📚 Energy Flow in Ecosystems | 生态系统中的能量流动
Energy flow is a fundamental concept in ecology that describes how energy is transferred from one organism to another within an ecosystem. All life depends on the input of energy, and in most ecosystems this energy originates from sunlight. Understanding how energy moves through food chains, the efficiency of these transfers, and the inevitable losses along the way is essential for IB Biology students.
能量流动是生态学的一个基本概念，描述了能量如何在生态系统内从一个生物体传递到另一个生物体。所有生命都依赖能量的输入，在大多数生态系统中，这些能量来自阳光。理解能量如何在食物链中流动、能量传递的效率以及沿途不可避免的损失，对 IB 生物学学生至关重要。
1. What is Energy Flow? | 什么是能量流动？
Energy flow refers to the passage of energy through the trophic levels of an ecosystem. Unlike chemical nutrients, which can be recycled through biogeochemical cycles, energy flows in a one‑way direction. It enters the ecosystem as light and is ultimately lost as heat.
能量流动是指能量通过生态系统各营养级的传递。与可通过生物地球化学循环回收的化学养分不同，能量以单向方式流动。它以光能形式进入生态系统，并最终以热能的形式散失。
The ultimate source of energy for nearly all ecosystems is the Sun. Producers capture this light energy and convert it into chemical energy stored in organic compounds. This chemical energy then fuels all other organisms.
几乎所有生态系统的最终能量来源都是太阳。生产者捕获光能并将其转化为储存在有机化合物中的化学能。这些化学能随后为所有其他生物提供能量。
Because energy transformations are never 100% efficient, the total amount of available energy decreases at each successive trophic level. This principle shapes community structure and limits the length of food chains.
由于能量转化永远不会达到 100% 的效率，可利用的能量总量在每一个连续的营养级都会减少。这一原理塑造了群落结构，并限制了食物链的长度。
2. Trophic Levels | 营养级
A trophic level is the position an organism occupies in a food chain. The first trophic level is always made up of producers (autotrophs). Primary consumers (herbivores) form the second trophic level, followed by secondary consumers (carnivores that eat herbivores), tertiary consumers (carnivores that eat other carnivores), and so on.
营养级是生物在食物链中所处的位置。第一营养级总是由生产者（自养生物）构成。初级消费者（食草动物）构成第二营养级，其次是次级消费者（以食草动物为食的食肉动物）、三级消费者（以其他食肉动物为食的食肉动物）等。
Decomposers, such as bacteria and fungi, are often not assigned a fixed trophic level because they break down dead organic matter from all levels. They play a vital role in releasing nutrients, but the chemical energy they harvest through respiration is ultimately lost as heat.
分解者（如细菌和真菌）通常不被赋予固定的营养级，因为它们分解来自所有营养级的死亡有机物质。它们在释放养分方面起着至关重要的作用，但它们通过呼吸作用获取的化学能最终仍以热能形式散失。
Only about 10% of the energy stored in one trophic level becomes incorporated into the biomass of the next level. The rest is used for respiration, lost as metabolic heat, or left as undigested waste.
一个营养级储存的能量中只有约 10% 会进入下一个营养级的生物量中。其余的能量用于呼吸作用、以代谢热的形式散失，或作为未消化的废物被排出。
3. Food Chains and Food Webs | 食物链与食物网
A food chain is a simple, linear sequence that shows the transfer of energy from one organism to the next. In reality, most organisms feed on more than one species, creating a complex network of feeding relationships known as a food web.
食物链是显示能量从一个生物传递到下一个生物的简单线性序列。实际上，大多数生物以不止一种物种为食，从而形成了一个复杂的取食关系网络，即食物网。
Food webs illustrate how energy can take multiple pathways through an ecosystem. A single species may occupy different trophic levels depending on what it eats. This interconnectedness adds stability to ecosystems because the removal of one species does not necessarily disrupt the entire energy flow.
食物网说明了能量在生态系统中可以通过多种途径流动。一个物种根据其食物可能占据不同的营养级。这种相互联系增加了生态系统的稳定性，因为一个物种的消失并不一定会破坏整个能量流动。
In IB Biology, you are expected to be able to construct and interpret food chains and food webs, identifying the trophic level of each organism and predicting the consequences if a particular species is removed.
