📚 Year 7 AQA Biology: Case Study Practice | Year 7 AQA 生物：案例分析实战演练
In this revision guide, we will work through a realistic case study based on a freshwater pond ecosystem. You will learn how to apply key Year 7 biology concepts – such as food chains, adaptations, bioaccumulation and population change – to unfamiliar data and scenarios. This mirrors the skills required in AQA end‑of‑topic tests and will build your confidence in interpreting biological information.
在这份复习指南中，我们将完成一个基于淡水池塘生态系统的真实案例研究。你将学会如何把七年级生物的核心概念——如食物链、适应、生物累积和种群变化——应用到陌生的数据和情境中。这反映了 AQA 单元测试中所需的技能，并将帮助你建立解读生物信息的信心。
1. The Case Study Scenario | 案例研究情境
Read the following description carefully. A group of students investigated a small freshwater pond in a local park. They recorded the organisms they found and measured the concentration of a toxic pesticide, Chemical X, in the tissues of several species. The pond is surrounded by grass and trees, and receives run‑off from a nearby housing estate. No fishing or boating is allowed, but ducks frequently visit the water.
仔细阅读以下描述。一组学生调查了当地公园里的一个小池塘。他们记录下发现的生物，并测量了几种生物体内有毒农药“化学品X”的浓度。池塘被草地和树木环绕，并接收附近住宅区的地表径流。池塘不允许钓鱼和划船，但鸭子经常来访。
2. Identifying Living Things and Using Keys | 识别生物并使用分类检索表
as legs → go to 2; 1b. No legs → go to 5. Such keys rely on clearly observable features like the number of legs, body segments or presence of wings. Practise using keys to identify common pond creatures such as dragonfly nymphs, water fleas (Daphnia), pond snails and sticklebacks.
在分析池塘生态系统之前，你必须能够识别这些生物。学生们使用了一个简单的二叉式检索表。例如：1a. 有腿 → 转到2；1b. 无腿 → 转到5。这类检索表依赖于清晰可观察的特征，如腿的数量、身体分节或是否有翅膀。练习使用检索表来识别常见的池塘生物，比如蜻蜓幼虫、水蚤（ Daphnia ）、池塘螺和三刺鱼。
3. Constructing Food Chains and Webs | 构建食物链和食物网
The students recorded the following feeding relationships: algae are eaten by water fleas; water fleas are eaten by sticklebacks and dragonfly nymphs; sticklebacks are eaten by herons; dragonfly nymphs are eaten by frogs; frogs are eaten by herons. Dead plant material is consumed by pond snails, which in turn are eaten by sticklebacks. Draw a food web that includes all these organisms. Remember that arrows show the direction of energy flow – from the organism being eaten to the organism that eats it.
学生们记录了以下摄食关系：藻类被水蚤吃；水蚤被三刺鱼和蜻蜓幼虫吃；三刺鱼被苍鹭吃；蜻蜓幼虫被青蛙吃；青蛙被苍鹭吃。死去的植物物质被池塘螺吃，而池塘螺又被三刺鱼吃。绘制一个包含所有这些生物的食物网。记住，箭头表示能量流动的方向——从被吃的生物指向取食的生物。
- Algae → Water flea → Stickleback → Heron
- Algae → Water flea → Dragonfly nymph → Frog → Heron
- Dead plant material → Pond snail → Stickleback → Heron
藻类 → 水蚤 → 三刺鱼 → 苍鹭；藻类 → 水蚤 → 蜻蜓幼虫 → 青蛙 → 苍鹭；死去的植物物质 → 池塘螺 → 三刺鱼 → 苍鹭。
4. Producers, Consumers and Predators | 生产者、消费者和捕食者
In this pond, algae and other aquatic plants are producers because they make their own food by photosynthesis. All other organisms are consumers. Primary consumers (herbivores) like water fleas and pond snails eat producers. Secondary consumers (carnivores) eat primary consumers: sticklebacks and dragonfly nymphs are examples. Tertiary consumers, such as herons, feed on secondary consumers. Identify the apex predator in this food web – the organism with no natural predators in this system.
在这个池塘里，藻类和其他水生植物是生产者，因为它们通过光合作用制造自己的食物。所有其他生物都是消费者。初级消费者（草食动物）如水蚤和池塘螺以生产者为食。次级消费者（肉食动物）吃初级消费者：三刺鱼和蜻蜓幼虫就是例子。三级消费者，如苍鹭，以次级消费者为食。找出这个食物网中的顶级捕食者——即在该系统中没有天敌的生物。
In this case, the heron is the apex predator. It is not eaten by any other organism within the pond community.
