Edexcel GCSE生物 生态系统 样方样带 碳循环
Topic 9 生态系统与物质循环是 Edexcel GCSE 生物 (1BI0) Paper 2 的最后一块内容。这份卷子 1 小时 45 分钟、100 分、占总成绩一半，抽样计算、生物量传递效率和分解速率三类题几乎每场都出现。下面按考试局官方 spec 的条目顺序走一遍，每个知识点配一个能直接写进答题线的说法。
Topic 9 sits at the end of the Edexcel GCSE Biology (1BI0) Paper 2, a 1 hour 45 minute paper worth 100 marks and half of the qualification. Sampling calculations, biomass transfer efficiency and decay rates appear in almost every series. Below, each specification point is paired with the wording that actually earns marks.
一、个体、种群、群落与生态系统的四级关系
生态学的组织层级从单个个体一直排到整个生态系统。同一物种在同一时间、同一地点生活的全部个体叫种群。不同物种的种群放在一起构成群落。群落再加上光照、温度、水、土壤这些非生物条件，才成为生态系统。群落只统计生物，生态系统必须把非生物部分也算进来，很多学生以为两者可以互换，写定义时就丢分。常见的错误是把单一树种的林地当成完整生态系统，其实人工林结构简单、物种数很少。答题时把时间、地点、物种这三条写全，种群的定义才站得住。
The levels of organisation run from a single organism up to a whole ecosystem, and the definitions only work if you keep the boundaries clear. A population is every individual of one species living in the same place at the same time, while a community adds the populations of other species on top. An ecosystem then adds the non-living side, light, temperature, water and soil, which is why examiners refuse to treat the two terms as interchangeable: a single-species plantation is a community with very little structure, not a full ecosystem.
二、非生物因素与生物因素怎样改变群落
群落里的物种组成会被两类因素推着变化。非生物因素包括温度、光照、水分和污染物，它们直接决定哪些物种活得下去。生物因素主要是竞争和捕食，资源有限时争夺光照或食物的个体被淘汰，捕食者数量上升时猎物种群随之下降。样带数据里最常见的情形是从树荫走到空地，植物高度和物种数目一起改变，这种沿环境梯度的分布叫带状分布。很多学生以为只有温度算非生物因素，写污染物时又把它归到生物因素里。判断方法很简单，因素本身不是活的就是非生物。考试里要求两类各举一例，例子写错就白丢一分。
Two families of factors shape which species survive in a community, and the split matters because the exam asks you to name the category as well as the factor. Temperature, light, water and pollutants are abiotic, so they act whether or not another organism is involved, whereas competition and predation are biotic because they depend on living neighbours. Walk a belt transect from deep shade into open ground and the height and number of plant species change together, which is zonation along an environmental gradient rather than random variation.
三、相互依存:食物网里少一个物种会怎样
群落里的物种互相依赖，这种依赖在食物网里看得最清楚。一个物种消失，它的捕食者可能没有替代食物，数量跟着下降。它的猎物失去天敌，短期内数量上升，随后因为食物不足又回落。整个链条因此重新调整，最后稳定在一个不同的物种组成上。很多学生以为食物网只考箭头方向，其实推理题要写的是数量变化的先后顺序。答题顺序建议写成三句，先写受影响的物种，再写原因，最后写新的平衡。常见的错误是把两个方向的变化写成同时发生，阅卷时这种答案通常只给一半分。
Species in a community depend on each other, and a food web is the diagram that makes those links visible. Remove one species and its predator may lose its main food supply, so its numbers fall, while its prey is briefly released from predation and then drops back as food runs short. The chain reorganises into a different mix of species, and the mark is awarded for the order of those changes rather than for the arrows themselves.
四、寄生与互利共生:两种不同的依赖
有些物种的生存完全依赖另一个物种，寄生和互利共生是两种典型关系。寄生物生活在宿主身上或体内，从宿主取得营养，宿主因此受损，绦虫和头虱都属于这一类。互利共生里双方都得到好处，豆科植物和根瘤菌是最常考的一对，细菌把大气中的氮转成植物能用的含氮化合物，植物把光合作用产生的糖供给细菌。很多学生以为互利共生就是双方都不受影响，其实双方都获益才算。寄生关系里宿主不会立刻死亡，这一点经常被写成错误答案。考试常要求两类各举一例，例子写对才给满分。把获益与受损这两个词用准，四分的对比题基本不会丢分。
Parasitism and mutualism are the two dependency relationships the specification names, and the difference comes down to who benefits. A parasite such as a tapeworm or head louse lives on or in its host and takes nutrients from it, so the host is harmed, though it usually survives long enough for the parasite to keep feeding. Mutualism works the other way: leguminous plants and rhizobia both gain, with the bacteria fixing atmospheric nitrogen into compounds the plant can use while the plant supplies sugars from photosynthesis.
