📚 Ecosystems | 生态系统
An ecosystem is a dynamic complex of living organisms interacting with each other and with the non-living components of their environment. In Cambridge A-Level Biology, ecosystems are studied through energy flow, nutrient cycling, population dynamics and succession, providing an integrated view of how natural systems function.
生态系统是生物有机体之间以及与其环境中非生物成分相互作用的动态复合体。在剑桥A-Level生物课程中，生态系统通过能量流动、营养循环、种群动态和演替进行研究，从而综合理解自然系统的运作方式。
1. Defining Ecosystems and Niches | 生态系统的定义与生态位
An ecosystem includes all the biotic (living) and abiotic (non-living) factors in a defined area. Biotic factors include producers, consumers and decomposers, while abiotic factors include temperature, light, water, soil pH and mineral availability.
生态系统包括某一特定区域内所有生物因子（有生命的）和非生物因子（无生命的）。生物因子包括生产者、消费者和分解者，而非生物因子包括温度、光照、水分、土壤pH和矿质元素的可用性。
A habitat is the place where an organism lives, whereas a niche describes its role in the ecosystem, including what it feeds on, when it is active and how it interacts with other species. Two species cannot occupy exactly the same niche indefinitely because of competitive exclusion.
栖息地是生物生活的地点，而生态位描述其在生态系统中的角色，包括其食物、活动时间以及与其他物种的相互作用。由于竞争排斥原理，两个物种不能无限期占据完全相同的生态位。
A population is a group of individuals of the same species in the same area, and a community is all populations living together. The ecosystem adds the abiotic environment to the community.
种群是同一区域内同一物种的个体集合，群落是共同生活的所有种群。生态系统则在群落基础上加上非生物环境。
2. Energy Flow and Trophic Levels | 能量流动与营养级
Energy enters most ecosystems as sunlight and is converted to chemical energy by photosynthesis in producers. This energy is then transferred through the ecosystem by feeding relationships.
能量以阳光形式进入大多数生态系统，并通过生产者的光合作用转化为化学能。这种能量随后通过捕食关系在生态系统中传递。
Organisms are assigned to trophic levels: producers (first trophic level), primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), tertiary consumers and decomposers. Decomposers release inorganic nutrients back to the environment.
生物被划分为不同的营养级：生产者（第一营养级）、初级消费者（食草动物）、次级消费者（以食草动物为食的食肉动物）、三级消费者和分解者。分解者将无机营养物释放回环境中。
Food chains show linear feeding sequences, but most feeding relationships form a food web because many consumers feed on more than one species.
食物链展示线性的取食顺序，但由于许多消费者以多种物种为食，大多数取食关系构成食物网。
3. Ecological Pyramids | 生态金字塔
Ecological pyramids represent the number, biomass or energy content of organisms at each trophic level. A pyramid of energy always remains upright because energy is lost at each transfer.
生态金字塔表示各营养级生物的数量、生物量或能量含量。能量金字塔始终保持正金字塔形，因为能量在每次传递中都会损失。
Pyramids of numbers can be inverted; for example, a single oak tree may support thousands of caterpillars. Pyramids of biomass can also be inverted in aquatic systems where phytoplankton are consumed rapidly by zooplankton.
数量金字塔可能倒置，例如一棵橡树可以供养数千只毛虫。在水生系统中，浮游植物被浮游动物快速消耗，因此生物量金字塔也可能倒置。
Energy pyramids are drawn to scale using units such as kJ m⁻² yr⁻¹, and the width of each bar is proportional to the energy available at that trophic level.
能量金字塔按比例绘制，使用单位如 kJ m⁻² yr⁻¹，每个柱的宽度与在该营养级可用的能量成正比。
4. Productivity: GPP and NPP | 生产力：总初级生产力与净初级生产力
Gross primary productivity (GPP) is the total rate at which producers convert light energy into chemical energy via photosynthesis. Net primary productivity (NPP) is the energy remaining after plant respiration.
总初级生产力（GPP）是生产者通过光合作用将光能转化为化学能的总速率。净初级生产力（NPP）是植物呼吸作用后剩余的能量。
NPP = GPP − R
where R is the energy lost through plant respiration. NPP represents the energy available to primary consumers and is typically expressed in kJ m⁻² yr⁻¹.
其中R为植物呼吸作用损失的能量。NPP代表可供初级消费者利用的能量，通常以kJ m⁻² yr⁻¹表示。
High NPP is found in warm, moist environments such as tropical rainforests and estuaries, whereas deserts and polar regions have low productivity.
温暖潮湿的环境（如热带雨林和河口）NPP较高，而沙漠和极地地区生产力较低。
5. Energy Transfer Efficiency | 能量传递效率
Only about 10% of the energy at one trophic level is converted into new biomass at the next trophic level. The rest is lost through respiration, movement, excretion and uneaten parts.
在一个营养级中，只有大约10%的能量转化为下一营养级的新生物量。其余能量通过呼吸作用、运动、排泄和未被取食的部分损失。
The efficiency of energy transfer can be calculated using the equation:
能量传递效率可用以下公式计算：
Efficiency = (energy available after transfer ÷ energy available before transfer) × 100
For example, if cattle consume 50 000 kJ of grass and retain 5 000 kJ in new tissue, efficiency is (5000 ÷ 50000) × 100 = 10%.
例如，如果牛摄入50 000 kJ的草，并在新组织中保留5 000 kJ，效率为 (5000 ÷ 50000) × 100 = 10%。
Low transfer efficiency limits the length of food chains, as too little energy remains to support further trophic levels.
