种又被多种捕食者捕食。

Food webs confer stability upon an ecosystem through alternative feeding pathways. If one prey species declines, a predator can switch to alternative prey, buffering the impact of population fluctuations. By contrast, a simple food chain is vulnerable: the removal of a single species can cause cascading extinction throughout the chain. This concept is directly tested in CIE examinations, so candidates should be able to predict the consequences of removing a particular species from a given food web.

食物网通过提供替代摄食途径赋予生态系统稳定性。若某种猎物数量下降，捕食者可转向替代猎物，从而缓冲种群波动的冲击。相比之下，简单的食物链很脆弱：单一物种的消失可在整条链中引发级联灭绝。这一概念在 CIE 考试中经常出现，考生应能够预测从给定食物网中移除某一特定物种所产生的后果。

6. Ecological Pyramids | 生态金字塔

Ecological pyramids are graphical representations of the number, biomass, or energy at successive trophic levels in an ecosystem. Each pyramid is drawn with the producers at the base and successive trophic levels stacked above, with each bar being proportional to the quantity measured.

生态金字塔是以图形方式表示生态系统中连续营养级的数量、生物量或能量的工具。每座金字塔以生产者作为基部，上方逐级堆叠更高的营养级，每个横条的长度与该营养级所测得的数量成正比。

Pyramids of numbers count individual organisms at each trophic level. They are often inverted in woodland ecosystems because a single large tree supports many thousands of insect herbivores. Pyramids of biomass represent the dry mass of organisms at each trophic level, measured in grams per square metre. Biomass pyramids are usually upright, although temporary inversions can occur in aquatic systems when phytoplankton are consumed faster than they accumulate.

数量金字塔统计各营养级的个体数量。在林地生态系统中，数量金字塔常呈倒置形态，因为单棵大树可支撑数以千计的植食性昆虫。生物量金字塔以每平方米的干重克数表示各营养级的有机物质量，通常为正金字塔，但在浮游植物被消耗速度快于其积累速度的水生系统中，可能出现暂时性倒置。

Pyramids of energy show the rate of energy flow through each trophic level, expressed as energy per unit area per unit time (e.g., kJ m⁻² year⁻¹). Energy pyramids are always upright because the second law of thermodynamics dictates that every energy transfer involves heat loss, so the total energy entering each successive trophic level must be less than that entering the previous level.

能量金字塔显示通过各营养级的能量流动速率，以单位面积单位时间的能量表示（如 kJ m⁻² year⁻¹）。能量金字塔总是正立的，因为热力学第二定律决定了每次能量传递都伴随热散失，因此进入每一连续营养级的总能量必然少于上一营养级。

Pyramid Type	Measurement	Shape	Key Limitation
Number	Organism count	Variable; may invert	Ignores organism size
Biomass	Dry mass per area	Usually upright	Snapshot; ignores productivity
Energy	Energy flow per time	Always upright	Requires complex measurement

In CIE examinations, candidates must understand that pyramids of energy are the most accurate representation of trophic structure because they reflect the rate of energy flow rather than a static snapshot, thereby accounting for differences in productivity and turnover time between trophic levels.

在 CIE 考试中，考生需明确能量金字塔是营养结构最准确的表示方式，因为它反映能量流动的速率而非静态快照，从而考虑到不同营养级之间生产力和周转时间的差异。

7. Energy Flow and the 10% Rule | 能量流动与 10% 定律

Solar energy enters the ecosystem through photosynthesis, converting approximately 1–3% of incident light energy into chemical energy stored in organic compounds. This energy is then transferred from trophic level to trophic level through feeding. At each transfer, substantial energy is lost through three main pathways: (1) the consumption of material that is not digested and is egested as faeces, (2) the excretion of nitrogenous waste following the deamination of excess amino acids, and (3) respiratory heat loss accompanying the oxidation of substrates to fuel life processes.

