A-Level CCEA Science: Ecosystems Revision Guide | A-Level CCEA 科学：生态系统考点精讲

📚 A-Level CCEA Science: Ecosystems Revision Guide | A-Level CCEA 科学：生态系统考点精讲

An ecosystem is a dynamic, self-sustaining system made up of living organisms interacting with each other and with their non-living environment. In the CCEA A-Level Science specification, understanding ecosystems means grasping how energy flows, how nutrients cycle, and how communities change over time. This guide covers the essential concepts you will need for the exam, from trophic levels and pyramids to succession and human impacts.

生态系统是一个动态、自我维持的系统，由生物体之间以及与其非生物环境相互作用组成。在 CCEA A-Level 科学考试大纲中，理解生态系统意味着掌握能量如何流动、物质如何循环以及群落如何随时间变化。本指南涵盖了考试所需的全部核心概念，从营养级和金字塔到演替及人类影响，逐一精讲。

1. Ecosystem Definition and Structure | 生态系统的定义与结构

An ecosystem comprises all the organisms (the community) living in a particular area, along with the physical environment. The community includes populations of different species, each occupying a specific ecological niche – the role it plays, including its habitat, its interactions, and its use of resources. The place where an organism lives is its habitat.

生态系统由生活在一定区域内的所有生物（群落）及其物理环境组成。群落包含不同物种的种群，每个物种占据一个特定的生态位——即它所扮演的角色，包括其栖息地、相互作用以及对资源的利用。生物体生活的场所就是它的栖息地。

2. Biotic and Abiotic Factors | 生物与非生物因素

Biotic factors are the living components of an ecosystem that affect other organisms. These include predation, competition (for food, light, space, mates), disease, and mutualism. Abiotic factors are the non-living physical and chemical elements, such as temperature, light intensity, water availability, soil pH, salinity, and oxygen concentration. Both sets of factors determine which species can survive and thrive in a given environment.

生物因素是指生态系统中影响其他生物的活体成分，包括捕食、竞争（食物、光照、空间、配偶）、疾病和互利共生。非生物因素则是非生命的物理和化学要素，如温度、光照强度、水分可用性、土壤 pH 值、盐度和氧浓度。这两类因素共同决定了哪些物种能够在特定环境中生存繁衍。

For example, in a freshwater pond, light availability influences the depth at which aquatic plants can photosynthesise; temperature affects the metabolic rate of fish; and competition between perch and roach for zooplankton controls their population sizes.

例如，在淡水池塘中，光照条件影响水生植物能进行光合作用的水深；温度影响鱼类的代谢速率；而河鲈与拟鲤对浮游动物的竞争控制着它们的种群数量。

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

Organisms are grouped into trophic levels based on their source of energy. Producers (autotrophs) such as green plants and algae convert light energy into chemical energy via photosynthesis. Primary consumers (herbivores) eat producers. Secondary consumers (carnivores) eat primary consumers, and tertiary consumers eat secondary consumers. Decomposers (bacteria, fungi) break down dead organic matter, returning nutrients to the soil.

生物根据其能量来源划分为营养级。生产者（自养生物），如绿色植物和藻类，通过光合作用将光能转化为化学能。初级消费者（草食动物）以生产者为食。次级消费者（肉食动物）以初级消费者为食，三级消费者又以次级消费者为食。分解者（细菌、真菌）分解死去的有机物，将养分归还土壤。

A food chain is a simple linear sequence showing ‘who eats whom’, but in reality, organisms feed at several trophic levels, forming a food web. Food webs are more stable because if one species declines, alternative food sources are available.

食物链是简单的线性序列，展示“谁吃谁”，但现实中生物往往在多个营养级上取食，形成食物网。食物网更加稳定，因为当某一物种数量下降时，有其他食物来源可供替代。

4. Energy Flow and Productivity | 能量流动与生产力

Energy enters most ecosystems as sunlight and is captured by producers. Gross primary productivity (GPP) is the total amount of chemical energy fixed by photosynthesis. Net primary productivity (NPP) is the energy remaining after producers have used some for respiration: NPP = GPP − R (where R is respiratory loss). Only NPP is available to the next trophic level.

