📚 Ecosystems and Processes | 生态系统与过程
Ecosystems are the fundamental functional units of the biosphere. This revision article examines how energy flows through ecosystems, how nutrients are recycled, and how natural and human processes shape ecosystem structure and drive change over time. The sequence follows a standard A-Level specification: definitions and components, biotic and abiotic factors, energy transfer and productivity, nutrient cycles, succession, stability and human impact.
生态系统是生物圈的基本功能单元。本文复习能量如何在生态系统中流动、养分如何循环，以及自然和人为过程如何塑造生态系统的结构并推动其随时间变化。文章顺序与 A-Level 考纲一致：定义与组分、生物与非生物因子、能量传递与生产力、养分循环、演替、稳定性与人类影响。
1. Defining Ecosystems: Structure and Components | 生态系统的定义：结构与组成
An ecosystem is a dynamic, interacting system comprising all living organisms (biotic components) and the physical environment (abiotic components) in a defined area. Because organisms and their surroundings constantly exchange energy and matter, an ecosystem is best pictured as a set of flows rather than a static list of species.
生态系统是一个动态的、相互作用的系统，由特定区域内所有生物（生物组分）和物理环境（非生物组分）组成。由于生物与环境之间不断交换能量与物质，最好把生态系统理解为一组"流"，而不是一份静态的物种清单。
Ecosystems exist at many scales. A pond, a woodland, a coral reef or a single rotting log can each be treated as an ecosystem, because each has inputs, outputs, internal cycling and boundaries chosen by the observer. The biosphere is the largest and most inclusive ecosystem of all.
生态系统存在于多种尺度上。一个池塘、一片林地、一座珊瑚礁，甚至一段正在腐烂的木头，都可以被当作一个生态系统，因为它们各有输入、输出、内部循环以及由研究者划定的边界。生物圈则是最大、最具包容性的生态系统。
A community contains only the living organisms; an ecosystem adds the abiotic environment and the measurable fluxes of energy and nutrients between them. This distinction is a common exam point when asked to compare the two terms.
群落只包含生物体；生态系统则额外包含非生物环境以及二者之间可测量的能量与养分通量。当题目要求比较这两个术语时，这一区分是常见考点。
2. Biotic Components: Producers, Consumers and Decomposers | 生物组分：生产者、消费者与分解者
Biotic components are usually classified into producers, consumers and decomposers. Producers (autotrophs) fix energy by photosynthesis or chemosynthesis; consumers (heterotrophs) obtain energy by feeding on other organisms; decomposers break down dead organic matter and release nutrients back into the soil.
生物组分通常分为生产者、消费者和分解者。生产者（自养生物）通过光合作用或化能合成固定能量；消费者（异养生物）通过取食其他生物获取能量；分解者分解死亡的有机物，并把养分释放回土壤。
Consumers are further divided. Primary consumers (herbivores) feed on producers; secondary and tertiary consumers feed on other animals; top predators occupy the highest trophic levels. Detritivores such as earthworms and woodlice ingest dead material and fragment it, greatly increasing the surface area available to bacteria and fungi.
消费者可进一步划分。初级消费者（植食动物）取食生产者；次级和三级消费者取食其他动物；顶级捕食者占据最高营养级。蚯蚓、潮虫等食碎屑动物摄食死亡物质并将其破碎，大大增加了细菌和真菌可利用的表面积。
Decomposers are essential for nutrient recycling rather than for energy supply, because the energy they release is ultimately lost as heat and cannot be reused by the community. They also return carbon dioxide to the atmosphere and mineral ions to the soil.
分解者对养分循环必不可少，但它并不提供能量，因为它释放的能量最终以热量散失，无法被群落再利用。分解者同时把二氧化碳归还大气，把矿质离子归还土壤。
Each species also occupies a niche: its role and position in the ecosystem, including the resources it uses, the conditions it tolerates and its interactions with other species. Two species cannot occupy exactly the same niche in a stable ecosystem, because competition will eventually exclude one of them.
每个物种还占据一定的生态位：即它在生态系统中的角色与位置，包括所利用的资源、能耐受的条件以及与其他物种的相互作用。在稳定的生态系统中，两个物种不可能占据完全相同的生态位，因为竞争最终会使其中一个被排除。
3. Abiotic Factors and Limiting Factors | 非生物因子与限制因子
Abiotic components include solar radiation, temperature, water availability, soil type and pH, together with oxygen concentration, salinity, wind and mineral content. These factors control which organisms can survive and how quickly ecosystem processes such as photosynthesis, decomposition and evaporation operate.
