📚 IB Biology: Food Chains Key Points | IB 生物：食物链 考点精讲
In IB Biology, understanding food chains is fundamental to ecology. A food chain shows the linear sequence of who eats whom in an ecosystem, illustrating the flow of energy and nutrients from one organism to another. Grasping the concepts of trophic levels, energy transfer efficiency, and ecological pyramids is essential for exam success and for appreciating the delicate balance of natural systems.
在 IB 生物中，理解食物链是生态学的基础。食物链显示了生态系统中谁吃谁的线性顺序，描述了能量和营养物质从一个生物体流向另一个的过程。掌握营养级、能量传递效率以及生态金字塔等概念，是考试成功的关键，也有助于理解自然系统的微妙平衡。
1. What is a Food Chain? | 什么是食物链？
rom the organism being eaten to the organism that eats it, indicating the direction of energy flow.
食物链是一种线性的生物序列，能量和营养物质通过一种生物被另一种生物吃掉而转移。它从生产者（将光能或化学能转化为有机物质的自养生物）开始，经过一系列消费者。箭头（→）从被吃的生物指向吃它的生物，表示能量流动的方向。
ixes solar energy, and that energy is passed along each level. Such chains rarely exist in isolation; they interconnect to form complex food webs.
一个典型的陆地食物链例子是：草 → 蚱蜢 → 青蛙 → 蛇 → 鹰。在这条链中，草固定了太阳能，能量沿着各个层次传递。这样的链很少孤立存在，它们相互连接形成复杂的食物网。
2. Trophic Levels | 营养级
Each step in a food chain is called a trophic level. The first trophic level is always occupied by producers (autotrophs). The second trophic level consists of primary consumers (herbivores) that feed directly on producers. Subsequent levels include secondary consumers (carnivores that eat herbivores) and tertiary consumers (carnivores that eat other carnivores). Some chains may have a quaternary consumer at the top.
食物链中的每一步称为一个营养级。第一营养级始终由生产者（自养生物）占据。第二营养级由直接取食生产者的初级消费者（草食动物）组成。后续级别包括次级消费者（吃草食动物的肉食动物）和三级消费者（吃其他肉食动物的肉食动物）。有些链的顶端可能有四级消费者。
Organisms can occupy different trophic levels in different food chains depending on what they are eating. For instance, an omnivorous bear may act as a primary consumer when eating berries and as a secondary consumer when eating fish. In IB Biology, you are expected to identify the trophic level of a given organism within a described feeding relationship.
生物在不同的食物链中可占据不同的营养级，这取决于它们吃什么。例如，杂食性的熊在吃浆果时可能是初级消费者，在吃鱼时可能是次级消费者。在 IB 生物中，你应能根据给定的摄食关系确定某种生物的营养级。
3. Producers and Consumers | 生产者与消费者
Producers, also known as autotrophs, include plants, algae, and some bacteria. They synthesise organic compounds from simple inorganic substances using energy from sunlight (photosynthesis) or from chemical reactions (chemosynthesis). They form the base of every food chain.
生产者，也称自养生物，包括植物、藻类和某些细菌。它们利用太阳能（光合作用）或化学反应的能量（化能合成作用）将简单的无机物合成有机化合物。它们构成了每条食物链的基础。
Consumers are heterotrophs that obtain energy by feeding on other organisms. Primary consumers are herbivores that eat producers. Secondary consumers are carnivores that eat primary consumers, and tertiary consumers eat secondary consumers. Some organisms, like vultures, feed on dead remains and are considered scavengers, but they are not a distinct trophic level—they fit into the consumer levels depending on what dead organism they consume.
消费者是通过取食其他生物获取能量的异养生物。初级消费者是吃生产者的草食动物。次级消费者是吃初级消费者的肉食动物，三级消费者吃次级消费者。有些生物，如秃鹫，以尸体为食，被视为食腐动物；但它们并不是一个独立的营养级——它们根据所吃的尸体属于哪个级别而归入相应的消费者层次。
4. Decomposers and Detritivores | 分解者与食碎屑者
Decomposers (mostly bacteria and fungi) and detritivores (such as earthworms and woodlice) break down dead organic matter and waste products. They are rarely shown in food chains but play a vital role in recycling nutrients back into the ecosystem, making them available for producers.
