📚 Food Chains in IGCSE Edexcel Biology | IGCSE Edexcel 生物：食物链考点精讲
In IGCSE Edexcel Biology, food chains form a fundamental part of ecology. They describe how energy is transferred between organisms in a linear sequence. This revision guide covers every key concept: producers, consumers, trophic levels, energy loss, pyramids of biomass, bioaccumulation, and common exam pitfalls. Use it to consolidate your knowledge and boost your exam performance.
在IGCSE Edexcel生物课程中，食物链是生态学的基础组成部分。它们描述了能量如何在生物之间以线性顺序传递。这份复习指南涵盖所有核心概念：生产者、消费者、营养级、能量损失、生物量金字塔、生物积累以及常见的考试误区。用它来巩固知识，提升考试成绩。
1. What is a Food Chain? | 什么是食物链？
A food chain is a diagram that shows a single pathway of energy flow through an ecosystem. It always begins with a producer and uses arrows (→) to indicate the direction of energy transfer, pointing from the food source to the eater.
食物链是显示生态系统中能量流动单一路径的图解。它总是以生产者开始，并使用箭头（→）表示能量传递的方向，从食物来源指向取食者。
For example, a simple grassland food chain: grass → grasshopper → frog → snake → hawk. The arrows do not mean ‘is eaten by’; they mean ‘is food for’ and show the movement of chemical energy stored in biomass.
例如，一条简单的草原食物链：草 → 蚱蜢 → 青蛙 → 蛇 → 鹰。箭头并非意为“被…吃”，而是“成为…的食物”，并显示储存在生物量中的化学能的流动。
It is essential to remember that a food chain represents only one possible feeding relationship; in reality, organisms often eat more than one type of food, which is where food webs come in.
必须记住，食物链只代表一种可能的摄食关系；实际上，生物通常不止吃一种食物，这就要用到食物网了。
2. Producers: The Start of the Chain | 生产者：食物链的起点
Producers are organisms that can make their own food using energy from sunlight. They are also called autotrophs. The most common producers are green plants and algae, which carry out photosynthesis.
生产者是能够利用太阳光能自己制造食物的生物，它们也称为自养生物。最常见的是绿色植物和藻类，它们进行光合作用。
The word equation for photosynthesis is: carbon dioxide + water → glucose + oxygen, with light energy absorbed by chlorophyll.
光合作用的文字方程式为：二氧化碳 + 水 → 葡萄糖 + 氧气，光能被叶绿素吸收。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Producers convert light energy into chemical energy stored in glucose and other organic molecules. This energy then forms the base of all food chains. Without producers, heterotrophs (consumers) would have no energy source.
生产者将光能转化为储存在葡萄糖和其他有机分子中的化学能。这些能量构成了所有食物链的基础。没有生产者，异养生物（消费者）就没有能量来源。
3. Consumers: Primary, Secondary and Tertiary | 消费者：初级、次级与三级消费者
Consumers are organisms that obtain energy by feeding on other organisms. They are heterotrophs. Based on their position in the food chain, they are classified as primary, secondary, or tertiary consumers.
消费者是通过取食其他生物来获取能量的生物，它们是异养生物。根据在食物链中的位置，它们被分为初级消费者、次级消费者和三级消费者。
- Primary consumers eat producers; they are herbivores (e.g. rabbit, grasshopper, zooplankton). 初级消费者取食生产者，它们是食草动物（如兔、蚱蜢、浮游动物）。
- Secondary consumers eat primary consumers; they may be carnivores or omnivores (e.g. frog, small fish). 次级消费者取食初级消费者，可能是食肉动物或杂食动物（如蛙、小鱼）。
- Tertiary consumers eat secondary consumers; they are typically top carnivores (e.g. hawk, shark). 三级消费者取食次级消费者，通常是顶级食肉动物（如鹰、鲨鱼）。
An organism can occupy different consumer levels in different food chains. For instance, a fox is a secondary consumer when it eats a rabbit, but becomes a tertiary consumer if it eats a snake that has eaten a mouse.
同一种生物在不同的食物链中可以处于不同的消费者级别。例如，狐狸吃兔子时是次级消费者，但如果它吃了一条吃过老鼠的蛇，就变成了三级消费者。
4. Trophic Levels Explained | 营养级解析
A trophic level is the position an organism occupies in a food chain. Each step in the chain represents a different trophic level, starting with level 1 for producers.
