📚 IB Science: Ecosystem Key Concepts | IB 科学：生态系统考点精讲
Ecology forms a core part of the IB Science curriculum, especially in Biology Topic 4 and in Environmental Systems and Societies (ESS). An ecosystem is a dynamic, interacting system comprising a community and its abiotic environment — and mastering it requires a clear understanding of energy flows, nutrient cycles, population dynamics, and community relationships. This guide distills the most exam-relevant concepts, complete with bilingual explanations to strengthen both your subject knowledge and scientific literacy.
生态学是 IB 科学课程的核心组成部分，尤其在生物学 Topic 4 和 环境系统与社会 (ESS) 中占有重要地位。生态系统是由群落及其非生物环境组成的动态交互系统——掌握它需要透彻理解能量流动、物质循环、种群动态和群落关系。本指南提炼了与考试最相关的概念，并附有双语讲解，以巩固您的学科知识与科学素养。
1. Ecosystem Structure and Components | 生态系统的结构与组成
An ecosystem consists of all the living organisms (biotic factors) in a given area, interacting with each other and with their non-living (abiotic) surroundings such as sunlight, temperature, water, and soil minerals.
生态系统由特定区域内的所有生物（生物因素）组成，它们彼此相互作用，并与阳光、温度、水和土壤矿物质等非生物（非生物因素）环境互动。
Biotic components are classified into producers (autotrophs), consumers (heterotrophs), and decomposers (detritivores and saprotrophs). Producers capture energy through photosynthesis or chemosynthesis; consumers obtain energy by feeding on other organisms; decomposers break down dead organic matter, recycling nutrients back into the ecosystem.
生物组分分为生产者（自养生物）、消费者（异养生物）和分解者（食碎屑生物和腐生生物）。生产者通过光合作用或化能合成作用捕获能量；消费者通过摄食其他生物获取能量；分解者分解死亡的有机物质，将养分循环回生态系统中。
Abiotic factors—such as pH, salinity, light intensity, and oxygen concentration—determine the types of organisms that can survive in an ecosystem. The niche of a species includes both its habitat and its functional role within that system.
非生物因素——如 pH、盐度、光照强度和氧气浓度——决定了能够在生态系统中生存的生物类型。一个物种的生态位包括其栖息地以及它在该系统中的功能角色。
2. Trophic Levels and Food Webs | 营养级与食物网
Trophic levels describe the feeding position of organisms in a food chain: level 1 – producers, level 2 – primary consumers (herbivores), level 3 – secondary consumers (carnivores that eat herbivores), and so on. Decomposers operate at all levels and are often omitted from simple food chains.
营养级描述的是生物在食物链中的摄食位置：第一营养级 – 生产者，第二营养级 – 初级消费者（食草动物），第三营养级 – 次级消费者（以食草动物为食的肉食动物），依此类推。分解者作用于所有营养级，在简单的食物链中常被省略。
A food web is a more realistic representation of feeding relationships, showing interconnected food chains within a community. It illustrates that most organisms consume or are consumed by multiple species, which increases ecosystem stability.
食物网是对摄食关系更为真实的呈现，展示了群落中相互连接的食物链。它表明大多数生物会摄食或被多种生物摄食，从而增加了生态系统的稳定性。
In IB exams, be able to construct a food web from given organisms and to predict the impacts of removing a keystone species. Also, remember that arrows in a food chain point in the direction of energy flow (towards the consumer).
在 IB 考试中，要能够根据给定的生物构建食物网，并预测移除关键种所产生的影响。同时记住，食物链中箭头的方向表示能量流动的方向（指向消费者）。
3. Energy Flow in Ecosystems | 生态系统中的能量流动
Energy enters most ecosystems as sunlight, which is converted by producers into chemical energy via photosynthesis. This energy then passes along the food chain through consumption, but with significant losses at each transfer.
