Food Chains | 食物链考点精讲

📚 Food Chains | 食物链考点精讲

A food chain is a linear sequence that shows how energy and nutrients are transferred from one organism to another within an ecosystem. In WJEC IGCSE Biology, you need to understand how food chains are constructed, the roles of different organisms, and the way energy flows through trophic levels. This topic also links to pyramids of numbers and biomass, bioaccumulation, and the effects of environmental change on feeding relationships.

食物链是显示能量和营养物质在生态系统中从一个生物传递到另一个生物的线性序列。在 WJEC IGCSE 生物考试中，你需要理解食物链的构建方式、不同生物的角色以及能量在营养级之间的流动规律。该主题还涉及数量金字塔与生物量金字塔、生物累积以及环境变化对捕食关系的影响。

1. What Is a Food Chain? | 什么是食物链？

A food chain always starts with a producer – an organism that makes its own food through photosynthesis. The arrows in a food chain point from the organism being eaten to the organism that eats it, showing the direction of energy transfer. For example: grass → rabbit → fox. The grass is the producer, the rabbit is a primary consumer, and the fox is a secondary consumer.

食物链总是从生产者开始——生产者是通过光合作用自己制造食物的生物。食物链中的箭头从被吃的生物指向吃它的生物，表示能量传递的方向。例如：草 → 兔 → 狐狸。草是生产者，兔是初级消费者，狐狸是次级消费者。

A food chain describes ‘who eats whom’ in a habitat. Each step in the chain is called a trophic level. Producers occupy the first trophic level, primary consumers the second, secondary consumers the third, and so on. Decomposers, such as bacteria and fungi, are not usually shown in food chains but they break down dead material at every trophic level, returning nutrients to the soil.

食物链描述了栖息地中“谁吃谁”的关系。链中的每一步都称为营养级。生产者占据第一营养级，初级消费者占据第二营养级，次级消费者占据第三营养级，依此类推。像细菌和真菌这样的分解者通常不显示在食物链中，但它们分解各个营养级的死物质，将养分归还给土壤。

2. Producers, Consumers and Decomposers | 生产者、消费者与分解者

Producers are green plants or algae that capture light energy from the sun and convert it into chemical energy through photosynthesis. They are autotrophic, meaning they produce their own food. In terrestrial ecosystems, typical producers include grasses, trees and shrubs; in aquatic ecosystems, phytoplankton are the main producers.

生产者是绿色植物或藻类，它们捕获来自太阳的光能，并通过光合作用将其转化为化学能。它们是自养生物，即自己生产食物。在陆地生态系统中，典型的生产者包括草、树和灌木；在水生生态系统中，浮游植物是主要的生产者。

Consumers are heterotrophic organisms that obtain energy by feeding on other organisms. Primary consumers (herbivores) eat producers; secondary consumers (carnivores) eat primary consumers; tertiary consumers eat secondary consumers. Some animals are omnivores and can act as consumers at more than one trophic level depending on what they eat.

消费者是异养生物，通过捕食其他生物获取能量。初级消费者（食草动物）吃生产者；次级消费者（食肉动物）吃初级消费者；三级消费者吃次级消费者。有些动物是杂食动物，根据所吃的食物可以在多个营养级中扮演消费者角色。

Decomposers are organisms, mainly bacteria and fungi, that break down dead organic matter and waste products. They secrete enzymes onto dead material and absorb the products of digestion. This process releases mineral ions back into the soil, which are then taken up by plants again. Decomposers are essential for nutrient cycling.

分解者主要是细菌和真菌，它们分解死的有机物和排泄物。它们向死物质分泌酶，然后吸收消化产物。这个过程将矿物离子释放回土壤中，再被植物吸收。分解者对营养循环至关重要。

3. Constructing Food Chains | 食物链的构建

When drawing a food chain, always begin with a producer. Write the names of organisms in order, and draw arrows between them. The arrow means ‘is eaten by’ or ‘transfers energy to’. A typical WJEC question might ask you to construct a food chain from a food web or from a list of organisms. Remember that the arrow points to the organism that receives the energy.

绘制食物链时，始终以生产者开始。按顺序写出生物的名称，并在它们之间画箭头。箭头表示“被……吃”或“将能量传递给……”。典型的 WJEC 考题可能要求你从食物网或一组生物中构建一条食物链。请记住，箭头指向获得能量的生物。

Food chains rarely have more than five trophic levels because so much energy is lost at each level that there is insufficient left to support another level. The longer the chain, the less energy is available to the top predator.

食物链很少超过五个营养级，因为在每个营养级上能量损失太大，剩余的能量不足以支撑下一个营养级。链越长，顶级捕食者可获得的能量就越少。

Example: phytoplankton → zooplankton → small fish → large fish → seal. Here phytoplankton are producers, zooplankton primary consumers, small fish secondary consumers, large fish tertiary consumers, seals quaternary consumers.