营养级是指生物在食物链中所处的位置。链中的每一步代表不同的营养级，从生产者的第1级开始。
The table below summarises the standard trophic levels with examples from a pond ecosystem:
下表总结了标准的营养级，并以池塘生态系统为例：
| Trophic Level | 营养级 | Organisms | 生物 |
|---|---|
| Level 1 (Producer) | 第1级（生产者） | Algae, pond weed | 藻类、水草 |
| Level 2 (Primary consumer) | 第2级（初级消费者） | Daphnia, tadpoles | 水蚤、蝌蚪 |
| Level 3 (Secondary consumer) | 第3级（次级消费者） | Small fish, newts | 小鱼、蝾螈 |
| Level 4 (Tertiary consumer) | 第4级（三级消费者） | Pike, heron | 狗鱼、苍鹭 |
Energy is lost at each transfer between trophic levels, which limits the length of food chains to usually four or five levels.
能量在营养级之间的每次传递中都会损失，这限制了食物链的长度通常为四到五级。
5. Food Webs: Interconnected Chains | 食物网：相互连接的链
A food web is a network of interconnected food chains. It shows multiple feeding relationships in an ecosystem, giving a more realistic picture of energy flow.
食物网是由相互连接的食物链组成的网络。它展示了一个生态系统中多种摄食关系，更真实地反映了能量流动。
In a food web, a single producer can be eaten by several primary consumers, and a top carnivore may feed on prey from different trophic levels. This complexity increases ecosystem stability; if one species declines, predators can switch to alternative prey.
在食物网中，一个生产者可以被多种初级消费者取食，而顶级食肉动物可能以不同营养级的猎物为食。这种复杂性增加了生态系统的稳定性；如果某一物种减少，捕食者可以转向其他猎物。
For example, in an oak woodland, leaves are eaten by caterpillars, aphids and beetles. Blue tits eat caterpillars and beetles, but also spiders, which themselves eat aphids. Removing blue tits would affect multiple populations.
例如，在橡树林中，叶子被毛虫、蚜虫和甲虫取食。蓝山雀吃毛虫和甲虫，也吃蜘蛛，而蜘蛛又吃蚜虫。移除蓝山雀将影响多个种群。
6. Energy Flow and the 10% Rule | 能量流动与10%法则
Energy enters most ecosystems as sunlight. Producers fix approximately 1-3% of that light energy into chemical energy. As energy passes from one trophic level to the next, roughly 90% is lost, and only about 10% is transferred to the next level.
能量以阳光的形式进入大多数生态系统。生产者将其中约1-3%的光能固定为化学能。当能量从一个营养级传递到下一个时，大约90%会损失，只有约10%被传递到下一级。
Efficiency = (Energy in new level / Energy in previous level) × 100%
效率 = （新营养级的能量 / 上一营养级的能量）× 100%
If a field of grass contains 10 000 kJ of energy, primary consumers (e.g. cows) might only gain 1 000 kJ; secondary consumers (e.g. humans eating beef) might receive 100 kJ. This is why food chains rarely exceed five trophic levels — there simply isn’t enough energy to support another level.
如果一片草地的能量为10 000千焦，初级消费者（如牛）可能只获得1 000千焦；次级消费者（如吃牛肉的人类）可能只得到100千焦。这就是食物链很少超过五个营养级的原因——根本没有足够的能量再支持一级。
| Trophic Level | 营养级 | Energy (kJ) | 能量（千焦） |
|---|---|
| Producer | 生产者 | 10 000 |
| Primary consumer | 初级消费者 | 1 000 |
| Secondary consumer | 次级消费者 | 100 |
| Tertiary consumer | 三级消费者 | 10 |
In exam calculations, always use the formula and give your answer to three significant figures unless stated otherwise.
在考试计算中，务必使用该公式，除非另有说明，否则答案给出三位有效数字。
7. Pyramids of Biomass | 生物量金字塔
A pyramid of biomass represents the dry mass of living material at each trophic level at a particular time, usually per unit area (e.g. g/m²). It is almost always pyramid-shaped because biomass decreases at higher levels due to energy losses.
