OCR生物 生态系统 生物量 能量转移 演替
OCR A Level 生物 A 把生态系统安排在模块 6.3.1，这一块的分值不大，题面变化却很多，从食物链上的能量计算到演替阶段排序都出现过。下面按生物量、生产效率、养分循环、演替与采样五条线索展开，每条都配上 OCR 评分方案里真正给分的措辞与数字。文中引用的卷面信息来自 OCR 官方 spec（H420，2015 年 9 月起施教，2017 年首次 A Level 考试）与 2024 年 6 月的真题卷。
Ecosystems occupies Module 6.3.1 of OCR A Level Biology A, a small slice of the marks that shows up in many shapes, from energy calculations along a food chain to ordering the stages of succession. The sections below follow five threads, namely biomass, productivity, nutrient cycling, succession and sampling, and each one keeps the wording examiners actually credit alongside the numbers. Paper references come from the OCR specification for H420 and the June 2024 examination paper.
生态系统内容在 H420/02 上的位置与分值
OCR A Level 生物 A 的生态内容集中在模块 6.3.1，卷面落点是 H420/02 Biological diversity。这一卷 2 小时 15 分、100 分，占整个 A Level 的 37%，同时抽考模块 1、2、4 和 6。Section A 是 15 分的选择题，生态部分在这里多以术语辨析出现，Section B 才是结构题与长答题。OCR examiner report 系列写明，A Level 生物各卷的数学技能约占 10%，生态题的数学成分主要落在生物量、生产效率与转移效率的计算上，单位写错整题不给分。2024 年 6 月 H420/02 的卷首就有一道题问食物链为什么很少超过五个营养级，给分答案是营养级之间生物量转移效率低，写成捕食者体型存在上限不得分。这类题只用一句话作答，关键词是 inefficient transfer。计算题的分值通常落在 2 到 3 分，写出公式、代入数字、给出单位三步齐全才拿满分。
Ecosystem content in OCR A Level Biology A sits in Module 6.3.1 and reaches the exam through Paper 2, the Biological diversity paper, which runs 2 hours 15 minutes for 100 marks and 37% of the qualification while sampling Modules 1, 2, 4 and 6. Section A is a 15-mark multiple choice section where ecology appears as term discrimination, and Section B carries the structured questions and the extended response; the examiners' reports record that mathematical skill stays at roughly 10% of the marks, mostly in biomass, productivity and transfer-efficiency calculations where a wrong unit costs the whole item. The June 2024 H420/02 paper opened with a question on why food chains rarely exceed five trophic levels, and the credited answer was the inefficiency of biomass transfer rather than any limit on predator size.
生态系统、群落、栖息地与生态位
生态系统的定义里要同时包含群落与非生物环境两部分，只写生物就是半句答案。群落指同一区域里全部种群的集合，种群指同一物种的一群个体，栖息地指生物生活的具体场所。分布与丰度受生物因素与非生物因素两重控制，捕食者、病原体与种间竞争属于生物因素，光照、温度、水分、pH 与土壤矿质属于非生物因素。生态位是一个物种在栖息地里占的全部角色，包含它与其他生物的相互作用，也包含它对非生物条件的耐受范围，两个物种不会长期占据完全一样的生态位。很多学生以为生态位就是栖息地，答题时把地点当成角色写，解释竞争的题目因此丢掉关键分。选择题常把这几个术语互相替换，题干写 habitat 时选 population 就错，先把每个术语的关键词圈出来再选。这份卷子的选择题每个只值 1 分，错一个术语就少 1 分。
An ecosystem is defined by its community together with the non-living conditions of the area, so a definition that stops at the organisms is only half an answer. A community is every population living in one place, a population is the individuals of one species, and a habitat is the physical place where an organism lives, while distribution and abundance are governed by biotic factors such as predators, pathogens and competition alongside abiotic factors such as light, temperature, water, pH and soil minerals. The niche is the full role a species plays in its habitat, covering its interactions with other organisms and its tolerance of abiotic conditions, and no two species hold the same niche for long; habitat and niche are the pair most often swapped in multiple choice items.