能量以阳光的形式进入大多数生态系统，并被生产者捕获。总初级生产力（GPP）是光合作用固定的化学能总量。净初级生产力（NPP）是生产者用于呼吸之后剩余的能量：NPP = GPP − R（其中 R 为呼吸消耗）。只有 NPP 可供下一营养级利用。

Energy transfer between trophic levels is inefficient – on average only about 10% of the energy in one level is converted into biomass at the next. The rest is lost through movement, heat, excretion, and indigestible parts. This limits the length of food chains (usually no more than 4–5 trophic levels).

营养级之间的能量传递效率很低——平均只有约 10%的能量转化为下一级的生物量。其余能量通过运动、散热、排泄和不可消化部分流失。这限制了食物链的长度（通常不超过 4-5 个营养级）。

5. Ecological Pyramids | 生态金字塔

Ecological pyramids are graphical representations of the structure of an ecosystem. Three main types are examined:

生态金字塔是生态系统结构的图形化表示。考试中涉及三种主要类型：

Pyramid Type 金字塔类型	What It Represents 表示内容	Typical Shape 典型形状
Pyramid of numbers	Number of individuals at each trophic level	Often upright, but can be inverted (e.g., one tree supports many insects)
Pyramid of biomass	Dry mass of living material per unit area	Usually upright; aquatic ecosystems may show an inverted pyramid if phytoplankton reproduce rapidly
Pyramid of energy	Energy content (kJ m⁻² yr⁻¹)	Always upright – energy is always lost at each transfer

The pyramid of energy is the most accurate representation of ecosystem structure because it accounts for the rate of production and cannot be inverted.

能量金字塔是生态系统结构最准确的表示，因为它考虑了生产速率，且永远不会倒置。

6. Nutrient Cycles: The Carbon Cycle | 物质循环：碳循环

Nutrients are recycled within ecosystems. The carbon cycle involves key processes:

营养物质在生态系统中循环。碳循环涉及以下关键过程：

Photosynthesis removes CO₂ from the atmosphere and fixes it into organic compounds.
光合作用从大气中吸收 CO₂ 并将其固定为有机化合物。
Respiration by plants, animals, and decomposers releases CO₂ back into the atmosphere.
植物、动物和分解者的呼吸作用将 CO₂ 释放回大气。
Decomposition of dead organic matter by microorganisms returns carbon compounds to the soil and atmosphere.
微生物对死亡有机物的分解将碳化合物归还土壤和大气。
Combustion of fossil fuels and biomass releases stored carbon as CO₂.
化石燃料和生物质的燃烧将储存的碳以 CO₂ 形式释放。
Oceans act as a carbon sink – CO₂ dissolves in water and is used by marine photosynthesizers.
海洋作为碳汇——CO₂ 溶于水，被海洋光合生物利用。
Formation of fossil fuels (coal, oil, gas) locks carbon away for millions of years.
化石燃料（煤、石油、天然气）的形成将碳封存数百万年。

The balanced equation for photosynthesis and respiration is central:

光合作用与呼吸作用的平衡方程式是核心：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (photosynthesis) | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (respiration)

7. Nutrient Cycles: The Nitrogen Cycle | 物质循环：氮循环

Nitrogen is essential for proteins and nucleic acids. Organisms cannot use atmospheric N₂ directly; it must be converted into a usable form through four main stages:

氮是蛋白质和核酸所必需的元素。生物不能直接利用大气中的 N₂，必须通过四个主要阶段将其转化为可吸收形式：

Nitrogen fixation: Free-living bacteria (e.g., Azotobacter) or mutualistic bacteria (Rhizobium in legume root nodules) convert N₂ into ammonia (NH₃) or ammonium ions (NH₄⁺). Lightning also fixes small amounts.

固氮作用：游离细菌（如 Azotobacter）或共生细菌（豆科植物根瘤中的 Rhizobium）将 N₂ 转化为氨 (NH₃) 或铵离子 (NH₄⁺)。闪电也能固定少量氮。

Ammonification: Decomposers break down proteins and urea from dead organisms and waste, releasing ammonium ions.

氨化作用：分解者分解死亡生物和排泄物中的蛋白质和尿素，释放铵离子。

Nitrification: Nitrifying bacteria oxidise ammonium ions first to nitrites (NO₂⁻) by Nitrosomonas, then to nitrates (NO₃⁻) by Nitrobacter. Nitrates are the form most readily absorbed by plant roots.