例如：浮游植物 → 浮游动物 → 小鱼 → 大鱼 → 海豹。其中浮游植物是生产者，浮游动物是初级消费者，小鱼是次级消费者，大鱼是三级消费者，海豹是四级消费者。

4. Trophic Levels and Energy Flow | 营养级与能量流动

A trophic level is the position an organism occupies in a food chain. Energy enters the food chain through producers, who convert approximately 1–3% of the light energy they receive into chemical energy. The rest is reflected, transmitted or lost as heat.

营养级是生物在食物链中所处的位置。能量通过生产者进入食物链，生产者将接收到的光能的大约 1–3% 转化为化学能。其余的能量被反射、透射或以热的形式散失。

At each trophic level, only about 10% of the energy is passed on to the next level. The remaining 90% is used for respiration, movement, growth, and is lost as heat or in waste products. This is often called the 10% rule. This inefficiency explains why biomass decreases along a food chain.

在每一个营养级，只有大约 10% 的能量传递给下一级。剩下的 90% 用于呼吸、运动、生长，并以热或废物的形式损失。这通常被称为十分之一法则。这种低效率解释了为什么生物量沿食物链递减。

Energy flow is always unidirectional – it moves from the sun to producers and then through consumers, and never goes backwards. Energy is not recycled in an ecosystem; nutrients are recycled but energy must be constantly supplied from the sun.

能量流动总是单向的——从太阳到生产者，再经消费者传递，绝不会回流。能量在生态系统中不被循环利用；营养物质可循环，但能量必须持续从太阳获取。

5. Food Webs | 食物网

A food web is a network of interconnected food chains. In reality, most organisms eat more than one type of food and are eaten by more than one predator. A food web gives a more realistic picture of feeding relationships in an ecosystem. If one organism is removed, the effect can be traced through the web.

食物网是由相互连接的食物链组成的网络。在现实中，大多数生物吃不止一种食物，并且被不止一种捕食者捕食。食物网更真实地反映了生态系统中的捕食关系。如果移除某种生物，其影响可以通过网络追踪。

When analysing a food web, you may be asked to identify producers, herbivores, carnivores, and omnivores. Also, you should be able to predict what would happen if a particular species disappeared. For instance, removing a top predator may cause an increase in its prey, which could then overgraze the producers, reducing biodiversity.

在分析食物网时，你可能需要确定生产者、食草动物、食肉动物和杂食动物。你还应该能够预测如果某个特定物种消失会发生什么。例如，移走一个顶级捕食者可能导致其猎物数量增加，进而可能过度啃食生产者，降低生物多样性。

WJEC often uses food webs to test understanding of interdependence and the knock-on effects of population changes. Remember: a change in one population can have indirect effects on organisms that are not directly connected in the web.

WJEC 经常使用食物网来考查对相互依赖关系以及种群数量变化连锁效应的理解。记住：一个种群的变化可能对食物网中并不直接相关的生物产生间接影响。

6. Pyramids of Numbers | 数量金字塔

A pyramid of numbers shows the count of individual organisms at each trophic level in a food chain. The width of each bar is proportional to the number of organisms. Usually, the producer bar is widest and it narrows towards the top. However, pyramids of numbers can look unusual. For example, one oak tree may support hundreds of insects, giving an inverted shape with a narrow base and a wider second level.

数量金字塔显示食物链中每个营养级的生物个体数量。每个条形图的宽度与生物数量成正比。通常，生产者条形图最宽，向顶端逐渐变窄。但数量金字塔可能看起来奇怪。例如，一棵橡树可以养活数百只昆虫，形成一个底部窄、第二级宽的倒置形状。

Because of such exceptions, pyramids of numbers are not always reliable for representing the structure of an ecosystem. WJEC expects you to recognise these limitations and understand why biomass pyramids are often more useful.

由于这类例外，数量金字塔并非总是可靠地代表生态系统结构。WJEC 期望你认识到这些局限性，并理解为什么生物量金字塔通常更有用。

In exams, you may be given a data table and asked to draw a pyramid of numbers to scale. Always label each bar with the name of the organism and its trophic level, and remember to draw bars of equal width with a scaled length.

考试中，你可能会被给一个数据表，要求按比例绘制一个数量金字塔。务必给每个条形图标明生物名称和营养级，并记住画出等宽、长度按比例的条形。

7. Pyramids of Biomass | 生物量金字塔

A pyramid of biomass represents the total dry mass of organisms at each trophic level, usually measured in grams per square metre (g/m²). Because biomass takes into account the size of organisms, it gives a more accurate picture of energy storage than numbers alone. A pyramid of biomass is almost always pyramid-shaped, with a broad producer base and progressively smaller tiers above.