较低的传递效率限制了食物链的长度，因为剩余的能量太少，无法支持更高的营养级。
6. The Carbon Cycle | 碳循环
Carbon is cycled between the atmosphere, living organisms, oceans and fossil reserves. Photosynthesis removes carbon dioxide from the atmosphere, while respiration, decomposition and combustion return it.
碳在大气、生物体、海洋和化石储备之间循环。光合作用从大气中吸收二氧化碳，而呼吸作用、分解作用和燃烧将其归还。
Decomposers such as fungi and bacteria break down dead organic matter, releasing CO₂ and mineral nutrients. Fossil fuels formed from ancient organic matter release stored carbon when burned.
真菌和细菌等分解者分解死亡有机物，释放CO₂和矿质营养。由古代有机物形成的化石燃料在燃烧时释放储存的碳。
Human activities, especially burning fossil fuels and deforestation, have increased atmospheric CO₂ concentration, contributing to enhanced greenhouse effect and climate change.
人类活动，尤其是燃烧化石燃料和砍伐森林，增加了大气CO₂浓度，导致温室效应增强和气候变化。
In aquatic systems, CO₂ dissolves in water and can form carbonate ions, linking atmospheric and oceanic carbon reservoirs.
在水生系统中，CO₂溶于水并可形成碳酸根离子，将大气和海洋碳库联系起来。
7. The Nitrogen Cycle | 氮循环
Nitrogen is essential for amino acids, proteins and nucleic acids. Most organisms cannot use atmospheric N₂ directly, so it must be fixed into ammonium or nitrate ions.
氮对氨基酸、蛋白质和核酸至关重要。大多数生物不能直接利用大气中的N₂，因此必须将其固定为铵离子或硝酸根离子。
Nitrogen fixation is carried out by free-living bacteria such as Azotobacter and by Rhizobium inside root nodules of legumes. The enzyme nitrogenase catalyses this reduction of N₂ to ammonia.
固氮由自由生活的细菌（如固氮菌）和豆科植物根瘤内的根瘤菌完成。固氮酶催化N₂还原为氨。
Nitrification is a two-step oxidation: ammonium to nitrite by Nitrosomonas, then nitrite to nitrate by Nitrobacter. Denitrification by Pseudomonas converts nitrate back to N₂ under anaerobic conditions.
硝化作用是两步氧化过程：亚硝化单胞菌将铵氧化为亚硝酸盐，硝化杆菌再将亚硝酸盐氧化为硝酸盐。假单胞菌在厌氧条件下进行反硝化，将硝酸盐还原为N₂。
Ammonification occurs when decomposers break down proteins and urea, releasing ammonium ions back into the soil. Farmers add nitrate fertilisers or plant legumes to maintain soil nitrogen.
氨化作用发生在分解者分解蛋白质和尿素时，将铵离子释放回土壤中。农民通过施用硝酸盐肥料或种植豆科植物来维持土壤氮素。
8. Population Growth and Carrying Capacity | 种群增长与容纳量
Populations can grow exponentially when resources are unlimited, producing a J-shaped curve. In reality, environmental resistance from limited food, disease, competition and predation slows growth.
当资源无限时，种群可呈指数增长，形成J形曲线。实际上，食物有限、疾病、竞争和捕食等环境阻力会减缓增长。
The logistic growth model gives an S-shaped curve, where growth rate declines as the population approaches carrying capacity (K), the maximum population size an environment can sustain.
逻辑斯蒂增长模型呈S形曲线，随着种群接近环境容纳量（K，即环境可维持的最大种群规模），增长率下降。
Predator-prey relationships show cyclical fluctuations. When prey numbers increase, predator numbers rise after a lag, causing prey numbers to fall, followed by a decline in predators.
捕食者-猎物关系表现出周期性波动。当猎物数量增加时，捕食者数量在滞后一段时间后上升，导致猎物数量下降，随后捕食者数量也下降。
r-selected species reproduce rapidly in unstable habitats, while K-selected species reproduce slowly and compete strongly near carrying capacity.
r-对策物种在不稳定栖息地中快速繁殖，而K-对策物种繁殖缓慢，在接近环境容纳量时竞争激烈。
9. Ecological Succession | 生态演替
Succession is the gradual, directional change in species composition of a community over time. Primary succession begins on bare, lifeless surfaces such as bare rock or sand dunes.
演替是群落物种组成随时间逐渐定向变化的过程。初级演替始于裸露、无生命的表面，如裸岩或沙丘。
Pioneer species such as lichens and mosses colonise bare rock, breaking it down and forming thin soil. Over time, grasses, shrubs and finally trees replace earlier communities, leading to a climax community.
地衣和苔藓等先锋物种在裸岩上定殖，分解岩石并形成薄层土壤。随着时间推移，草本、灌木最终被树木取代，形成顶极群落。
Secondary succession occurs after a disturbance such as fire or land clearance, where soil already exists. It proceeds faster than primary succession because seeds and organic matter remain.
次级演替发生在火灾或土地清理等干扰之后，此时土壤已经存在。由于保留了种子和有机质，其进程比初级演替更快。
In the UK, primary succession can be seen on sand dunes or after glacial retreat, with climax woodland dominated by oak or beech depending on climate and soil.
在英国，沙丘或冰川退缩后的裸地可观察到初级演替，顶极林地以橡树或山毛榉为主，取决于气候
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