能量多以阳光的形式进入生态系统，被生产者通过光合作用转化为化学能。随后，这种能量通过捕食沿着食物链传递，但每一次传递都伴随着巨大的损失。
On average, only about 10% of the energy at one trophic level is converted into biomass at the next level. The remaining ~90% is lost mainly as heat from respiration, through undigested waste, and via uneaten remains.
平均而言，一个营养级的能量只有大约 10% 转化为下一个营养级的生物量。其余约 90% 的能量主要以呼吸作用产热、未被消化的废物和未被食用的残体等形式损耗。
This low transfer efficiency explains why food chains rarely exceed four or five trophic levels and why there is a limit to the biomass that higher trophic levels can support. Energy flow is strictly one-way; it cannot be recycled.
这种低传递效率解释了为什么食物链很少超过四或五个营养级，也解释了为什么高营养级能够支持的生物量存在上限。能量流动是严格单向的，无法循环利用。
4. Productivity: GPP and NPP | 生产力：总初级生产力与净初级生产力
Gross primary productivity (GPP) is the total amount of chemical energy fixed by photosynthesis per unit area per unit time. A portion of this energy is used by plants for their own respiration (R). The energy remaining—available for growth, reproduction, and for consumers—is called net primary productivity (NPP).
总初级生产力 (GPP) 是单位面积单位时间内通过光合作用固定的总化学能量。其中一部分能量被植物用于自身呼吸作用 (R)。剩余的能量——可用于生长、繁殖和供给消费者——称为净初级生产力 (NPP)。
NPP = GPP − R
NPP represents the rate at which biomass accumulates in an ecosystem and is a crucial measure for comparing the productivity of different biomes, such as tropical rainforests (high NPP) and deserts (low NPP).
NPP 表示生态系统中生物量积累的速率，是比较不同生物群落生产力的关键指标，例如热带雨林（高 NPP）和沙漠（低 NPP）。
Be prepared to interpret graphs of GPP, NPP, and respiration, and to explain how environmental factors like light, water, and nutrient availability affect primary productivity.
准备好解读有关 GPP、NPP 和呼吸作用的图表，并能够解释光照、水分和养分可利用性等环境因素如何影响初级生产力。
5. Nutrient Cycling: Carbon Cycle | 物质循环：碳循环
The carbon cycle is a global biogeochemical cycle in which carbon moves between major reservoirs: the atmosphere (as CO₂), oceans, fossil fuels, sediments, and living biomass. Key fluxes include photosynthesis, respiration, combustion, and diffusion between the atmosphere and oceans.
碳循环是一个全球性的生物地球化学循环，碳在主要储库之间移动：大气（以 CO₂ 形式）、海洋、化石燃料、沉积物和生物量。关键的流通过程包括光合作用、呼吸作用、燃烧以及大气与海洋之间的扩散。
Photosynthesis removes CO₂ from the atmosphere, converting it into organic compounds; respiration returns CO₂. Decomposition by microorganisms releases carbon back into the soil and atmosphere. Over geological time, partially decomposed organic matter can form fossil fuels.
光合作用从大气中吸收 CO₂，将其转化为有机化合物；呼吸作用则将 CO₂ 释放回大气。微生物的分解作用将碳释放回土壤和大气中。在漫长的地质时间里，部分分解的有机质可形成化石燃料。
Human activities, particularly the burning of fossil fuels and deforestation, have significantly increased atmospheric CO₂ concentrations, intensifying the greenhouse effect and driving climate change. IB questions often ask you to outline carbon fluxes and evaluate their impacts.
人类活动，尤其是化石燃料燃烧和森林砍伐，显著增加了大气中的 CO₂ 浓度，加剧了温室效应并推动气候变化。IB 考题常要求你概述碳流通及其影响。
6. Nutrient Cycling: Nitrogen Cycle | 物质循环：氮循环
Although the atmosphere is 78% nitrogen gas (N₂), most organisms cannot use it directly. The nitrogen cycle transforms nitrogen into usable forms through four main processes: nitrogen fixation, nitrification, assimilation, and denitrification, with ammonification also playing a key role.