五、样方与样带:把看到的数量换成一平方米多少
野外调查没法数清整片草地，所以要抽样。样方是一块固定面积的方框，通常是 0.25 平方米或 1 平方米，随机摆放若干次，数出每框里的个体数。算出平均值后乘以总面积与样方面积的比值，就得到整片区域的估计数。样带是沿环境梯度拉一条线，每隔固定距离放一个样方，用来记录物种如何随位置变化。很多学生以为样方摆得越多越准，其实随机摆放比单纯增加数量更重要，专挑长得密的地方放会系统性偏高。常见的错误是忘记把样方面积换算成平方米，最后单位写错，整道计算题的分全丢。样带数据要按距离画成图，横轴是距离，纵轴是数量或高度。
Fieldwork never counts a whole habitat, so ecologists sample it, and quadrats are the tool that turns a few squares into an estimate for the site. A quadrat of known area is placed at random several times, the individuals inside are counted, and the mean is scaled up by total area divided by quadrat area. A belt transect instead runs a line along an environmental gradient with quadrats at fixed intervals, which shows how the community changes with position rather than how many organisms live there overall.
六、生物量金字塔为什么越往上越窄
能量沿着食物链传递时会大量损失。生物呼吸把一部分能量转成热散失，运动与维持体温也在消耗，排泄物和未消化的部分带走的能量不会被上一级利用。能传到下一个营养级的只是很小一部分，所以营养级越往上，能支撑的个体数与生物量越少。这决定了食物链不会太长，通常四到五级就到顶。很多学生以为顶级捕食者稀少是因为繁殖慢，其实更直接的原因是可用能量少。金字塔的形状因此总是底部宽、顶部窄，倒金字塔只出现在浮游生物季节性消长的特殊水体里。写解释题时把散热、运动、排泄三个去向分开写，比笼统说浪费能量更容易拿分。
Energy is lost at every step up a food chain, and that loss is what shapes the pyramid. Respiration releases energy as heat, movement and keeping warm use more, and the undigested remains leave the chain entirely, so only a small share of the energy at one level reaches the next. That is why chains rarely run beyond four or five levels and why the pyramid is always broad at the base: a top predator is scarce because there is little energy left to support it, not simply because it breeds slowly.
七、传递效率的百分比怎么算
效率题的分几乎全在格式上。公式是下一个营养级的生物量除以本营养级的生物量，再乘 100%。单位必须两边一致，克每平方米对克每平方米，千焦对千焦。算出的小数保留两位，百分号不要漏。很多学生以为效率可以超过 100%，其实只要把两个营养级的位置写反就会出现这种不可能的结果，这也是自查的好办法。常见的错误是拿个体数代替生物量，个体大小差异大时算出来的数没有意义。写答案时把公式、代入过程、结果三行分开，阅卷按步给分，过程分比结果分更稳。
Efficiency questions are marked mainly on method, so the layout matters as much as the arithmetic. Divide the biomass at the higher trophic level by the biomass at the level below it and multiply by 100, keeping both masses in the same unit and the percentage sign on the answer. If the figure comes out above 100 you have divided the wrong way round, and using counts of individuals instead of biomass gives a meaningless result whenever body sizes differ.
八、碳循环:四个过程把碳送回大气
碳在生物与非生物部分之间来回移动。光合作用把大气中的二氧化碳固定成有机物，是碳进入生物体的入口。生物呼吸和分解者的呼吸把有机物里的碳重新放回大气。燃烧化石燃料把封存了很久的碳一次性释放。食物链里的摄食把碳从一个营养级搬到下一个营养级，动物遗体与排泄物再由微生物分解。很多学生以为植物只在白天释放二氧化碳，其实呼吸昼夜都在进行，只是白天光合作用更强，净效果才是吸收。把光合作用、呼吸、燃烧、分解这四个过程写全，碳循环的六分题通常就稳了。
Carbon moves between the living and non-living parts of an ecosystem through a small set of processes. Photosynthesis is the route in, fixing carbon dioxide into organic compounds, while respiration in living organisms and in decomposers releases it back to the atmosphere. Combustion of fossil fuels adds carbon that had been locked away for millions of years, and feeding passes carbon along the food chain until dead bodies and waste are broken down by microorganisms.
