Edexcel生物 生态系统 能量传递 GPP NPP 演替
Edexcel A level Biology A（Salters-Nuffield，spec 代码 9BN0）把生态学放在 Topic 5，能量流动、初级生产力、演替与气候变化证据都在这一章，Paper 1 与 Paper 3 都会考。很多学生以为生态题靠背定义，真正拉开差距的是拿到一张能量流图或一组采样数据以后，能不能算、能不能解释传递效率为什么那么低。这篇按 Edexcel 官方 spec 的 5.1 到 5.12 与 Core Practical 10 的落点，把生态系统这块的考点一次讲清。
Topic 5 of Edexcel A level Biology A (Salters-Nuffield, 9BN0) is where ecology, energy transfer, succession and climate evidence sit together, and it is examined on both Paper 1 and Paper 3. Ecology questions are rarely pure recall: you are handed an energy flow diagram, a food web or a set of sampling data and asked to calculate, compare and explain. This piece works through specification points 5.1 to 5.12, with the calculations and the wording examiners expect.
生态系统、生境、种群与群落四个词的边界
生态学研究的最小完整单位是生态系统，它包含一片区域内所有的生物以及它们所处的非生物环境。非生物因子指气候、土壤类型、光照与水分这类物理化学条件，生物因子指捕食、竞争与疾病这类由生物造成的影响。单个物种在同一区域内的全部个体组成种群，同一生境内所有种群一起构成群落，群落加上非生物环境才是生态系统，判断边界时看它能不能基本自我维持。Edexcel 官方 spec 的 5.1 要求你能用这些术语描述一个具体生境，比如一片白垩草地包含哪些种群、哪些因子在起限制作用。常见的错误是把种群和群落混着写，题干问群落时只写一个物种的数量变化，阅卷时这类答案拿不到分。
An ecosystem is a community of organisms together with the abiotic conditions they live in, and the specification expects you to apply these four terms rather than recite them; abiotic factors cover climate, soil type, light and water availability, while biotic factors include predation, competition and disease. A population is all the individuals of one species in an area, and the community is every population sharing that habitat. When a question asks about a community, describing the rise and fall of a single species will not answer it; examiners look for the limiting factor you chose and the reason it matters.
生态位与资源分配，为什么同域物种要分开取食
生态位描述一个物种在群落里怎么生活，包括它吃什么、在哪活动、什么时候活动、与哪些物种发生关系。同一生境里两个物种的生态位如果高度重叠，竞争长期会把其中一个挤出去，共存的前提是资源被分开使用。森林里不同种类的林莺在不同高度取食昆虫，就是分层利用的典型例子。研究生态位不只看它现在吃什么，还要看季节变化和食物短缺时它转向什么。Edexcel 的 5.1 要求把生态位与物种的丰度和分布联系起来，答题时写出起作用的限制因子比列出物种名单更重要。很多学生会把生态位等同于生境，生境是地址，生态位是它在这个地址里的活法。
A niche is the role a species plays in a community: what it eats, where and when it is active, and which species it interacts with. Two species whose niches overlap heavily cannot coexist for long, because competition removes the weaker one from that part of the habitat; warblers feeding at different heights in the same woodland show how resources get partitioned. Distribution and abundance follow from these differences, so a good answer names the limiting factor instead of listing species. Habitat and niche are not interchangeable: the habitat is the address, the niche is how the organism makes its living there.
能量怎么进入生态系统，GPP 与 NPP 的关系
能量从太阳进入生态系统只有一条路，就是被生产者固定成有机物中的化学能。绿色植物、藻类与部分细菌属于生产者，也叫自养生物，它们能用无机物合成有机物。单位时间单位面积内生产者固定能量的速率叫初级生产力，其中固定在有机物里的总量是总初级生产力，也就是 GPP。生产者自己也要呼吸，呼吸消耗掉的那部分要从 GPP 里扣掉，剩下的才是净初级生产力 NPP，也就是能进入下一营养级的能量。Edexcel spec 的 5.10 把两者的关系写成 NPP 等于 GPP 减去植物呼吸损失，考试常给一张能量流图让你往里填数。常见的错误是把 NPP 当成生产者制造的有机物总量，那样会把呼吸损耗算两遍，后面的传递效率自然全错。
Energy enters an ecosystem in one way only: producers capture light and store it as chemical energy in organic molecules; green plants, algae and some bacteria are autotrophs, able to build organic compounds from inorganic ones. Primary productivity is the rate at which that energy is fixed per unit area per unit time, and gross primary productivity, GPP, is the total fixed. Producers respire as well, and those losses are subtracted from GPP to leave net primary productivity, NPP, the energy actually available to the next trophic level; the relationship is written as NPP equals GPP minus respiratory losses, and questions often hand you an energy flow diagram to complete. Treating NPP as everything the plant made double-counts respiration and throws off the efficiency calculation later.