虽然大气中 78% 是氮气 (N₂)，但大多数生物无法直接利用。氮循环通过四个主要过程将氮转化为可利用的形式：固氮作用、硝化作用、同化作用和反硝化作用，同时氨化作用也起着关键作用。
Nitrogen fixation is the conversion of N₂ into ammonia (NH₃) or ammonium ions (NH₄⁺) by free-living bacteria (e.g., Azotobacter), symbiotic bacteria (Rhizobium in root nodules of legumes), or via lightning and industrial processes (Haber-Bosch).
固氮作用是指通过自由生活的细菌（如固氮菌）、共生细菌（豆科植物根瘤中的根瘤菌）、闪电或工业过程（哈伯-博斯法）将 N₂ 转化为氨 (NH₃) 或铵离子 (NH₄⁺)。
Nitrification then converts NH₄⁺ to nitrites (NO₂⁻) and nitrates (NO₃⁻) by nitrifying bacteria in aerobic soil. Plants assimilate nitrates to synthesise proteins. Denitrification returns N₂ to the atmosphere under anaerobic conditions. Ammonification releases NH₄⁺ from organic waste.
然后，硝化作用由好氧土壤中的硝化细菌将 NH₄⁺ 转化为亚硝酸盐 (NO₂⁻) 和硝酸盐 (NO₃⁻)。植物同化硝酸盐以合成蛋白质。反硝化作用在厌氧条件下将 N₂ 返回大气。氨化作用从有机废物中释放出 NH₄⁺。
7. Ecological Pyramids | 生态金字塔
Ecological pyramids graphically represent the structure of an ecosystem. The three main types are pyramids of numbers, biomass, and energy. Pyramids of energy are always upright because energy is lost at each trophic level; they depict energy content (kJ m⁻² yr⁻¹).
生态金字塔以图形方式表示生态系统的结构。三种主要类型是数量金字塔、生物量金字塔和能量金字塔。能量金字塔总是呈正金字塔形，因为每个营养级都会损失能量；它描绘的是能量含量（kJ m⁻² yr⁻¹）。
Pyramids of biomass typically narrow towards the top, but can be inverted in aquatic ecosystems where phytoplankton reproduce rapidly and have a small standing biomass at any given time while supporting a larger biomass of zooplankton.
生物量金字塔通常向上变窄，但在水生生态系统中可能出现倒置，因为浮游植物繁殖迅速，在任意时间点的现存量很小，却能支撑起更大的浮游动物生物量。
Pyramids of numbers can also be inverted (e.g., a single tree supporting many insects). For IB exams, be able to draw and label each pyramid type, explain shape differences, and identify which pyramid provides the most accurate picture of energy flow.
数量金字塔也可能出现倒置（例如一棵大树支撑着大量昆虫）。在 IB 考试中，要能绘制并标注各类金字塔，解释形状差异，并辨识出哪种金字塔最能准确反映能量流动。
8. Population Ecology and Growth Models | 种群生态与增长模型
A population is a group of individuals of the same species living in the same area at the same time. Key population characteristics include size, density, dispersion, and age structure. Population growth is governed by birth rate, death rate, immigration, and emigration.
种群是指同一时期生活在同一区域内的同种个体集合。关键的种群特征包括大小、密度、分布和年龄结构。种群增长受出生率、死亡率、迁入和迁出的制约。
When resources are unlimited, populations exhibit exponential growth, described by the equation dN/dt = rN, where N is population size and r is the intrinsic rate of increase. This yields a J-shaped curve.
当资源无限时，种群呈指数增长，用方程 dN/dt = rN 描述，其中 N 为种群大小，r 为内禀增长率。这会产生 J 形曲线。
dN/dt = rN
In reality, environmental resistance limits growth, leading to logistic growth, expressed as dN/dt = rN(1 − N/K), where K is the carrying capacity. This produces an S-shaped (sigmoid) curve, levelling off at K.
