📚 477: Energy Transfer in Ecosystems | 477：生态系统中的能量传递
The movement of energy through an ecosystem is a core concept in IGCSE Biology and Combined Science. This article breaks down the processes, the reasons for energy loss, and the key diagrams you need to master for your Edexcel exam.
能量在生态系统中的传递是IGCSE生物和综合科学的核心概念。本文详细讲解能量传递的过程、能量损失的原因以及Edexcel考试中需要掌握的关键图表。
1. The Sun as the Ultimate Energy Source | 太阳作为最终能量来源
Nearly all living organisms ultimately depend on sunlight for energy. Green plants and some bacteria capture light energy during photosynthesis to convert carbon dioxide and water into glucose and oxygen.
几乎所有生物最终都依赖阳光获取能量。绿色植物和一些细菌在光合作用中捕获光能，将二氧化碳和水转化为葡萄糖和氧气。
The chemical energy stored in glucose becomes the energy available to other organisms when the plant is eaten. This makes plants the producers of the ecosystem.
当植物被取食时，储存在葡萄糖中的化学能就可供其他生物利用。这使得植物成为生态系统的生产者。
2. Food Chains and Trophic Levels | 食物链与营养级
A food chain shows the feeding relationship between organisms and the transfer of energy from one organism to another. Each step in a food chain is called a trophic level.
食物链展示了生物之间的取食关系以及能量从一种生物向另一种生物的传递。食物链中的每一步称为一个营养级。
A typical example is: grass → grasshopper → frog → snake. Here, grass is the producer (first trophic level), grasshopper is the primary consumer (second trophic level), frog is the secondary consumer (third), and snake is the tertiary consumer (fourth).
一个典型的例子是：草 → 蝗虫 → 青蛙 → 蛇。其中，草是生产者（第一营养级），蝗虫是初级消费者（第二营养级），青蛙是次级消费者（第三营养级），蛇是三级消费者（第四营养级）。
- Producer: makes its own food, usually plants
- Primary consumer: eats plants (herbivore)
- Secondary consumer: eats primary consumers
- Tertiary consumer: eats secondary consumers
- 生产者：自己制造食物，通常是植物
- 初级消费者：吃植物（植食性动物）
- 次级消费者：吃初级消费者
- 三级消费者：吃次级消费者
3. Energy Transfer Between Trophic Levels | 营养级之间的能量传递
When an organism is eaten, not all of the energy stored in its biomass is passed on to the consumer. In fact, only about 10% of the energy at one trophic level is transferred to the next level.
当一个生物被吃掉时，其生物量中储存的能量并不会全部传递给捕食者。事实上，一个营养级中大约只有10%的能量能传递到下一营养级。
This is why food chains rarely have more than four or five trophic levels – too little energy remains to support additional consumers.
这就是为什么食物链通常只有四到五个营养级——剩余能量太少，不足以支持更多的消费者。
Energy available to next level ≈ 10% of previous level
可传递到下一营养级的能量 ≈ 前一营养级能量的10%
4. Why Is Energy Lost? | 能量为什么会损失？
Energy is lost between trophic levels for several reasons. Understanding these reasons helps explain the shape of ecological pyramids.
能量在营养级之间损失的原因有多种。理解这些原因有助于解释生态金字塔的形状。
| Reason | Explanation |
| Respiration (呼吸作用) | Used to release energy from glucose; much is lost as heat. |
| Movement (运动) | Energy is used for muscle contraction and other activities. |
| Not eaten (未被食用) | Parts like bones, roots and fur are not consumed by the next trophic level. |
| Excretion and egestion (排泄与排遗) | Undigested food leaves as faeces; waste urea and excess water are removed. |
All these losses mean that only a small fraction of the energy consumed becomes new biomass in the consumer.
这些损失意味着，只有一小部分被摄入的能量能转化为消费者体内的新生物量。
5. Pyramids of Energy and Biomass | 能量金字塔与生物量金字塔
A pyramid of biomass shows the amount of biomass (living material) present at each trophic level at a given time. It is usually upright because producers have the greatest biomass.
生物量金字塔表示某一时刻各营养级中生物量（活体物质）的总量。它通常呈下宽上窄的正金字塔形，因为生产者的生物量最大。
A pyramid of energy shows the total energy flowing through each trophic level per unit time. It is always upright because energy is always lost between levels.