在这个例子中，苍鹭是顶级捕食者。在池塘群落内，它不被任何其他生物取食。
5. Energy Flow and Loss | 能量流动与损失
At each trophic level, only about 10% of the energy is passed on to the next level. The rest is lost as heat from respiration, used for movement and growth, or excreted as waste. This explains why food chains are rarely longer than four or five trophic levels – there simply is not enough energy to support another level. Draw a pyramid of numbers for the pond: typically, many algae support fewer water fleas, even fewer sticklebacks and very few herons.
在每个营养级，大约只有10%的能量传递到下一级。其余的能量以呼吸产热的形式散失，用于运动和生长，或作为废物排出。这就解释了为什么食物链很少有超过四到五个营养级的——根本就没有足够的能量来支撑额外的层级。为池塘绘制一个数量金字塔：通常，许多藻类支撑了较少的水蚤，更少的三刺鱼和极少的苍鹭。
6. Adaptations of Pond Organisms | 池塘生物的适应
Each species is adapted to survive in its habitat. Water fleas have transparent bodies that make them hard for predators to spot. Dragonfly nymphs possess extendable jaws to snatch prey quickly. Sticklebacks have streamlined bodies and fins for rapid swimming. Frogs have moist skin for cutaneous respiration and powerful hind legs for jumping. Explain how one of these adaptations increases the organism’s chance of survival and reproduction.
每个物种都适应了在其栖息地中生存。水蚤身体透明，使捕食者难以发现。蜻蜓幼虫拥有可伸展的下颚，能迅速抓取猎物。三刺鱼拥有流线型的身体和鱼鳍，便于快速游泳。青蛙有湿润的皮肤可进行皮肤呼吸，并有强壮的后腿用于跳跃。解释其中一种适应如何增加该生物的生存和繁殖机会。
For example, the transparent body of a water flea makes it less visible to sticklebacks, reducing the chance of being eaten. This means the water flea can survive longer and produce more offspring, passing on its genes for transparency.
例如，水蚤透明的身体使其不易被三刺鱼发现，降低了被吃掉的可能性。这意味着水蚤可以存活更长时间，产生更多后代，将透明度这一基因传递下去。
7. Bioaccumulation of Chemical X | 化学品X的生物累积
The students measured the concentration of Chemical X in the water and in the tissues of several species. Their results are shown in the table below. Pesticides that do not break down easily can accumulate in the bodies of organisms. This process is called bioaccumulation. As you move up the food chain, the concentration of the pesticide increases, a phenomenon known as biomagnification.
学生们测量了水中以及几种生物组织内化学品X的浓度。结果如下表所示。不易分解的农药会在生物体内累积。这一过程称为生物累积。随着你沿食物链向上移动，农药的浓度增加，这种现象称为生物放大。
| Organism / 生物 | Trophic level / 营养级 | Chemical X concentration (ppm) / 化学品X浓度 (ppm) |
|---|---|---|
| Pond water / 池塘水 | - | 0.02 |
| Algae / 藻类 | Producer / 生产者 | 0.25 |
| Water flea / 水蚤 | Primary consumer / 初级消费者 | 2.1 |
| Stickleback / 三刺鱼 | Secondary consumer / 次级消费者 | 18.5 |
| Heron / 苍鹭 | Tertiary consumer / 三级消费者 | 152.0 |
Explain why the heron has the highest concentration of Chemical X. As it eats many sticklebacks, each containing 18.5 ppm, the pesticide accumulates in its body over time and is not easily excreted. Top predators are therefore most at risk from toxic pollutants.
解释为什么苍鹭体内化学品X的浓度最高。因为它吃掉许多三刺鱼，每条鱼含有18.5 ppm，农药随时间在其体内累积且不易排出。因此，顶级捕食者最容易受到有毒污染物的威胁。
8. Population Changes and Interdependence | 种群变化与相互依存
Imagine that a disease kills most of the frogs in the pond. Predict and explain how this might affect the populations of dragonfly nymphs, sticklebacks and herons. Use the food web you constructed. Without frogs, the heron loses a food source, so heron numbers may decline. Dragonfly nymphs are no longer eaten by frogs, so their population could increase. More dragonfly nymphs eat more water fleas, reducing the food available for sticklebacks, which may then decrease. These knock‑on effects illustrate interdependence.