实际上，环境阻力会限制增长，导致逻辑斯蒂增长，表示为 dN/dt = rN(1 − N/K)，其中 K 是环境容纳量。这会形成 S 形（S 型）曲线，在 K 值处趋于平稳。
dN/dt = rN(1 − N/K)
Methods for estimating population size include quadrat sampling (for sessile organisms) and the mark-release-recapture (Lincoln Index): N = (n₁ × n₂)/n₃, where n₁ = number first captured and marked, n₂ = total in second capture, n₃ = number marked in second capture.
估算种群大小的方法包括样方法（适用于固着生物）和标志重捕法（林肯指数）：N = (n₁ × n₂)/n₃，其中 n₁ 为第一次标记并释放的数量，n₂ 为第二次捕获总数，n₃ 为第二次捕获中带有标记的数量。
9. Community Interactions: Competition, Predation, Symbiosis | 群落相互作用：竞争、捕食与共生
Interspecific interactions shape community structure. Competition occurs when two species rely on the same limited resource. The competitive exclusion principle states that two species with identical ecological niches cannot coexist indefinitely; one will outcompete the other, leading to resource partitioning or local extinction.
种间相互作用塑造了群落结构。当两个物种依赖相同的有限资源时，就会发生竞争。竞争排斥原理指出，生态位完全相同的两个物种无法长久共存；一个会胜出，导致资源分割或局地灭绝。
Predation is a feeding relationship where one organism (predator) kills and consumes another (prey). Predator-prey dynamics often show coupled oscillations in population sizes. Herbivory is a form of predation where the plant is not necessarily killed.
捕食是一种摄食关系，其中一种生物（捕食者）杀死并吃掉另一种（猎物）。捕食者-猎物种群动态常表现为耦合振荡。植食是一种捕食形式，但植物不一定会被杀死。
Symbiosis refers to close, prolonged associations between species. Mutualism benefits both (e.g., corals and zooxanthellae), commensalism benefits one without affecting the other (e.g., barnacles on whales), and parasitism benefits one at the expense of the host (e.g., tapeworms).
共生指物种间紧密、持久的联系。互利共生对双方都有利（如珊瑚和虫黄藻），偏利共生使一方受益而对另一方无影响（如鲸鱼身上的藤壶），寄生则是一方受益而损害宿主（如绦虫）。
10. Ecological Succession | 生态演替
Ecological succession is the predictable, directional change in community composition over time. Primary succession begins in lifeless areas with no soil, such as bare rock exposed after a volcanic eruption. Pioneer species like lichens and mosses colonise the rock, breaking it down to form thin soil.
生态演替是群落组成随时间发生的有规律的定向变化。初级演替起始于没有土壤的无生命区域，比如火山喷发后裸露的岩石。地衣和苔藓等先锋物种首先在岩石上定居，将其分解形成薄薄的土壤。
Over time, grasses, shrubs, and eventually trees establish, leading to a climax community—a relatively stable, self-perpetuating assemblage of species. Secondary succession occurs where an existing community has been disturbed but soil remains (e.g., after a forest fire), and it proceeds much faster.
随着时间的推移，草本植物、灌木直至乔木相继建立，最终形成顶级群落——一个相对稳定、能自我延续的物种组合。次级演替则发生在原有群落遭到破坏但土壤尚存的区域（如森林火灾后），演替速度要快得多。
During succession, species diversity, biomass, and niche specialisation generally increase. Be prepared to describe the seral stages and to analyse changes in abiotic factors (e.g., soil depth, light availability) across a successional sequence.
演替过程中，物种多样性、生物量和生态位特化通常都会增加。准备好描述演替系列阶段，并分析演替序列中非生物因素（如土壤深度、光照可获得性）的变化。
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