生物量金字塔表示每个营养级生物的总干重，通常以克每平方米（g/m²）为单位。因为生物量考虑了生物的大小，它比单纯的数量更准确地反映能量储存状况。生物量金字塔几乎总是呈金字塔形，生产者基部宽大，向上逐级减小。

To draw a pyramid of biomass, you need to calculate the dry mass of organisms. Data is often provided. Draw horizontal bars of equal width, with the length representing biomass. Label each level. Unlike numbers pyramids, inverted biomass pyramids are very rare; they can occur in aquatic ecosystems during a short period when phytoplankton reproduce rapidly while zooplankton biomass lags behind, but for IGCSE, you should consider the typical upright shape.

要绘制生物量金字塔，你需要计算生物体的干重。数据通常会提供。画出等宽的横向条形图，长度代表生物量。标记每一级。与数量金字塔不同，倒置的生物量金字塔非常罕见；在水生生态系统中，浮游植物迅速繁殖而浮游动物生物量暂时滞后的短暂时期可能出现，但在 IGCSE 中，你应该考虑典型的直立形状。

WJEC questions may ask you to explain why biomass decreases at higher trophic levels. Key reasons include: not all of the organism is eaten, some parts are indigestible and lost as faeces, and most energy taken in is used in respiration and lost as heat.

WJEC 题目可能会要求你解释为什么在更高营养级生物量会减少。关键原因包括：并非生物的所有部分都被吃掉，有些部分无法消化而作为粪便流失，摄入的大部分能量用于呼吸并以热的形式损失。

8. Energy Loss and Efficiency Calculations | 能量损失与效率计算

Energy is lost between trophic levels due to several processes: respiration, movement, heat loss, uneaten parts (e.g. bones, roots), and excretion. This is why food chains are short and why the biomass of top predators is small.

能量在营养级之间因多种过程而损失：呼吸、运动、热量散失、未被食用的部分（如骨头、根）和排泄。这就是为什么食物链很短，且顶级捕食者的生物量很小。

You may be required to calculate the efficiency of energy transfer using the formula:

Efficiency (%) = (energy transferred to next level ÷ total energy in) × 100

你可能需要计算能量传递效率，使用公式：

效率 (%) = (传递给下一级的能量 ÷ 总输入能量) × 100

For example, if a plant fixes 20 000 kJ of energy by photosynthesis and a rabbit feeding on it gains 2 000 kJ, the transfer efficiency is (2 000 ÷ 20 000) × 100 = 10%. WJEC expects you to show all steps and use correct units.

例如，如果一株植物通过光合作用固定了 20 000 kJ 能量，而以它为食的兔子获得 2 000 kJ，则传递效率为 (2 000 ÷ 20 000) × 100 = 10%。WJEC 期望你展示所有步骤并使用正确的单位。

You may also be asked to explain low efficiency in terms of energy uses: energy is used for metabolic processes such as maintaining body temperature (in mammals and birds), active transport, muscle contraction, and is lost as heat to the environment. Only a small fraction is converted into new biomass that can be passed on.

你也可能需要从能量用途角度解释低效率：能量用于代谢过程，例如维持体温（哺乳动物和鸟类）、主动运输、肌肉收缩，并以热的形式散失到环境中。只有一小部分转化为新的生物量供下一级利用。

9. Pyramids of Energy | 能量金字塔

A pyramid of energy shows the energy content at each trophic level, usually in kilojoules per square metre per year (kJ/m²/yr). These pyramids are always upright because energy is always lost as it flows through the ecosystem. They provide the most accurate representation of energy flow but are less commonly used in IGCSE because data is difficult to collect.

能量金字塔显示每个营养级的能量含量，通常以千焦每平方米每年（kJ/m²/年）为单位。这种金字塔总是直立的，因为能量在生态系统中流动时总是会损失。它们提供了能量流动最准确的表示，但在 IGCSE 中较少使用，因为数据难以收集。

WJEC may present a pyramid of energy in a data-handling question. You should be able to compare it with biomass and number pyramids and state why it never appears inverted. The laws of thermodynamics dictate that energy conversions are never 100% efficient, so each subsequent level must contain less energy.

WJEC 可能在数据处理题目中呈现能量金字塔。你应该能够将其与生物量金字塔和数量金字塔进行比较，并说明为何它绝不会倒置。热力学定律规定能量转换永远不会 100% 有效，因此每一个后续营养级包含的能量必然更少。

10. Bioaccumulation and Biomagnification | 生物累积与生物放大

Bioaccumulation is the build-up of a non-biodegradable substance, such as a pesticide or heavy metal, inside an individual organism over time. Biomagnification refers to the increase in concentration of such substances as you move up the food chain. These concepts frequently appear in WJEC exams.