在 IB 生物学中，你需要能够构建和解读食物链与食物网，确定每种生物的营养级，并预测如果某个特定物种被移除会带来怎样的后果。
4. Producers: The Energy Source | 生产者：能量之源
Producers, or autotrophs, are organisms that can manufacture their own organic molecules using an external energy source. Photoautotrophs—such as plants, algae, and cyanobacteria—use sunlight to drive photosynthesis. The overall equation can be summarised as:
生产者（自养生物）是指能够利用外部能源制造自身有机分子的生物。光合自养生物（如植物、藻类和蓝细菌）利用阳光驱动光合作用。总反应方程式可概括为：
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
The chemical energy stored in glucose (and subsequently in starch, cellulose, and other carbohydrates) becomes available to the rest of the ecosystem when consumers eat the producers.
储存在葡萄糖（以及随后的淀粉、纤维素和其他碳水化合物）中的化学能，当消费者取食生产者时，便可为生态系统的其余部分所用。
Some ecosystems, such as deep‑sea hydrothermal vents, rely on chemoautotrophs. These bacteria oxidise inorganic molecules (e.g. hydrogen sulfide) to obtain energy, but for the IB Biology syllabus the focus remains on sunlight‑driven systems.
某些生态系统（如深海热液喷口）依赖化能自养生物。这些细菌通过氧化无机分子（如硫化氢）获取能量，但在 IB 生物学课程中，重点仍然是阳光驱动的生态系统。
5. Consumers: Primary, Secondary, Tertiary | 消费者：初级、次级、三级
Consumers (heterotrophs) cannot produce their own food and must obtain energy by ingesting other organisms. Primary consumers are herbivores that feed directly on producers. Secondary consumers are carnivores that eat primary consumers, while tertiary consumers are carnivores that prey on secondary consumers.
消费者（异养生物）无法自行制造食物，必须通过摄入其他生物来获取能量。初级消费者是以生产者直接为食的食草动物。次级消费者是捕食初级消费者的食肉动物，而三级消费者则是捕食次级消费者的食肉动物。
Some organisms, called omnivores, feed at multiple trophic levels. For example, a human who eats both vegetables and beef functions simultaneously as a primary and a secondary consumer.
一些被称为杂食动物的生物在多个营养级上取食。例如，一个既吃蔬菜又吃牛肉的人同时扮演初级消费者和次级消费者的角色。
At each step, the energy passed on is not the total chemical energy contained in the food. A large proportion is lost through respiration, undigested material, and heat. This is why consumer biomass is always less than the biomass of the level below.
在每一步中，传递的能量并不是食物中所含的全部化学能。很大一部分通过呼吸作用、未消化物质和热量而散失。这就是为什么消费者的生物量总是低于下一营养级的生物量。
6. Energy Transfer Efficiency: The 10% Rule | 能量传递效率：百分之十定律
Ecological efficiency, often referred to as the 10% rule, states that on average only about 10% of the energy at one trophic level is transferred to the next level. The remaining 90% is used for metabolic processes or lost as waste and heat.
生态效率（通常称为百分之十定律）指出，平均而言，一个营养级中只有约 10% 的能量被传递到下一个营养级。剩下的 90% 被用于代谢过程或作为废物和热量散失。
The actual percentage can vary between 5% and 20% depending on the ecosystem and the types of organisms involved. IB examination questions frequently ask students to calculate energy transfer efficiency using the formula:
实际的百分比可能随生态系统和所涉及生物的类型在 5% 到 20% 之间变化。IB 考试经常要求学生使用以下公式计算能量传递效率：
Energy transfer efficiency (%) = (Energy at higher trophic level / Energy at lower trophic level) × 100%
For example, if producers fix 50 000 kJ m⁻² yr⁻¹ of solar energy and primary consumers assimilate 5 000 kJ m⁻² yr⁻¹, the efficiency is (5 000 / 50 000) × 100% = 10%.
例如，如果生产者每年每平方米固定了 50 000 kJ 的太阳能，而初级消费者同化了 5 000 kJ m⁻² yr⁻¹，则效率为 (5 000 / 50 000) × 100% = 10%。
This low efficiency is the fundamental reason why food chains rarely exceed four or five trophic levels. There simply is not enough energy left to support an additional level.
这种低效率是食物链很少超过四到五个营养级的根本原因。根本没有足够的能量剩余来支撑一个额外的营养级。
7. Pyramids of Energy | 能量金字塔
A pyramid of energy shows the total energy content at each trophic level over a given period, usually expressed in units such as kJ m⁻² yr⁻¹. Because energy is lost at every transfer, the pyramid of energy is always upright—it can never be inverted.