分解者（主要是细菌和真菌）和食碎屑者（如蚯蚓和潮虫）分解死亡的有机物和排泄物。它们很少在食物链中被标示出来，但在将养分循环回生态系统、使其能被生产者重新利用方面起到至关重要的作用。
IB exam questions sometimes ask why decomposers are not usually included in a drawn food chain. The main reason is that they feed on detritus from all trophic levels, so placing them at a single level would misrepresent the constant recycling of materials. Nevertheless, they can be represented in more detailed diagrams of nutrient cycles.
IB 考题有时会问为什么分解者通常不被画在食物链中。主要原因是它们以各营养级的碎屑为食，因此把它们放在单一层次会错误地反映物质的持续循环。不过，它们可以在更详细的物质循环图中表示出来。
5. Food Webs | 食物网
A food web is a network of interconnected food chains that shows the more realistic feeding relationships in an ecosystem. Most organisms eat more than one type of food and can be eaten by several different predators. A food web thus provides a more complete picture of energy flow.
食物网是由相互连接的食物链组成的网络，显示了生态系统中更真实的摄食关系。大多数生物不止吃一种食物，也可能被多种不同的捕食者吃掉。因此，食物网提供了能量流动的更完整图景。
Food webs increase ecosystem stability. If one species declines, consumers that rely on it can switch to alternative prey, reducing the risk of collapse. In IB Biology, you may be asked to construct a food web from given data and identify how many trophic levels or feeding links are present.
食物网增强了生态系统的稳定性。如果某物种数量下降，依赖它的消费者可以转而捕食其他猎物，从而降低崩溃的风险。在 IB 生物中，你可能会被要求根据给定数据构建食物网，并指出存在多少个营养级或摄食联系。
6. Energy Flow and the 10% Rule | 能量流动与 10% 规则
As energy passes along a food chain, much of it is lost at each trophic level. Only about 10% of the energy stored in one trophic level is converted into biomass at the next level. The remaining 90% is used for respiration, movement, growth, and is lost as heat or in undigested wastes.
能量沿着食物链传递时，每个营养级都会损失大量能量。只有大约 10% 的能量从某一营养级转化为下一营养级的生物量。其余 90% 用于呼吸、运动、生长，并以热或未消化的废物形式损失掉。
Eₙ₊₁ ≈ 0.1 × Eₙ
This 10% rule explains why food chains rarely have more than four or five trophic levels: insufficient energy remains to support another level. The efficiency of energy transfer can vary between 5% and 20% depending on the ecosystem and the organisms involved.
这条 10% 规则解释了为什么食物链很少超过四到五个营养级：剩下的能量不足以支持更高级别。能量传递效率依生态系统和所涉及生物的不同，可在 5% 到 20% 之间变化。
In IB exam calculations, you might be asked to determine the energy available at a given trophic level if you know the energy input at the producer level. For example, if producers capture 20,000 kJ m-2 yr-1, the primary consumers would only have about 2,000 kJ m-2 yr-1 available.
在 IB 考试的计算中，你可能会被要求根据生产者的能量输入，求特定营养级的可用能量。例如，若生产者每年每平方米固定 20 000 kJ 能量，那么初级消费者只能获得约 2 000 kJ m-2 yr-1。
7. Ecological Pyramids | 生态金字塔
Ecological pyramids are graphical representations of the trophic structure of an ecosystem. The three main types are the pyramid of numbers, the pyramid of biomass, and the pyramid of energy. Each type displays different information and can be used to analyse feeding relationships.
生态金字塔是对生态系统营养结构的图形化表示。三种主要类型是数量金字塔、生物量金字塔和能量金字塔。每种类型展示不同的信息，可用于分析摄食关系。
| Pyramid type / 金字塔类型 | What it represents / 表示内容 | Units / 单位 | Can be inverted? / 可否倒置？ |
|---|---|---|---|
| Pyramid of numbers / 数量金字塔 | Number of organisms at each trophic level / 各营养级的生物数量 | Individuals per unit area / 单位面积个体数 | Yes / 可以 (e.g. a single tree supports many insects / 如一棵树支持许多昆虫) |
| Pyramid of biomass / 生物量金字塔 | Total dry mass of living material at each trophic level / 各营养级活组织的总干重 | g m-2 or kg ha-1 / 克每平方米或千克每公顷 | Sometimes / 有时 (e.g. in aquatic ecosystems during algal blooms / 如藻华期的水生生态系统) |
| Pyramid of energy / 能量金字塔 | Flow of energy through each trophic level over time / 各营养级在一段时间内的能量流 | kJ m-2 yr-1 / 千焦每平方米每年 | Never / 永不 (always upright because of energy loss / 因能量损失总是正立) |
For IB Biology, it is important that you can explain the advantages of an energy pyramid over the other two types. Because it accounts for time and productivity, it never appears inverted and gives a truer picture of ecosystem function.