生物量的定义与两种测法
生物量指某一区域里生物物质的量，OCR 的写法里它既可以用干重表示，也可以用含碳量表示，含碳量常按干重的 50% 估算。测量生物量必须先烘干样品，湿重含有水分，不同个体的含水量又不一样，拿湿重比较两个样方会得出错误结论。干重样品随后放入量热计燃烧，样品放出的热量传给已知体积的水，水温上升幅度换算出化学能，单位是焦耳或千焦。干重的单位是每单位面积的质量，例如克每平方米，化学能的单位要多一个时间项，例如千焦每平方米每年。真题里出现过单位限定题，湖泊这类水体生态系统的生产量按体积计，单位要写成千焦每立方米每年。常见的错误是单位混着写，把克每平方米写成克每平方米每年，时间项一错整题不给分。
Biomass is the quantity of living material in an area, and OCR accepts either dry mass or mass of carbon, with carbon generally taken as 50% of the dry mass. Samples have to be dried before weighing because wet mass carries variable water content, which makes comparisons between quadrats unreliable; dry samples are then burned in a calorimeter, and the temperature rise in a known volume of water gives an estimate of the chemical energy stored, measured in joules or kilojoules. Dry mass carries units of mass per area such as grams per square metre, while energy content needs a time term as well, and for a lake the question expects kilojoules per cubic metre per year rather than per square metre.
能量沿营养级传递与食物链长度
能量以光的形式进入大多数生态系统，光合作用把它固定在有机物中，之后沿生产者、初级消费者、次级消费者的顺序传递。营养级从生产者开始编号，生产者是第一营养级，吃植物的动物是第二营养级，从消费者开始编号是常见的手滑。传不下去的能量有四条去路：呼吸作用以热的形式散失、随粪便与尿液排出、成为未被摄食的部分、死亡后留给分解者。2024 年 6 月的真题里，食物链很少超过五个营养级的正确解释就是营养级之间生物量转移效率低，写成植食动物吃得少不给分，这类题只值 1 分。长食物链的末端可用的能量太少，无法维持再高一级的种群。答题时按题面情景挑去路，题干讲草场与奶牛就写呼吸损失与粪便，题干讲海洋时写未被摄食的部分更贴切。
Energy enters most ecosystems as light and is fixed into organic matter by photosynthesis, after which it moves through producers, primary consumers and secondary consumers in turn, and trophic levels are numbered from the producer rather than from the first consumer. Energy fails to pass on in four ways, namely respiratory loss as heat, loss in faeces and urine, the parts of an organism never eaten, and the fraction left to decomposers after death. The June 2024 paper credited the inefficiency of biomass transfer as the reason food chains rarely exceed five trophic levels, which is why the far end of a long chain supports too little energy for another population.
GPP、NPP 与消费者的净生产量
总初级生产力 GPP 指单位时间单位面积里植物在光合作用中固定的化学能总量，净初级生产力 NPP 是扣掉呼吸损失之后留在植物生物量里的部分，公式写成 NPP = GPP 减 R。R 代表呼吸损失，这部分能量以热的形式离开生态系统，下一营养级拿不到。消费者的净生产量写成 N 等于 I 减去括号里的 F 加 R，I 是摄入食物中的化学能，F 是粪便与尿液带走的能量，R 是呼吸损失，括号里两项相加后整体从 I 里扣除。食物链传递的是净生产量而不是总生产量，两个量混用会让后面的效率计算全错。给出一组数字就能验算，GPP 为 48000 千焦每平方米每年、呼吸损失 19000，NPP 就是 29000，这一步在评分方案里通常占 1 分。中括号写错位置也是失分点，写成 I 减 F 加 R 等于把粪便那一项加了回去。
Gross primary productivity is the total chemical energy fixed by photosynthesis per unit area per unit time, and net primary productivity is what remains in plant biomass once respiratory loss is deducted, so NPP equals GPP minus R. That respiratory fraction leaves the ecosystem as heat and stays out of reach of the next trophic level, and the net production of consumers follows the same logic as N equals I minus the sum of F and R, where I is the chemical energy in ingested food, F is the energy lost in faeces and urine, and R is respiration. Food chains pass on net rather than gross production, so confusing the two throws out every later calculation; with a GPP of 48000 kilojoules per square metre per year and respiration of 19000, the NPP is 29000.