太阳能通过光合作用进入生态系统，约 1–3% 的入射光能被转化为储存在有机化合物中的化学能。随后，能量通过摄食在营养级之间反复传递。在每一次传递中，能量主要通过三条途径发生大量损耗：（1）未被消化而以粪便形式排出的物质；（2）过量氨基酸脱氨基后随含氮废物排出的能量；（3）底物氧化以驱动生命过程时伴随呼吸作用散失的热量。

Energy transfer efficiency = (Energy available at trophic level ₙ₊₁ ÷ Energy available at trophic level ₙ) × 100%

Typically, only about 10% of the energy available at one trophic level is incorporated into the biomass of the next trophic level; this is often called the 10% law. The low efficiency explains why food chains are short and why top carnivores have the smallest populations and the greatest risk of extinction when ecosystems are disturbed.

通常，某一营养级可获得的能量中仅有约 10% 被整合到下一营养级的生物量中，此即常说的 10% 定律。较低的传递效率解释了为何食物链较短，以及为何顶级肉食动物的种群规模最小，当生态系统受到干扰时灭绝风险最大。

8. Productivity: GPP, NPP, and Secondary Productivity | 生产力：总初级生产力、净初级生产力与次级生产力

Primary productivity is the rate at which producers convert light energy into chemical energy. Gross primary productivity (GPP) is the total quantity of chemical energy fixed by photosynthesis within a given area per unit time. However, a portion of this fixed energy is consumed by the producers’ own respiration for maintenance, growth, and reproduction.

初级生产力是生产者将光能转化为化学能的速率。总初级生产力（GPP）是单位时间单位面积内经光合作用固定的化学能总量。然而，固定能量的一部分被生产者自身的呼吸作用消耗，用于维持、生长和繁殖。

Net primary productivity (NPP) represents the energy remaining after respiratory losses, and corresponds to the plant biomass available to primary consumers. The relationship may be expressed as:

净初级生产力（NPP）是扣除呼吸消耗后剩余的能量，即供给初级消费者利用的植物生物量。其关系可表示为：

NPP = GPP − R

where R represents respiratory losses. Secondary productivity is the rate at which consumers convert the chemical energy in their food into their own biomass. It is always lower than primary productivity in the same ecosystem because of the digestive inefficiencies, excretion, and respiratory heat loss described above. Global patterns show that tropical rainforests have the highest NPP, closely followed by estuaries and coral reefs, while deserts and open oceans have the lowest.

其中 R 代表呼吸损失。次级生产力是消费者将食物中的化学能转变为自身生物量的速率。由于上述消化效率低下、排泄损失和呼吸热散失等因素，同一生态系统中次级生产力总是低于初级生产力。全球格局表明，热带雨林的 NPP 最高，河口和珊瑚礁紧随其后，而沙漠和开阔大洋的 NPP 最低。

9. The Carbon Cycle | 碳循环

The carbon cycle is a global biogeochemical cycle through which carbon atoms are exchanged among the atmosphere, hydrosphere, biosphere, and geosphere. The major reservoirs of carbon include atmospheric carbon dioxide, dissolved bicarbonate ions in the oceans, fossil fuels, sedimentary rocks such as limestone, and organic matter in living organisms and soils.

碳循环是全球性的生物地球化学循环，碳原子在大气圈、水圈、生物圈和岩石圈之间不断交换。碳的主要储库包括大气中的二氧化碳、海洋中溶解的碳酸氢根离子、化石燃料、石灰岩等沉积岩，以及生物体和土壤中的有机物质。

Photosynthesis removes carbon dioxide from the atmosphere and fixes it into carbohydrate. Respiration in all aerobic organisms returns carbon dioxide to the atmosphere. Combustion of fossil fuels and biomass releases stored carbon rapidly. Decomposition of dead organic matter by saprotrophic bacteria and fungi releases CO₂. Marine organisms fix dissolved carbon into calcium carbonate shells, and over geological time these deposits form limestone. In the oceans, dissolved CO₂ establishes an equilibrium with atmospheric CO₂, so the oceans act as a major carbon sink.