生物量金字塔表示在某一特定时间，各营养级单位面积（如克/平方米）内生物的干重。它几乎总是呈金字塔形，因为能量损失导致较高营养级的生物量减少。
Using dry mass avoids the problem of varying water content in different organisms. To obtain dry mass, you place the sample in an oven at a low temperature until all water has evaporated, then weigh the remaining solid.
使用干重可以避免不同生物因含水量不同而导致的问题。获得干重的方法是将样本放入低温烘箱中，直到所有水分蒸发，然后称量剩余固体。
A typical pyramid of biomass for a grassland might show grass forming the wide base, herbivores a narrower bar, and carnivores an even narrower top bar, all drawn to scale.
草原的典型生物量金字塔可能显示草构成宽大的底部，食草动物构成较窄的条形，食肉动物构成更窄的顶部条形，并且均按比例绘制。
Pyramids of biomass can occasionally be inverted for some aquatic ecosystems over short periods, but for Edexcel IGCSE, you are expected to interpret and draw straight-sided pyramids that are normally upright.
在某些水生生态系统的短期内，生物量金字塔偶尔可能倒置，但在Edexcel IGCSE中，你需要能够解释和绘制通常为正立的直边金字塔。
8. Why Energy is Lost at Each Level | 为何每个营养级都有能量损失
Energy is lost at every trophic level due to several processes. These losses explain why only about 10% of energy is passed on and why food chains are short.
由于多种过程，能量在每一个营养级都会损失。这些损失解释了为什么只有约10%的能量被传递，以及为什么食物链很短。
Key reasons for energy loss:
能量损失的主要原因：
- Respiration releases energy as heat during metabolism. 呼吸作用在代谢过程中以热的形式释放能量。
- Movement and other life processes use up energy. 运动及其他生命活动消耗能量。
- Not all of the organism is eaten (e.g. bones, roots, fur). 生物的部分组织没被吃掉（如骨头、根、毛皮）。
- Energy is lost in urine and faeces; indigestible materials are egested. 能量随尿液和粪便损失；无法消化的物质被排出体外。
- Some energy is used to maintain body temperature in endotherms. 恒温动物需消耗部分能量来维持体温。
Only the energy stored in the new biomass (growth) is available to the next trophic level. Because so much energy is lost as heat to the surroundings, energy transfer is inefficient, and energy cannot be recycled — it must be continually supplied by the sun.
只有储存在新生生物量（生长）中的能量才能供下一营养级使用。由于大量能量以热的形式散失到环境中，能量传递效率很低，并且能量无法循环——必须有太阳不断供给。
9. Bioaccumulation and Biomagnification | 生物积累与生物放大
Bioaccumulation is the build-up of toxic substances, such as pesticides or heavy metals, in the tissues of an organism. If the substance cannot be broken down or excreted, it accumulates over the organism’s lifetime.
生物积累是指有毒物质（如杀虫剂或重金属）在生物组织内的积聚。如果该物质无法被分解或排泄，它就会在生物一生中不断累积。
Biomagnification describes how the concentration of these toxins increases at successive trophic levels. Top carnivores end up with the highest concentrations because they eat many contaminated prey items, each containing the toxin.
生物放大描述的是这些毒素在连续营养级中浓度增加的现象。顶级食肉动物体内最终浓度最高，因为它们吃了许多含有毒素的受污染猎物。
A classic example is DDT, a pesticide used to control insects. DDT leached into water bodies: plankton → small fish → large fish → osprey. While plankton absorbed tiny amounts, ospreys accumulated DDT concentrations high enough to thin eggshells, causing population decline.
经典例子是DDT，一种用于控制昆虫的杀虫剂。DDT渗入水体：浮游生物 → 小鱼 → 大鱼 → 鱼鹰。虽然浮游生物吸收了微量DDT，但鱼鹰体内积累的DDT浓度高到足以使蛋壳变薄，导致种群数量下降。
When answering exam questions, always use the term biomagnification correctly and explicitly link the increasing concentration to the position in the food chain.
在回答考试题目时，务必正确使用术语“生物放大”，并明确将浓度增加与食物链中的位置联系起来。
10. Decomposers and Nutrient Recycling | 分解者与营养素循环
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