转移效率的计算与 10% 的经验值
转移效率的公式是生物量转移量除以生物量摄入量再乘 100，分子取传到上一营养级的部分，分母取低营养级被吃掉的量，两个量取反答案就会大于 100%。实际效率一般在 10% 左右，题干给的数字才是唯一依据，凭记忆套固定百分数容易算错。若草地净生产量为 24000 千焦每平方米每年、奶牛摄入其中的 8000、奶牛净生产量 800，那么奶牛吃掉了草地净生产量的三分之一，而奶牛净生产量相对草地净生产量只有约 3.3%。两种口径的答案差三倍，题干问哪一种要先看清，评分方案里对应的说法是净生产量的百分比。阅卷时看到只写 10% 而不列式子的答案，通常只给公式分，不给计算分。单位也要跟着写，效率本身没有单位，写成百分比后不必再加千焦。
Efficiency of transfer is biomass transferred divided by biomass intake, multiplied by 100, with the transferred biomass in the numerator and the intake from the lower trophic level in the denominator, so swapping them gives an answer above 100%. Transfer sits near 10% on average, yet the figures in the question are the only evidence worth using; where grass has a net production of 24000 kilojoules per square metre per year, cattle ingest 8000 and the cattle net production is 800, the cattle take in a third of the grass production while their own net production is only about 3.3% of it. Those two readings differ threefold, and an answer that quotes 10% without showing the working tends to earn the formula mark only.
分解者与氮循环的四个环节
分解者由细菌与真菌组成，它们把尸体、落叶与粪便里的有机物分解，养分以简单化合物的形式回到土壤，碳和氮由此回到循环。氮是蛋白质与核酸的必需元素，植物却不能利用空气中的氮气，得先把它固定成铵离子或硝酸盐。根瘤菌与豆科植物共生，豌豆这类植物的根瘤里，细菌把氮气还原成铵离子，植物回赠糖类，这种关系在评分方案里写成互惠共生。土壤里的硝化分两步完成，亚硝化单胞菌把铵离子氧化成亚硝酸盐，硝化杆菌再把亚硝酸盐氧化成硝酸盐，植物吸收硝酸盐用于合成核苷酸与氨基酸。淹水缺氧的土壤里，反硝化细菌把硝酸盐还原成氮气，这部分氮离开生态系统，农田排水不良时氮肥肥效下降就是这个原因。写氮循环题按固定顺序排四个环节，顺序写错会丢掉后面的连锁分。
Decomposers are the bacteria and fungi that break down dead organisms, leaf litter and faeces, returning nutrients to the soil in simple forms and driving both the carbon and nitrogen cycles. Nitrogen is needed for proteins and nucleic acids, yet plants cannot use nitrogen gas, so it has to be fixed into ammonium ions or nitrates; Rhizobium in the root nodules of legumes such as peas reduces nitrogen gas to ammonium ions and receives sugars in return, a relationship the mark scheme calls mutualistic. Nitrification then runs in two steps, with Nitrosomonas oxidising ammonium to nitrite and Nitrobacter oxidising nitrite to nitrate, while in waterlogged, oxygen-poor soil denitrifying bacteria reduce nitrate back to nitrogen gas and take that nitrogen out of the ecosystem.
碳循环与人为通量的比例
碳循环的四个环节是光合作用、摄食、呼吸与分解，碳在有机物与二氧化碳之间来回转换，任何一个环节的速率变化都会改变大气里的二氧化碳浓度。燃烧化石燃料把封存了上亿年的碳在短时间内释放出来，这条路径绕过了光合作用这个入口，大气二氧化碳上升的速度因此快于自然循环吸收的速度。森林砍伐从两端推高浓度，砍掉的树不再进行光合作用，分解与焚烧又把储存的碳释放出来。结构题常给一张碳通量图，让你读出数值最大的通量，或者算出人为来源占的比例，图上的单位多是每年十亿吨碳。读图时先分清自然通量与人为通量，这一步通常占 1 分。写这类答案只描述图上的数字就够，不必展开气候影响，题干没有问到的不加分。
The carbon cycle runs through photosynthesis, feeding, respiration and decomposition, moving carbon between organic molecules and carbon dioxide so that a change in the rate of any one step shifts the atmospheric concentration. Burning fossil fuels releases carbon that was locked away for hundreds of millions of years and bypasses photosynthesis, which is why atmospheric carbon dioxide rises faster than the natural cycle can absorb it; deforestation pushes from both ends, since felled trees stop photosynthesising while decomposition and burning release stored carbon. A flux diagram in the exam usually asks which flux is largest or what share is anthropogenic, and the units are usually gigatonnes of carbon per year, so sorting natural from human fluxes earns the first mark.