九、水循环与饮用水:蒸发、蒸腾与脱盐
水循环的动力来自太阳。海水、湖水和土壤表面的水蒸发进入大气，植物通过蒸腾把根系吸收的水从叶片散失出去。水汽在高空冷却凝结成云，再以降水形式回到地面。落到地面的水一部分形成地表径流，一部分下渗进入地下水，最终回到海洋。很多学生以为蒸腾对水循环贡献很小，其实陆地上的水汽有相当一部分来自植物，砍伐森林会改变当地降水。干旱地区要得到饮用水只能脱盐，蒸馏和反渗透是两种常用做法，成本与能耗是主要限制。考卷常把水循环和碳循环放在一起问，两个循环的驱动力不同，不要混着答。
The water cycle is driven by the sun. Water evaporates from the sea, lakes and soil, plants lose water through transpiration, and the vapour cools and condenses into clouds before returning as precipitation. Some of that water runs off the surface, some percolates into the ground and eventually reaches the sea, and in drought areas drinking water has to come from desalination by distillation or reverse osmosis, where cost and energy use are the limiting factors.
十、氮:化肥、轮作与细菌的作用
植物需要硝酸盐来合成氨基酸和蛋白质，而大气中的氮气不能被直接利用。固氮细菌把氮气转成含氮化合物，豆科植物根部的根瘤菌是典型例子。分解者把动植物遗体中的含氮有机物分解成氨，硝化细菌再把氨逐步氧化成亚硝酸盐和硝酸盐，植物才能吸收。反硝化细菌把硝酸盐还原成氮气，氮回到大气，循环闭合。农业上施化肥能快速补充硝酸盐，过量则随雨水进入水体引起富营养化。轮作豆科作物可以让土壤中的硝酸盐自然恢复，减少化肥依赖，这一条常作为可持续做法的得分点。很多学生以为植物能直接吸收氮气，写氮循环时这一句会把整段的分带偏。
Plants need nitrate to build amino acids and proteins, yet the nitrogen in the air is unavailable to them directly. Nitrogen-fixing bacteria such as rhizobia in legume root nodules convert atmospheric nitrogen into usable compounds, decomposers release ammonia from dead material, and nitrifying bacteria oxidise that ammonia into nitrites and then nitrates. Fertilisers top up soil nitrate quickly but leach into water when over-applied, and rotating legumes into the sequence restores nitrate without the same input.
十一、分解速率:温度、水分与氧气的三条线
分解是微生物把有机物拆成无机物，速率受三个条件控制。温度在某一范围之内升高，酶活性上升，分解加快，超过最适温度后微生物死亡，速率反而下降。水分决定微生物能否活动，干燥的食物之所以能保存，原因就是缺水。氧气决定进行有氧还是无氧分解，密封、真空与充氮包装都在切断氧气。堆肥的做法正相反，要通气、保温并保持湿润，让分解尽可能快。速率计算用变化量除以时间，例如质量减少的克数每天多少克，单位写清楚。常见的错误是只看温度忽略水分，数据图里两条曲线一对比就会露出来。
Decomposition is the breakdown of organic material by microorganisms, and three variables control how fast it runs. Warmer conditions raise enzyme activity up to an optimum, after which the microbes are killed and the rate falls, while the water content decides whether they can work at all, which is why drying preserves food. Oxygen availability separates aerobic from anaerobic decay, so composting does the opposite of preservation: it keeps the heap aerated, warm and moist to break material down quickly.
十二、人类活动、生物多样性与指示物种
人类活动对生态系统既有正面也有负面影响。养殖鱼类提供蛋白质，而密集养殖会排放废物、传播海虱，逃逸个体还会与野生种群杂交。引进外来物种常常没有天敌，灰松鼠与日本虎杖在英国都挤掉了本地物种。化肥和污水带来的硝酸盐会造成富营养化，藻类暴发遮住光照，水下植物死亡，分解者耗掉氧气，鱼类随之死亡。维护生物多样性保住了食物网里的关键环节，重新造林能恢复栖息地并固定碳。指示物种可以用来判断污染程度，血虫和颤蚓指示水质差，淡水虾和石蝇指示水质好，地衣种类与月季黑斑病反映空气质量。很多学生以为富营养化的直接原因是藻类有毒，其实杀鱼的是缺氧这一步。
Human activity cuts both ways in an ecosystem. Fish farming supplies protein, yet dense cages release waste, spread sea lice and let escapees interbreed with wild stocks, while introduced species often arrive without natural enemies, as grey squirrels and Japanese knotweed did in Britain. Nitrate from fertiliser and sewage drives eutrophication, where an algal bloom blocks light, submerged plants die and decomposers strip the oxygen that fish need; keeping biodiversity intact protects the key links in a food web, and indicator species such as bloodworm, freshwater shrimp, stonefly and lichens turn that pollution into evidence.
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