算净初级生产力时先看单位再看方向
算 NPP 之前先看清题目给的是速率还是总量，单位里带 per year 的是速率，带 per unit area 的才需要除以面积。单位通常写成 J m-2 yr-1，水生生态系统按体积算就用 J m-3 yr-1，单位写错在 Edexcel 的计算题里一样扣分。给了一张表的时候，先把每一列的单位抄在草稿纸上，再把需要的两列相减或相除，不要跳步。算式之后补一句结论，说明这个结果代表多少能量可以进入下一营养级，或者与另一块样地相比生产力高出多少。错得最多的是数字算对但单位漏写、方向写反的答案，两分只剩一分。
Before calculating, check whether the data are rates or totals: a value per year is a rate, and anything quoted per unit area has to be scaled by the area you are given. NPP is expressed in J m-2 yr-1 on land, and in J m-3 yr-1 where the habitat is a volume of water; in Edexcel calculations the unit carries a mark, so write it down with the number. Work column by column, subtract or divide only the two quantities the formula needs, and finish with a sentence saying what the figure means for the next trophic level; answers that are numerically right but directionally wrong lose the second mark.
营养级之间的能量与生物量传递效率怎么算
两个相邻营养级之间，后一级得到的能量或生物量除以前一级的，再乘百分之百，就是这个传递的效率。用生物量算的优点是野外称一称就能得到，缺点是含水量与热值不同，不同物种之间不便直接比较；用能量算需要把干重换算成焦耳，但能说清有多少能量真正被同化。Edexcel 的 5.11 同时要求两种算法，题目给什么就用什么，不要在一条食物链里混用两套数据。算出来的效率通常很低，草食动物这一级的数字往往最难看，因为大部分植物组织根本吃不到或者消化不了。答题时把分子分母写清，标上单位与年份，别只丢一个百分数，很多学生把效率倒着算成前一级除以后一级，结果大于百分之百还照写不误。
Transfer efficiency between two trophic levels is the energy or biomass gained at the higher level divided by that available at the lower level, expressed as a percentage. Biomass is quicker to measure in the field, though water content and calorific value vary between organisms; energy requires converting dry mass into joules but shows how much is actually assimilated; the specification asks for both routes, so use whichever currency the question supplies and never mix them along one food chain. The values are low, and the herbivore step is usually the weakest because much plant material is never eaten or cannot be digested; write the numerator and denominator explicitly, keep the units, and remember that a figure above 100 per cent means the fraction has been inverted.
传递效率低的四个去向：没吃、没消化、呼吸与排泄
效率低的原因要分清楚四个去向。一部分组织没被取食，根、木质部和带刺的叶子往往留在原地；一部分被吃下但没被消化，随粪便排出体外，这部分能量属于分解者的食物，不算被消费者同化；被同化的能量还要在一次呼吸中以热的形式散失，哺乳动物和鸟类维持体温的代价尤其高；剩下的才用于生长，成为后一级能取到的生物量。因为这个原因，食物链很少超过四到五个营养级，末端能支撑的生物量太小。写答案时先写去向再补数值，题干给的摄入量要减掉粪便才是同化量，把粪便里的能量算成同化能量是这里最常见的错误。
Four routes explain the losses. Some plant tissue is never eaten, and roots, wood and defended leaves stay in place; of what is eaten, an indigestible fraction leaves as faeces, which feeds decomposers rather than the consumer; assimilated energy is then spent on respiration and lost as heat, a heavy cost for birds and mammals holding a constant body temperature; only what remains supports growth and becomes available to the next level; that is why food chains rarely run beyond four or five levels: the biomass left at the end is simply too small; counting the energy in faeces as assimilated is the commonest slip at this point.
初级演替：先锋物种怎么把裸岩变成土壤
初级演替发生在从来没有群落的新生地，比如岩浆冷却后的岩石表面、退去的冰川留下的裸地，或者海边新形成的沙丘。最先落脚的先锋物种通常是地衣和苔藓，它们能耐受养分和水分的极端缺乏，靠风化岩石与自身死亡后的分解慢慢积出薄薄一层土壤。土层变厚以后，小草本与草类的种子才能站住，它们的根系把土壤固定住，减少流失。植被越复杂，遮阴与土壤水分条件越适合灌木和小树，前一阶段的物种在自己改造出来的环境里反而竞争力下降，于是被替换掉。演替最终到达的稳定群落叫顶级群落，温带地区常见的是阔叶林。Edexcel spec 提到设得兰群岛的 Keen of Hamar，那里因为土壤与母岩的特性，稳定群落并不是森林，顶级群落并不处处都是树林。
Primary succession begins on land that has never carried a community, such as rock cooled from magma, ground exposed by a retreating glacier, or a newly formed sand dune. Pioneer species are usually lichens and mosses, able to tolerate very low nutrient and water availability and to build a thin soil from weathered rock and their own decay; as the soil deepens, grasses and small herbs can establish, and their roots hold it in place. Shade and soil moisture then favour shrubs and young trees, while the earlier species lose out in the very conditions they created. The stable end point is the climax community, usually broadleaf woodland in temperate regions; the specification points to the Keen of Hamar in Shetland, where soil and underlying rock leave a stable community without trees, so a climax is not automatically a forest.