想象一下，一种疾病杀死了池塘里的大部分青蛙。预测并解释这将如何影响蜻蜓幼虫、三刺鱼和苍鹭的种群数量。使用你构建的食物网。没有了青蛙，苍鹭失去了一个食物来源，因此苍鹭数量可能下降。蜻蜓幼虫不再被青蛙捕食，因此其数量可能增加。更多的蜻蜓幼虫吃掉更多的水蚤，减少了三刺鱼可获取的食物，三刺鱼的数量可能因此减少。这些连锁反应说明了生物的相互依存。
9. Sampling Techniques and Data Analysis | 取样技术与数据分析
To estimate the population of pond snails, the students used a quadrat and a kick‑ nd counted the snails within it. They repeated this 10 times and calculated a mean. Explain why using a large number of samples makes the estimate more reliable. More samples reduce the effect of chance variation and give a mean that is closer to the true population size.
为了估算池塘螺的数量，学生们使用了样方和踢网采样法。他们随机在浅水区放置0.5 m × 0.5 m的样方，并计数样方内的螺。他们重复了10次并计算平均值。解释为什么使用大量样本能使估计更可靠。更多的样本降低了偶然变异的影响，得出的平均值更接近真实的种群大小。
(total pond area / quadrat area). Always show your working and remember to include units.
他们还使用公式计算了估算的总数量：总螺数 =（每个样方平均数）×（池塘总面积 / 样方面积）。务必展示你的计算过程，并记住写上单位。
Total snails = mean × (pond area ÷ quadrat area)
总螺数 = 平均数 ×（池塘面积 ÷ 样方面积）
10. Designing a Follow‑Up Investigation | 设计后续调查
A student suggests that the pesticide enters the pond from the housing estate run‑off. Describe how you could test this hypothesis. You would need to sample water at different points: upstream of the estate, immediately after the run‑off enters the pond, and in the centre of the pond. Measure the concentration of Chemical X in each sample. A gradient of concentration decreasing from the run‑off point would support the hypothesis. Remember to control variables such as time of day and sampling depth.
一名学生提出，农药是从住宅区的径流进入池塘的。描述你如何检验这一假说。你需要在不同地点采集水样：住宅区上游、径流刚进入池塘处、以及池塘中心。测量每个样品中化学品X的浓度。如果浓度从径流入口向外递减，就能支持该假说。记住控制变量，如一天中的同一时间和采样深度。
11. Human Impact and Conservation | 人类影响与自然保护
The pond ecosystem is affected by human activities. Litter, fertilisers and pesticides from gardens can alter the balance. Fertilisers cause eutrophication: extra nutrients lead to algal blooms, which block sunlight and use up oxygen when they decompose, killing fish and other organisms. Suggest ways to reduce negative human impact. Possible solutions include creating a buffer zone of vegetation to absorb run‑off, using organic fertilisers, and educating the community about responsible disposal of chemicals.
池塘生态系统受到人类活动的影响。来自花园的垃圾、肥料和农药会改变平衡。肥料导致富营养化：额外的养分引起藻类大量繁殖，遮挡阳光，并在分解时消耗氧气，杀死鱼类和其他生物。提出减少负面人类影响的方法。可能的解决方案包括：建立植被缓冲带以吸收径流，使用有机肥料，以及教育社区负责任地处理化学品。
12. Common Mistakes and Examiner Tips | 常见错误与考官提示
When answering case study questions, avoid these pitfalls: (1) Drawing food chains with arrows pointing the wrong way – always from food to feeder. (2) Forgetting to label axes and units when plotting graphs. (3) Confusing bioaccumulation with biodegradation – bioaccumulation means building up, not breaking down. (4) Describing a population change without explaining it using the food web. Always refer back to the data and use scientific vocabulary such as 'consumer', 'predator', 'prey' and 'interdependence'.
在回答案例分析问题时，要避免这些陷阱：（1）绘制食物链时箭头方向错误——总是从食物指向取食者。（2）绘制图表时忘记标注坐标轴和单位。（3）将生物累积与生物降解混淆——生物累积是指积累，而不是分解。（4）描述种群变化时没有用食物网加以解释。始终要引用数据，并使用科学词汇，如“消费者”、“捕食者”、“猎物”和“相互依存”。
Practise by writing a full paragraph explaining why a change in one population can affect several others. Use the pond scenario and include links to at least two other species.
通过写一个完整的段落来练习，解释为什么一个种群的变化会影响其他几个种群。使用池塘情景，并包含至少与其他两种生物的联系。
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