能量金字塔表示单位时间内流经各营养级的总能量。它总是呈正金字塔形，因为能量在营养级之间总是有损失的。
Producer biomass ≈ 10 × primary consumer biomass ≈ 100 × secondary consumer biomass
生产者生物量 ≈ 10 × 初级消费者生物量 ≈ 100 × 次级消费者生物量
6. The Role of Decomposers and Detritivores | 分解者与食碎屑者的作用
Decomposers, such as bacteria and fungi, break down dead organisms and waste matter. They use the remaining energy for their own respiration and release nutrients back into the soil.
分解者，如细菌和真菌，将死亡的生物体和废物分解。它们利用其中剩余的能量供自身呼吸，并将营养物质释放回土壤。
Detritivores such as earthworms and woodlice feed on dead material, increasing its surface area so decomposers can act more efficiently.
蚯蚓和木虱等食碎屑者取食死亡物质，增加其表面积，使分解者能更高效地作用。
Decomposition is essential for recycling carbon and nitrogen in ecosystems, which you will need to explain in the carbon cycle and nitrogen cycle questions.
分解对生态系统中碳和氮的循环至关重要，这是回答碳循环和氮循环问题时需要解释的要点。
7. Human Impact: Shorter Food Chains Are More Efficient | 人类影响：较短的食物链更高效
Because energy is lost at each trophic level, the total energy available for human consumption is greater if we eat plants directly rather than animals that have eaten plants.
由于每个营养级都会损失能量，如果人类直接吃植物，而不是吃吃了植物的动物，那么可供人类利用的总能量会更多。
For example, 1000 kJ of sunlight intercepted by wheat may produce about 100 kJ of wheat biomass. If a cow eats the wheat, only about 10 kJ enters the cow’s biomass. If a human eats the cow, only about 1 kJ remains.
例如，小麦拦截的1000千焦日光能可能产生约100千焦的小麦生物量。如果牛吃小麦，只有约10千焦进入牛体内。如果人吃牛，只剩下约1千焦。
This is why intensive agriculture often uses shorter food chains to maximise energy yield for humans.
这就是为什么集约化农业常采用较短的食物链，以最大化人类可获得的能量产出。
8. Exam Tips for Edexcel IGCSE | Edexcel IGCSE 考试提示
When answering questions on energy transfer, always mention the specific reasons for loss: respiration, movement, heat loss, undigested material, and excretion.
回答能量传递问题时，一定要具体说出损失原因：呼吸作用、运动、散热、未消化的物质和排泄。
- Draw arrows in the direction of energy flow, not from predator to prey.
- Label trophic levels clearly.
- Use correct units (kJ m⁻² year⁻¹) for energy values.
- Remember biomass units are grams or kg per square metre.
- 箭头方向表示能量流动方向，不要从捕食者指向猎物。
- 清晰标注各营养级。
- 能量数值要使用正确的单位（kJ m⁻² year⁻¹）。
- 生物量单位是克或千克每平方米。
Also, be prepared to explain why a pyramid of biomass may appear inverted in some aquatic ecosystems (because producers reproduce very quickly).
同时，要准备好解释为什么在某些水生生态系统中生物量金字塔可能呈现倒置（因为生产者繁殖非常快）。
9. Common Misconceptions | 常见误区
Many students confuse biomass with energy. Biomass is the mass of living material, while energy is the capacity to do work. The diagrams are linked but measure different quantities.
许多学生混淆生物量和能量。生物量是活体物质的质量，而能量是做功的能力。两者相关但衡量的量不同。
Another misconception is that decomposers only appear at the end of food chains. In reality, they break down material at all trophic levels.
另一个误区是认为分解者只出现在食物链末端。实际上，分解者在所有营养级都在分解物质。
10. Quick Revision Summary | 快速复习总结
Energy enters ecosystems through photosynthesis, flows through trophic levels, and decreases by about 90% at each step because of respiration and other losses.
能量通过光合作用进入生态系统，流经各营养级，并因呼吸作用和其他损失在每个营养级减少约90%。
Pyramids of energy are always upright, while biomass pyramids may occasionally be inverted. Decomposers recycle nutrients, making them essential for ecosystem stability.
能量金字塔总是正立，而生物量金字塔偶尔可能倒置。分解者循环利用养分，对生态系统稳定性至关重要。
Key equation: Energy transfer efficiency = (energy available after transfer ÷ energy available before transfer) × 100%
核心公式：能量传递效率 =（传递后的能量 ÷ 传递前的能量）× 100%
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