光合作用从大气中吸收二氧化碳并将其固定为碳水化合物。所有需氧生物的呼吸作用将二氧化碳返回大气。化石燃料和生物质的燃烧快速释放储存的碳。腐生细菌和真菌对死亡有机物的分解释放 CO₂。海洋生物将溶解碳固定为碳酸钙外壳，经过地质时间尺度沉积形成石灰岩。海洋中溶解的 CO₂ 与大气 CO₂ 建立平衡，因此海洋是重要的碳汇。

Human activities, particularly the burning of fossil fuels and deforestation, have shifted the carbon balance, increasing atmospheric CO₂ concentrations and driving climate change. In the examination context, candidates should be prepared to label a diagram of the carbon cycle and explain how each arrow corresponds to a specific biological or physical process.

人类活动，特别是化石燃料燃烧和毁林，已使碳平衡发生偏移，大气 CO₂ 浓度上升并驱动气候变化。在考试背景下，考生应能标注碳循环示意图中的每个箭头，并解释每个箭头对应的具体生物或物理过程。

10. The Nitrogen Cycle | 氮循环

The nitrogen cycle describes the transformations of nitrogen among atmospheric nitrogen (N₂), organic nitrogen in proteins and nucleic acids, ammonium ions (NH₄⁺), nitrate ions (NO₃⁻), and nitrite ions (NO₂⁻). Nitrogen is an essential component of amino acids, nucleotides, ATP, and chlorophyll, yet atmospheric N₂ is metabolically inaccessible to most organisms because the triple bond between nitrogen atoms is extremely stable.

氮循环描述了氮在大气氮（N₂）、蛋白质和核酸中的有机氮、铵离子（NH₄⁺）、硝酸根离子（NO₃⁻）和亚硝酸根离子（NO₂⁻）之间的转化过程。氮是氨基酸、核苷酸、ATP 和叶绿素的重要组成元素，但大气中的 N₂ 对大多数生物而言无法代谢利用，因为氮原子之间的三键极其稳定。

The key stages in the nitrogen cycle are:

氮循环的关键阶段包括：

Process	Transformation	Organisms Involved
Nitrogen fixation	N₂ → NH₄⁺	Rhizobium, Azotobacter, cyanobacteria
Nitrification	NH₄⁺ → NO₂⁻ → NO₃⁻	Nitrosomonas, Nitrobacter
Assimilation	NO₃⁻ / NH₄⁺ → organic N	Plants and other producers
Ammonification	Organic N → NH₄⁺	Decomposing bacteria and fungi
Denitrification	NO₃⁻ → N₂	Pseudomonas (anaerobic)

Nitrogen fixation is carried out by free-living soil bacteria such as Azotobacter, symbiotic bacteria such as Rhizobium within root nodules of legumes, and cyanobacteria in aquatic environments. Nitrification requires aerobic conditions and proceeds in two steps: ammonia is first oxidised to nitrite by Nitrosomonas, then nitrite is oxidised to nitrate by Nitrobacter. Nitrate is the most readily absorbed form of nitrogen for most plants. Denitrification occurs under anaerobic waterlogged conditions, when facultative anaerobic bacteria use nitrate as a terminal electron acceptor, releasing N₂ back to the atmosphere.

固氮作用由固氮菌（如 Azotobacter）等自生土壤细菌、豆科植物根瘤中的共生细菌（如 Rhizobium）以及水生环境中的蓝细菌完成。硝化作用需要好氧条件，分两步进行：氨首先被 Nitrosomonas 氧化为亚硝酸盐，然后亚硝酸盐被 Nitrobacter 氧化为硝酸盐。硝酸盐是多数植物最易吸收的氮形态。反硝化作用发生在厌氧渍水条件下，兼性厌氧细菌以硝酸盐作为末端电子受体，将氮气释放回大气。

11. Ecological Succession | 生态演替

Ecological succession is the directional, progressive change in the species composition of a community over time. Primary succession occurs on substrates that are newly exposed or newly formed and initially devoid of soil, such as bare rock, lava flows, or freshly deposited glacial till. Secondary succession occurs in areas where an existing community has been disturbed or removed but soil remains intact, such as abandoned farmland or clear-felled woodland.