初生演替与次生演替
演替是一个群落被另一个群落取代的过程，初生演替发生在从未有过生物的区域，火山喷发后的裸岩是标准例子。先锋物种先定居，地衣能耐受干旱与温差，它的分泌物与死亡后的残体被微生物分解，逐渐积累腐殖质并形成薄层土壤。土壤变厚以后苔藓、草本与灌木依次进入，物种丰富度随演替推进上升，最后形成自维持的顶极群落。次生演替发生在原有植被被清除、土壤还在的区域，森林火灾后的地段属于这一类，土层已经存在，演替从更后面的阶段开始，恢复速度明显更快。答题时把先锋物种、腐殖质积累、土壤形成与顶极群落四个阶段写全，次生演替再多写一句土壤已存在，这种题一般 4 到 6 分。顶极群落的措辞要落在稳定与自维持两点上，只写树长得高大拿不到描述分。
Succession is the replacement of one community by another, and primary succession starts on ground that has never supported life, with bare rock after a volcanic eruption the standard example. Pioneer species such as lichens tolerate drought and wide temperature swings, and as they die the decomposing remains build humus and gradually form a thin soil; as the soil deepens, mosses, grasses and shrubs arrive in turn and species richness rises until a self-sustaining climax community establishes. Secondary succession starts where vegetation has been cleared but soil remains, as after a forest fire, so it begins at a later seral stage and proceeds faster, and a full answer names the pioneer species, humus accumulation, soil formation and the climax community.
样方、样带与偏差控制
丰度的测量靠取样，样方是一个已知面积的方框，随机放置后数清框内每个物种的个体数；样带沿一条拉直的卷尺记录与线接触的物种；带样带用两条卷尺，按固定间隔在两条线之间取样。随机取样的目的只有一个，让每个位置被选中的机会相等，从而把主观偏差压到最低，位置用随机数或随机坐标决定。样方尺寸要适配对象，草本用 0.25 到 1 平方米，灌木与树木要大得多，尺寸用错会算出荒谬的密度。重复次数通常每个样地 10 次以上，只取一个样方的数据看不出任何趋势，题干给出 12 个样方时就按 12 组数据算平均。写答案时把随机、重复、合适尺寸三个词都写进去，这三点在评分方案里各占 1 分。
Abundance is measured by sampling, with a quadrat of known area dropped at random positions so the individuals of each species inside can be counted, a line transect recording the species touching a stretched tape, and a belt transect sampling between two tapes at fixed intervals. Random placement exists to give every spot an equal chance of selection and keep subjective bias out, so positions come from random numbers or random coordinates rather than from where the vegetation looks interesting. Quadrat size has to suit the organisms, since 0.25 to 1 square metre suits grasses while shrubs and trees need far more, and a single quadrat per site shows no trend at all; examiners look for random, repeated and appropriately sized in the same answer.
阅卷视角下的四处常规失分
生态部分失分最多的地方是单位，生物量写克每平方米、生产量写千焦每平方米每年，两者互换就是零分。把生物量与生物量转移量当成同一个量也会出问题，效率算出来会大于 100%，这种答案阅卷时直接判错。公式里的符号同样要交代，NPP = GPP 减 R 里的 R 得写明是呼吸损失，只写字母不给解释分。演替题常见的跳跃是从先锋物种直接写到顶极群落，中间漏掉腐殖质积累与土壤形成，这类答案通常只能拿到一半分。很多学生以为生态部分靠背单词，实际上一半的分值落在计算与单位上，这四处在实考中都直接扣分。
Four faults account for most of the lost marks in ecology: units, since dry mass is quoted per area while productivity needs per area per year; concept mixing, where biomass and biomass transferred are treated as one quantity so the efficiency comes out above 100% and examiners mark it wrong on sight; undefined symbols, because the R in NPP equals GPP minus R has to be named as respiratory loss; and skipped stages in succession, where jumping from pioneer species to climax community without humus accumulation and soil formation tends to earn half marks. Many students treat ecology as vocabulary revision, yet about half the marks sit in calculations and units.
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