能量金字塔显示了在一定时期内每个营养级的总能量含量，通常以 kJ m⁻² yr⁻¹ 等为单位表示。由于每次传递都会损失能量，能量金字塔总是正立的——它永远不可能倒置。
When drawing or interpreting a pyramid of energy, each bar should be drawn to scale, and the width of each bar represents the relative amount of energy. The base (producers) is the widest, and the top consumer is the narrowest.
在绘制或解读能量金字塔时，每个条形应按比例绘制，条形的宽度代表能量含量的相对大小。底部（生产者）最宽，顶级消费者最窄。
The pyramid of energy provides the most accurate picture of community structure because it accounts for the rate of energy production and avoids distortions caused by differences in organism size or longevity.
能量金字塔提供了最准确的群落结构图景，因为它考虑了能量生产的速率，避免了由于生物体大小或寿命差异造成的失真。
8. Pyramids of Biomass and Numbers | 生物量金字塔与数量金字塔
A pyramid of biomass represents the total dry mass of living tissue at each trophic level, typically measured in g m⁻² or kg m⁻². In many terrestrial ecosystems, the pyramid of biomass is upright. However, in some aquatic ecosystems, the biomass of phytoplankton (producers) can be smaller than the biomass of zooplankton (consumers) at a given moment, leading to an inverted pyramid of biomass.
生物量金字塔表示每个营养级中活组织的总干重，通常以 g m⁻² 或 kg m⁻² 计量。在许多陆地生态系统中，生物量金字塔是正立的。然而在一些水生生态系统中，浮游植物（生产者）的生物量在某一时刻可能小于浮游动物（消费者）的生物量，从而形成倒置的生物量金字塔。
The pyramid of numbers simply counts the number of organisms at each level. It can be upright (grassland) or inverted (one large tree supporting numerous insects). Because it ignores size and metabolic rate, it is the least reliable representation of energy flow.
数量金字塔只统计每个营养级的生物个体数量。它可以是正立的（草原），也可以是倒置的（一棵大树支撑着大量昆虫）。由于它忽略了生物体的大小和代谢速率，它是反映能量流动最不可靠的表示方式。
For IB Biology, it is important to understand why only the pyramid of energy is guaranteed to be upright and to be able to interpret data from all three types of ecological pyramids.
对于 IB 生物学，理解为什么只有能量金字塔保证是正立的，并能够解读来自所有三种生态金字塔的数据，是非常重要的。
9. Productivity: GPP and NPP | 生产力：总初级生产力和净初级生产力
Gross primary productivity (GPP) is the total amount of chemical energy fixed by photosynthesis per unit area per unit time. Plants, however, use some of this energy for their own respiration (R). The energy that remains and is available to the next trophic level is the net primary productivity (NPP).
总初级生产力（GPP）是光合作用在单位面积、单位时间内固定的化学能总量。然而，植物会将其中一部分能量用于自身的呼吸作用（R）。剩余并可提供给下一营养级使用的能量即为净初级生产力（NPP）。
The relationship is expressed by the simple equation:
它们之间的关系可用简单的等式表示：
NPP = GPP − R
NPP represents the energy that is actually incorporated into plant biomass and is therefore the energy available to primary consumers. In IB exams, you may be required to calculate NPP, GPP, or the percentage efficiency of energy transfer between trophic levels.
NPP 代表了真正进入植物生物量的能量，因此也是可供初级消费者利用的能量。在 IB 考试中，你可能需要计算 NPP、GPP 或各营养级之间能量传递的效率百分比。
Secondary productivity refers to the rate at which consumers convert the energy they ingest into their own biomass. Much of the ingested energy is lost in faeces and respiration, so secondary productivity is always lower than the consumed primary productivity.
次级生产力指消费者将摄入的能量转化为自身生物量的速率。摄入的能量中有很大部分随粪便和呼吸作用散失，因此次级生产力总是低于所消耗的初级生产力。
10. Energy Losses in Ecosystems | 生态系统中的能量损耗
Why is so much energy lost between trophic levels? The main reasons include:
为什么营养级之间会损失如此多的能量？主要原因包括：
Not all parts of a food organism are eaten (e.g. bones, roots, shells).
不是食物生物的所有部分都被食用（例如骨骼、根部、外壳）。
Some ingested material cannot be digested and is lost as faeces. This energy becomes available to decomposers rather than the consumer.