生态演替是群落物种组成随时间发生方向性、渐进式变化的过程。原生演替发生在裸露基岩、熔岩流或新沉积冰川淤泥等新暴露或新形成且最初无土壤的基质上。次生演替发生在原有群落被干扰或移除但土壤仍得以保留的区域，如弃耕农田和皆伐林地。

During primary succession on bare rock, pioneer species such as lichens and mosses colonise the surface. Their weathering action and organic secretion gradually form a thin layer of soil. As soil depth and nutrient content increase, larger plants such as grasses and ferns establish, followed by shrubs and eventually trees, culminating in a climax community. Each seral stage modifies the environment in ways that make conditions less suitable for its own persistence and more suitable for subsequent colonists, a mechanism known as facilitation.

在裸岩上发生原生演替时，地衣和苔藓等先锋物种首先定殖。它们对岩石的风化作用和有机分泌物逐渐形成薄层土壤。随着土壤厚度和养分含量的增加，禾草和蕨类等大型植物得以建立，随后是灌木，最终出现乔木，直至形成顶极群落。每一个演替系列阶段都以某种方式改造环境，使其自身持续性下降，同时为后续定殖者创造更适宜条件，这一机制称为促进作用。

Succession is accompanied by several consistent trends: increasing species diversity, increasing total biomass, lengthening food chains, and greater ecosystem stability. In the absence of further disturbance, the community reaches a climax state in equilibrium with the prevailing climate. The concept of succession is of great applied importance in conservation biology, where managing disturbance regimes can maintain plagioclimax communities dominated by particular species.

演替伴随若干一致的趋势：物种多样性增加、总生物量增加、食物链延长以及生态系统稳定性增强。若没有进一步干扰，群落将到达与当地气候保持平衡的顶极状态。演替概念在保护生物学中具有重大应用价值，通过管理干扰体系可维持以特定物种为主的偏途顶极群落。

12. Ecosystem Stability and Human Impact | 生态系统稳定性与人类影响

Ecosystem stability comprises two distinct properties: resistance, the ability to withstand disturbance and remain unchanged, and resilience, the speed at which an ecosystem returns to equilibrium after a disturbance. In general, ecosystems with high species diversity and complex food webs show greater resilience because alternative pathways buffer energy flow and nutrient cycling against the loss of individual species.

生态系统稳定性包含两个不同的属性：抵抗力（抵御干扰并保持不变的能⼒）和恢复力（受到干扰后恢复到平衡状态的速度）。一般而言，物种多样性和食物网复杂程度较高的生态系统具有更强的恢复力，因为替代途径可缓冲单一物种消失对能量流动和养分循环的冲击。

Human activities frequently compromise ecosystem stability. Deforestation reduces habitat complexity and disrupts the hydrological cycle; agricultural intensification simplifies food webs and diminishes soil biodiversity; pollution — particularly eutrophication caused by nitrate and phosphate run-off — substantially alters nutrient cycling and can lead to algal blooms, oxygen depletion, and fish death. Over-exploitation of top predators removes key trophic regulators, releasing prey populations from top-down control and triggering trophic cascades.

人类活动经常削弱生态系统的稳定性。毁林降低栖息地复杂性并干扰水循环；农业集约化简化食物网并减少土壤生物多样性；污染（特别是硝酸盐和磷酸盐径流引起的富营养化）极大改变养分循环，可导致藻类暴发、水体缺氧和鱼类死亡。对顶级捕食者的过度开发移除了关键营养调节者，使猎物种群摆脱自上而下的控制，引发营养级联效应。

For the CIE examination, candidates should be able to evaluate the effects of specific human activities on ecosystem structure and function, interpret data on species abundance and primary productivity, and propose reasoned management strategies that maintain ecological balance. Familiarity with the carbon and nitrogen cycles is essential for explaining how anthropogenic perturbations propagate through global biogeochemical systems.

就 CIE 考试而言，考生应能够评估特定人类活动对生态系统结构和功能的影响，解读物种丰度和初级生产力数据，并提出维持生态平衡的合理管理策略。深刻理解碳循环和氮循环对于解释人为扰动如何在全球生物地球化学系统中传播至关重要。

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Ecosystem Structure and Function | 生态系统的结构与功能