非生物组分包括太阳辐射、温度、水分、土壤类型和酸碱度，以及氧气浓度、盐度、风和矿物质含量。这些因素控制着哪些生物能够生存，以及光合作用、分解作用和蒸发等生态系统过程的速率。
Where a factor is in short supply it becomes a limiting factor. The rate of photosynthesis, for example, may be restricted by light intensity, carbon dioxide concentration, water supply or the availability of nitrate ions. Liebig's law of the minimum states that growth is governed by the resource in shortest supply relative to demand.
当某种因子供给不足时，它就成为限制因子。例如光合作用速率可能受光照强度、二氧化碳浓度、水分供应或硝酸根离子可用性的限制。李比希最小因子定律指出，生长受相对于需求最缺乏的那种资源支配。
Shelford's law of tolerance adds that too much of a factor can also be harmful, so each species has an optimum range rather than simply a minimum requirement. Excess salt, waterlogging or high temperature can reduce growth just as effectively as a shortage.
谢尔福德的耐性定律补充说明，某因子过量同样有害，因此每种生物具有最适范围，而不仅仅是最低需求。盐分过高、积水或高温对生长的抑制效果，可能与资源短缺一样明显。
Abiotic factors rarely act alone. Temperature affects both enzyme-controlled metabolism and evaporation, and the two together determine how much water remains available in the soil; pH controls the solubility of mineral ions. Ecologists therefore monitor factors over time and use correlation with species distribution rather than relying on single readings.
非生物因子很少单独起作用。温度既影响酶控制的新陈代谢，也影响蒸发，二者共同决定土壤中剩余的可用水量；pH 则控制矿质离子的可溶性。因此生态学家会长期监测这些因子，并用它们与物种分布之间的相关性来推断影响，而不是依赖单次读数。
4. Energy Flow: Food Chains and Trophic Levels | 能量流动：食物链与营养级
Energy enters most ecosystems as solar radiation. Producers convert it into chemical energy during photosynthesis:
生产者把太阳辐射能转化为化学能，反应式如下：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
is finally lost from the system as heat. A food chain shows the direction of energy transfer and the feeding sequence, while a food web shows overlapping chains and gives a more realistic picture of a community.
能量随后沿食物链传递，例如 草 → 兔 → 狐，最终以热量形式离开系统。食物链表示能量传递的方向和取食顺序；食物网表示相互重叠的多条食物链，能更真实地反映群落状况。
Each stage of a chain is a trophic level. Producers form the first trophic level, herbivores the second, and carnivores the third and above. Omnivores feed at more than one level, so a single species may appear at several points in a food web.
食物链的每一环节是一个营养级。生产者构成第一营养级，植食动物为第二营养级，肉食动物为第三及更高营养级。杂食动物在多个营养级取食，因此同一物种可能在食物网中出现在多个位置。
Energy is lost at every transfer because organisms respire, excrete and produce heat; much of the plant material is never eaten, and some of what is eaten is not digested. Unlike nutrients, energy does not cycle - it flows in one direction and leaves the ecosystem as heat, so a continuous solar input is required.
每次传递都会损失能量，因为生物会呼吸、排泄并产生热量；大量植物材料从未被取食，被取食的部分也有一些无法消化。与养分不同，能量不循环——它单向流动并以热量形式离开生态系统，因此需要持续的太阳能输入。
5. Primary Productivity and Ecological Efficiency | 初级生产力与生态效率
Gross primary production (GPP) is the total chemical energy fixed by producers in a given area and time. Net primary production (NPP) is what remains after producers have met their own respiratory costs, so:
总初级生产量（GPP）是生产者在单位面积、单位时间内固定的化学能总量。净初级生产量（NPP）是生产者扣除自身呼吸消耗后剩余的部分，因此：
NPP = GPP − R
NPP is the energy actually available to consumers and decomposers, and it is usually expressed in kJ m⁻² y⁻¹ or g m⁻² y⁻¹. Tropical rainforests, estuaries and coral reefs have high NPP; deserts, tundra and open oceans have low NPP per unit area, although the ocean is vast in total.
NPP 是真正可供消费者和分解者利用的能量，通常以 kJ
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