一些摄入的物质无法被消化，随粪便排出。这些能量变成了分解者的可利用能量，而没有进入消费者体内。
A large fraction of assimilated energy is used for cellular respiration to power movement, growth, and maintenance, and is ultimately released as metabolic heat.
同化后的能量中有很大一部分用于细胞呼吸，以驱动运动、生长和维持生命，最终以代谢热的形式释放。
In endotherms (warm‑blooded animals), a particularly high proportion of energy is spent on maintaining a constant body temperature, making their net productivity relatively low compared with ectotherms.
在恒温动物中，维持恒定体温消耗的能量占比特别高，这使得它们的净生产力相对于变温动物较低。
These unavoidable losses explain why the amount of energy available decreases sharply at each successive trophic level, limiting the total number of levels in a food chain.
这些不可避免的能量损耗解释了为什么可利用的能量在每个连续的营养级急剧减少，从而限制了食物链的总级数。
11. Calculations and Data Interpretation | 计算与数据解读
IB Biology frequently presents data tables showing energy flow (in kJ m⁻² yr⁻¹) for different trophic levels. You may be asked to:
IB 生物学经常给出显示不同营养级能量流动（单位为 kJ m⁻² yr⁻¹）的数据表。你可能会被要求：
Calculate the percentage of energy transferred from one level to the next.
计算从一个营养级到下一个营养级的能量传递百分比。
Determine GPP, NPP, or R given the other variables.
在已知其他变量的情况下，确定 GPP、NPP 或 R。
Draw a pyramid of energy to scale.
按比例绘制能量金字塔。
Compare the efficiency of energy transfer in different food chains and suggest reasons for any differences.
比较不同食物链中的能量传递效率，并提出造成差异的原因。
Always show your working clearly and give your answer to an appropriate number of significant figures. Remember that efficiencies are often expressed as percentages to one decimal place.
始终清晰地展示计算过程，并给出适当有效数字的答案。记住效率通常以百分比表示，保留一位小数。
For example, if a field receives 4.0 × 10⁶ kJ of sunlight energy per square metre per year and the NPP is 4.0 × 10⁴ kJ m⁻² yr⁻¹, the photosynthetic efficiency is (4.0 × 10⁴ / 4.0 × 10⁶) × 100% = 1.0%. This low figure is typical for real‑world producers.
例如，如果某田地每年每平方米接收 4.0 × 10⁶ kJ 的太阳光能，而 NPP 为 4.0 × 10⁴ kJ m⁻² yr⁻¹，则光合作用效率为 (4.0 × 10⁴ / 4.0 × 10⁶) × 100% = 1.0%。这个低数值在现实生产者中是很典型的。
12. Human Impact on Energy Flow | 人类对能量流动的影响
Human activities can significantly alter the patterns of energy flow in ecosystems. Agriculture, for instance, aims to maximise the energy that reaches humans by shortening food chains. Eating plants directly transfers far more energy than eating animals that have consumed plants, because one trophic level is avoided.
人类活动能显著改变生态系统中的能量流动模式。例如，农业旨在通过缩短食物链，最大限度地将能量传递给人类。直接食用植物可比食用以植物为食的动物传递多得多的能量，因为省去了一个营养级。
The burning of fossil fuels releases stored chemical energy that originally came from ancient photosynthesis. This energy bypasses natural food chains and is used directly for human purposes, but it also contributes to climate change and disrupts ecosystem energy budgets.
化石燃料的燃烧释放了最初来自远古光合作用的储存化学能。这些能量绕过了自然食物链，直接为人类所用，但它也加剧了气候变化，扰乱了生态系统的能量收支。
Intensive livestock farming requires large inputs of grain and water, which themselves represent energy. The conversion efficiency is low, meaning that producing meat is far less energy‑efficient than growing crops for direct human consumption. Understanding energy flow helps inform discussions about food security and sustainability.
集约化畜牧业需要投入大量的谷物和水，这些本身代表着能量。转化效率很低，这意味着生产肉类的能量效率远低于种植直接供人类食用的农作物。理解能量流动有助于对粮食安全和可持续性展开讨论。
In IB Biology, you should be able to evaluate the efficiency of different food production systems using your knowledge of trophic levels and energy losses. This connects ecological principles to real‑world environmental issues.
在 IB 生物学中，你应该能够运用有关营养级和能量损耗的知识，评估不同食物生产系统的效率。这将生态学原理与现实世界的环境问题联系了起来。
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