硝化作用：硝化细菌首先将铵离子氧化为亚硝酸盐 (NO₂⁻)（由 Nitrosomonas 作用），再氧化为硝酸盐 (NO₃⁻)（由 Nitrobacter 作用）。硝酸盐是植物根系最容易吸收的形式。

Denitrification: Under anaerobic conditions (e.g., waterlogged soils), denitrifying bacteria convert nitrates back into N₂ gas, returning it to the atmosphere.

反硝化作用：在厌氧条件下（如渍水土壤），反硝化细菌将硝酸盐还原为 N₂ 气体，返回大气。

8. Primary Succession | 初生演替

Primary succession occurs in a lifeless area where no soil exists, such as bare rock after a volcanic eruption or a retreating glacier. The sequence involves:

初生演替发生在没有土壤的无生命区域，如火山喷发后的裸露岩石或冰川消退后的地面。其序列包括：

Pioneer species (e.g., lichens, mosses) colonise the bare rock. They withstand extreme conditions and begin breaking down the rock surface by weathering and accumulating tiny amounts of organic matter.
先锋物种（如地衣、苔藓）在裸岩上定居。它们耐受极端条件，通过风化和积累少量有机质开始分解岩石表面。
As these organisms die, their remains form a thin, primitive soil. This allows small, fast-growing plants (e.g., grasses, ferns) to establish.
随着这些生物死亡，其残体形成薄薄的原始土壤。这使小型快速生长的植物（如草本、蕨类）得以立足。
Over time, soil depth and nutrient content increase. Shrubs and then small trees appear. Each community alters the environment, making it less suitable for itself but more suitable for the next community – this is facilitation.
随着时间推移，土壤深度和养分含量增加。灌木出现，随后小乔木生长。每个群落改变环境，使其变得不太适合自身，却更适合下一个群落——这就是促进效应。
Eventually, a stable climax community is reached, dominated by large, shade-tolerant trees (e.g., oak woodland). The climax is determined by climate and soil conditions.
最终达到稳定的顶级群落，以大型耐阴乔木（如橡树林）为主。顶级群落由气候和土壤条件决定。

9. Secondary Succession and Climax Communities | 次生演替与顶级群落

Secondary succession occurs in areas where an existing community has been cleared by a disturbance (e.g., fire, farming, deforestation) but soil remains. It is faster than primary succession because the soil already contains seeds, nutrients, and microorganisms. The stages are similar: pioneer plants → grasses → shrubs → trees, eventually re-forming a climax community.

次生演替发生在原有群落因干扰（如火灾、耕作、砍伐森林）被清除，但土壤仍存在的区域。它比初生演替更快，因为土壤中已经含有种子、养分和微生物。其阶段相似：先锋植物 → 草本 → 灌木 → 乔木，最终重新形成顶级群落。

A climax community is not a single fixed endpoint. In the UK, the climatic climax is deciduous woodland, but other factors such as grazing, fire, or waterlogging can produce a plagioclimax (a community prevented from reaching the climatic climax by human or animal activity, e.g., heather moorland maintained by burning).

顶级群落并非单一固定的终点。在英国，气候顶级群落是落叶林，但放牧、火灾或渍水等其他因素可能产生偏途顶级群落（由于人类或动物活动阻止达到气候顶级的群落，例如通过烧荒维持的石南灌丛）。

10. Human Impact on Ecosystems | 人类对生态系统的影响

Human activities can disrupt ecosystems in many ways. Deforestation reduces biodiversity, destroys habitats, and releases CO₂, contributing to climate change. Overfishing can deplete fish stocks and disrupt marine food webs. Eutrophication – caused by fertiliser run-off or sewage – leads to algal blooms, oxygen depletion, and death of aquatic organisms. Conservation strategies such as replanting, setting fishing quotas, buffer strips, and protected areas aim to maintain balanced ecosystems and preserve biodiversity for future generations.

人类活动可以多种方式破坏生态系统。森林砍伐降低生物多样性、破坏栖息地并释放 CO₂，加剧气候变化。过度捕捞会耗尽鱼类资源并扰乱海洋食物网。富营养化——由肥料径流或污水引起——导致藻类大量繁殖、水体缺氧以及水生生物死亡。诸如再造林、设定捕捞配额、缓冲带和保护区等保护策略，旨在维持生态系统平衡，并为子孙后代保存生物多样性。

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A-Level CCEA Science: Ecosystems Revision Guide | A-Level CCEA 科学：生态系统 考点精讲