次级演替与偏途演替：放牧、割草和火烧改写终点
次级演替发生在原有群落被破坏、但土壤与种子库还在的地方，比如火烧过的石楠荒原、废弃的农田或者被风暴吹倒一片的林地。因为土壤已经形成，次级演替的速度比初级演替快得多，从草地回到林地可能只要几十年。如果人为活动持续把演替压在中间阶段，群落就会停在那里，这种状态叫偏途演替。英国南部的白垩草地和石楠荒原都需要放牧、割草或周期性火烧来维持，一旦停止管理，灌木侵入，依赖低矮植被的物种就会消失。干预方式会影响演替走向，割草带走养分，放牧把养分以粪便形式留在原地，火烧把氮带走并促使喜火物种的种子萌发。答这类题要写清具体手段与它对物种组成的影响，只写一句人类活动破坏生态拿不到分。
Secondary succession follows disturbance where soil and a seed bank survive, for example on burnt heather moorland, abandoned farmland or woodland flattened by a storm. Because the soil is already there, it runs much faster than primary succession, and grassland can return to woodland within decades; where human activity holds the community at an intermediate stage the result is deflected succession. Chalk grassland and heather moor in Britain depend on grazing, mowing or periodic burning to stay open, and once management stops, scrub invades and the species of short vegetation disappear. The method matters: mowing removes nutrients, grazing returns them in dung, and burning drives off nitrogen while triggering the germination of fire-adapted seeds; marks come from naming the intervention and its effect on species composition.
样方与样线：Core Practical 10 的抽样与数据处理
野外生态学的数据来自抽样，整片生境没法数一遍。样方适合测量不移动的生物，用随机方式铺若干个，记录每个样方里的物种与个体数；样线适合观察环境梯度上的变化，沿一条线每隔固定距离记录一次。样方的大小和数量取决于研究对象，草本群落半米见方往往够用，灌木和乔木要几米见方；样方太少，平均值会随一两个偶然出现的个体大幅摆动。Edexcel 的 Core Practical 10 要你观察分布模式并从抽样数据给出结论，所以方法必须写全：随机化怎么做的、样方多大、用了多少个、记录了什么指标。常见的错误是先把样方摆在长得最好的地方，数据看着漂亮，实际带有主观偏差。数据出来以后要算频率或密度，再与光照、土壤水分这类非生物因子作比较。
Field ecology relies on sampling, because counting an entire habitat is not possible. Quadrats suit organisms that do not move, and a number of them placed at random give a record of the species and individuals present; a transect is better for change along an environmental gradient, with records taken at fixed intervals along the line. Quadrat size and number depend on what is being studied: half a metre square may be enough for a herb community, while shrubs and trees need metres; too few quadrats and one chance individual swings the mean. Core Practical 10 asks for distribution patterns explained from sampled data, so the method must be reported in full: how positions were randomised, the quadrat size, how many were used and what was recorded; placing quadrats where the vegetation looks best introduces a subjective bias that no volume of data can repair.
泥炭孢粉与树木年轮，气候证据链与相关不等于因果
判断生态系统正在怎么变，靠的是一组能互相印证的长期记录。大气二氧化碳浓度自 1958 年起在夏威夷莫纳罗亚观测站连续测量，这段由美国国家海洋和大气管理局公布的曲线是最常被引用的直接证据。温度记录来自全球气象站与海洋浮标，时间跨度更长，但站点分布不均，需要用统计方法处理。更长的时间尺度靠代用指标，泥炭沼泽里的花粉层层堆积，能还原当地植被从耐寒草本到阔叶林的变化；树木年轮宽度在水分或温度受限的地区记录了逐年的生长条件。Edexcel 的 5.12 要求区分相关与因果，二氧化碳与温度同步上升只是相关，要判断因果还需要机制上的证据，比如二氧化碳吸收红外辐射的实验室测量。答题时把每类证据能说明什么、局限在哪里写出来，比只写气候在变暖得分高得多。
Evidence for how ecosystems are changing comes from long records that corroborate one another; atmospheric carbon dioxide has been measured continuously at the Mauna Loa Observatory in Hawaii since 1958, and the curve published by the US National Oceanic and Atmospheric Administration is the most directly quoted evidence. Temperature records come from weather stations and ocean buoys; they cover a longer period but are unevenly distributed and need statistical treatment; older changes are reconstructed from proxies: pollen layers in peat bogs reveal a shift from cold-tolerant herbs to broadleaf woodland, and tree ring widths record year-by-year growth where water or temperature is limiting. Specification point 5.12 asks you to separate correlation from causation; carbon dioxide and temperature rising together is a correlation, while a causal claim also needs a mechanism, such as laboratory measurements of infrared absorption. Marks come from saying what each type of evidence shows and where its limits lie.
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