生物累积是指一种不可生物降解的物质（如杀虫剂或重金属）在个体生物体内随时间积累的过程。生物放大是指这些物质的浓度沿食物链上升而增加的现象。这些概念在 WJEC 考试中经常出现。

Suppose a farmer sprays a pesticide on crops. Small amounts are absorbed by plants. Primary consumers eat many plants and accumulate the pesticide in their tissues because it cannot be excreted easily. Secondary consumers eat many primary consumers, receiving an even higher dose. Top predators end up with dangerously high concentrations, which can affect reproduction or cause death.

假设农民在作物上喷洒了一种杀虫剂。少量杀虫剂被植物吸收。初级消费者吃掉许多植物，由于杀虫剂不易排泄，便在体内组织积累。次级消费者吃掉许多初级消费者，接收了更高的剂量。顶级捕食者最终含有危险的高浓度杀虫剂，可能影响繁殖或导致死亡。

The classic example is DDT, which caused thinning of eggshells in birds of prey, leading to population declines. Another example is mercury accumulation in fish. WJEC may ask you to interpret data showing concentration increases at higher trophic levels.

经典案例是 DDT，它导致猛禽蛋壳变薄，造成种群数量下降。另一个例子是鱼类体内汞的积累。WJEC 可能要求你解读显示较高营养级浓度增加的数据。

11. Impact of Environmental Change on Food Chains | 环境变化对食物链的影响

Food chains and webs are sensitive to environmental changes such as temperature shifts, pollution, introduction of non-native species, and habitat destruction. These factors can alter population sizes and disrupt energy flow. For instance, a decrease in the availability of a producer due to drought will reduce the numbers of herbivores, which will in turn reduce carnivore populations.

食物链和食物网对温度变化、污染、外来物种引入以及栖息地破坏等环境变化十分敏感。这些因素可改变种群数量并扰乱能量流动。例如，干旱导致生产者减少将降低食草动物数量，进而降低食肉动物数量。

Eutrophication is another relevant process. When fertilisers run off into water bodies, they cause an algal bloom. Algae block light, causing aquatic plants to die. Decomposers break down the dead matter, using up oxygen, which leads to the death of fish and other consumers. This disrupts the entire aquatic food chain.

富营养化是另一个相关过程。当化肥流入水体时，会引起藻类疯长。藻类遮挡光线，导致水生植物死亡。分解者分解死物质时消耗氧气，导致鱼类和其他消费者死亡。这扰乱了整个水生食物链。

In your exam, you may be asked to explain how a specific event, such as deforestation or an oil spill, affects a given food web. Always trace the impact from the producer level upwards, and mention both direct and indirect effects, including the potential for bioaccumulation if chemicals are involved.

考试中，你可能需要解释某一特定事件，如毁林或石油泄漏，如何影响给定的食物网。务必从生产者层级开始向上追溯影响，并提及直接和间接效应，如果涉及化学物质，还要提及生物累积的潜力。

12. Key Terminology and Exam Tips | 核心术语与备考技巧

Make sure you can define and use the following terms accurately: ecosystem, habitat, population, community, producer, consumer (primary, secondary, tertiary), decomposer, trophic level, food chain, food web, biomass, bioaccumulation, biomagnification, pyramid of numbers, pyramid of biomass, and energy transfer efficiency.

确保你能准确定义和使用以下术语：生态系统、栖息地、种群、群落、生产者、消费者（初级、次级、三级）、分解者、营养级、食物链、食物网、生物量、生物累积、生物放大、数量金字塔、生物量金字塔和能量传递效率。

Common exam mistakes include placing arrows in the wrong direction, confusing pyramids of numbers with pyramids of biomass, forgetting to label axes or bars, and misidentifying organisms in a food web. Practice drawing pyramids to scale and calculating energy transfer efficiencies. Read questions carefully – WJEC often embeds the food chain topic within data analysis or environmental context questions.

常见考试错误包括：箭头方向画错、混淆数量金字塔和生物量金字塔、忘记给坐标轴或条形图加标签、以及错误识别食物网中的生物。练习按比例绘制金字塔和计算能量传递效率。仔细阅读题目——WJEC 常将食物链主题嵌入数据分析或环境背景题中。

When explaining energy loss, always link to respiration, heat loss and waste. For bioaccumulation, stress the adjectives ‘non-biodegradable’ and ‘fat-soluble’, as these substances accumulate in fatty tissues. Time management is key: give yourself enough time to interpret diagrams and construct clear, well-labelled pyramids.

在解释能量损失时，始终联系呼吸、散热和排泄物。对于生物累积，强调形容词“不可生物降解的”和“脂溶性的”，因为这些物质在脂肪组织中积累。时间管理很重要：给自己留出足够的时间解读图表并构建清晰、标注完整的金字塔。

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Food Chains | 食物链 考点精讲