对于 IB 生物，重要的是你能解释能量金字塔相较于其他两种类型的优点。由于它考虑了时间和生产力因素，它永远不会倒置，从而能更真实地反映生态系统的功能。
8. Why Only the Energy Pyramid is Always Upright | 为何只有能量金字塔始终直立
The pyramid of energy is always upright because of the laws of thermodynamics. Energy is lost as heat at every trophic level, so the energy available always decreases from producers to top consumers. This fundamental loss means the upper level can never contain more energy than the level below it.
能量金字塔始终是直立的正形，这是由热力学定律决定的。能量在每个营养级都以热的形式损失，所以从生产者到顶级消费者，可用能量总是递减的。这种根本性的损失意味着上层绝不可能比下层含有更多能量。
In contrast, a pyramid of numbers can be inverted when numerous small organisms feed on one large producer, such as hundreds of aphids on a single oak tree. A pyramid of biomass may also invert in aquatic environments where phytoplankton have a low standing biomass but reproduce so quickly that they sustain a higher biomass of zooplankton over the year.
相比之下，数量金字塔可能在无数小生物取食一个大个体生产者时倒置，比如一棵橡树上的数百只蚜虫。生物量金字塔也可能在水生环境中倒置，因为浮游植物的现存量很低，但它们繁殖极快，足以在全年维持更高生物量的浮游动物。
IB often asks students to analyse given pyramid data and explain why some pyramids appear inverted. The key is to note that standing biomass at one moment differs from productivity over time.
IB 经常要求学生分析给定的金字塔数据，并解释为什么有些金字塔看起来是倒置的。关键在于，某一时刻的现存生物量与一段时间内的生产力是不同的。
9. Biomagnification | 生物放大
Biomagnification is the increasing concentration of a persistent, non-biodegradable substance in the tissues of organisms at successively higher trophic levels. A classic example is the pesticide DDT, which was used widely to control insects. Although applied at low concentrations, DDT accumulated in the fatty tissues of consumers and became more concentrated up the food chain.
生物放大是指一种持久性、不可生物降解的物质在更高营养级生物体组织中的浓度不断增加。一个经典例子是曾广泛用于防治害虫的杀虫剂 DDT。尽管施用浓度很低，DDT 会在消费者的脂肪组织中积累，并沿食物链逐级浓缩。
birds (25 ppm). The high concentration in top predators caused eggshell thinning and population declines in birds like ospreys and eagles. This illustrates the far-reaching impact of toxic substances in food chains.
在典型的水生食物链中：浮游生物（0.04 ppm DDT）→ 小型鱼（0.5 ppm）→ 大型鱼（2 ppm）→ 食鱼鸟类（25 ppm）。顶级捕食者体内的高浓度导致鸟蛋壳变薄，使鱼鹰和鹰等鸟类数量下降。这说明了食物链中毒性物质的深远影响。
IB students should be able to explain how biomagnification affects apex predators and why these substances pose a risk even when released in low quantities. It is also a strong argument for monitoring pollutants and preserving top predator populations as indicators of ecosystem health.
IB 学生应能解释生物放大如何影响顶级捕食者，以及为什么即使低量释放这些物质也会构成风险。这也是监测污染物并保护顶级捕食者种群作为生态系统健康指示物种的有力论据。
10. Limitations of Food Chains and Food Webs | 食物链与食物网的局限性
Food chains are oversimplified models. They represent only a single feeding pathway and ignore omnivory, detritivory, and seasonal diet changes. They do not show the quantitative flow of energy or biomass unless additional data are attached.
食物链是过度简化的模型。它们只表示单一的摄食路径，忽略了杂食性、食碎屑性和季节性食性变化。除非附加额外数据，否则它们无法显示能量或生物量的定量流动。
Food webs, while more realistic, can also become extremely complex and difficult to analyse. Moreover, both models assume static feeding relationships and do not easily incorporate environmental changes, such as the removal of a keystone species or the arrival of an invasive species. In IB Biology, you should discuss these limitations when evaluating ecological studies.
食物网虽然更真实，但也可能变得极其复杂而难以分析。此外，两种模型都假设摄食关系是静态的，不容易纳入环境变化的影响，如关键种的消失或入侵种的到来。在 IB 生物中，评估生态学研究时你应讨论这些局限性。
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