OCR生物 生物多样性 物种指数 采样 考点
生物多样性 (Biodiversity) 是 OCR A-Level Biology (H420) Module 4 "Biodiversity, evolution and disease" 的核心考点，也是历年 Paper 2 与 Paper 3 简答题与数据分析题的高频来源。很多同学能背出定义，却在辛普森多样性指数的计算、采样方案的合理性评价、以及保护措施的分类上丢分。本文从概念层次讲到定量计算，再讲到 Required Practical 的采样细节，最后归纳影响因素与保护策略，帮你把这一章的分析分拿满。
Biodiversity is one of the most heavily examined topics in OCR A-Level Biology (H420) Module 4, "Biodiversity, evolution and disease". It appears in Paper 2 short-answer questions and in Paper 3 data-analysis questions almost every series. Students usually memorise the definition without difficulty, yet they lose marks on three predictable points: calculating Simpson's Index of Diversity, justifying a sampling method, and classifying conservation strategies. This article moves from definitions to quantitative calculation, then to the required practical and finally to the factors that threaten biodiversity, so that you can answer both recall and analysis questions with confidence.
1. 生物多样性的三个层次
生物多样性不是一个单一指标，它包括三个层次。第一是遗传多样性 (genetic diversity)：同一物种内部等位基因 (alleles) 的差异，例如同一片林地中大麦草的不同抗病等位基因。第二是物种多样性 (species diversity)：一个生态系统中物种的数量与各物种个体数的相对比例，它同时取决于物种丰富度 (species richness) 与均匀度 (evenness)。第三是生态系统多样性 (ecosystem diversity)：不同栖息地类型与生态过程的差异，例如同一区域内湿地、林地与草甸的共存。考试中若问 "state what is meant by biodiversity"，标准答案必须同时点出遗传、物种与生态系统三个层次，只写 species 一层最多拿一半分。遗传多样性高的种群在环境剧变时更可能存活，因为某些等位基因可能带来耐受性的优势，这正是自然选择 (natural selection) 的原料。
Biodiversity is not a single measurement. It has three components. Genetic diversity describes the variety of alleles within a single species; a population of the same grass species can carry different disease-resistance alleles. Species diversity describes both the number of species in a habitat and the relative abundance of each species, which means it depends on both species richness and evenness. Ecosystem diversity describes the range of habitats and ecological processes present, such as wetland, woodland and meadow existing side by side. If an exam question asks you to state what is meant by biodiversity, a full-mark answer must mention all three components: genetic, species and ecosystem. A population with high genetic diversity is more likely to survive an environmental change, because at least some alleles may confer tolerance. This is why genetic diversity matters: it is the raw material of natural selection.
2. 物种丰富度与均匀度
物种丰富度 (species richness) 指一个区域内不同物种的数目，它只统计 "有多少种"，不关心每种有多少个个体。均匀度 (species evenness) 指各物种个体数目的接近程度。两块样地可能物种丰富度完全相同，但均匀度差别很大。例如样地 A 有 10 种，每种各 20 个个体，那么它非常均匀；样地 B 也有 10 种，但其中一种有 190 个个体，其余 9 种各只有 1 个，那么它极不均匀。均匀度低的群落通常说明存在竞争排斥或人类干扰，少数物种占据主导。这里有一个常见误区：物种数目多并不等于生物多样性高，如果某一个物种压倒性占据资源，群落抵抗干扰的能力反而更弱，因为一旦病原体针对该优势种，整个群落结构就会崩塌。
Species richness is the number of different species present in an area. It counts how many species there are and ignores how many individuals each species contributes. Species evenness describes how close the population sizes of those species are to one another. Two quadrats can have identical species richness but very different evenness. Suppose site A contains 10 species, each with 20 individuals: it is highly even. Site B also contains 10 species, but one species has 190 individuals while the other nine have only one each: it is extremely uneven. Low evenness usually indicates competitive exclusion or human disturbance, with a small number of species dominating the community. A common misconception is to treat species richness as equivalent to biodiversity. It is not. If one species dominates the resource base, the community may be less resilient, because a pathogen that targets the dominant species can collapse the whole structure.
3. 辛普森多样性指数的计算
辛普森多样性指数 (Simpson's Index of Diversity) 用公式 D = 1 - Σ(n/N)² 计算，其中 n 是某一物种的个体数，N 是所有物种个体总数，Σ 表示对所有物种求和。D 的取值范围在 0 到 1 之间，越接近 1 表示多样性越高。计算步骤只有四步：先求总数 N，再逐个求 n/N，再平方并求和，最后用 1 减去该和。举例：样地中有三个物种，个体数分别为 20、30、50，则 N = 100，n/N 分别等于 0.2、0.3、0.5，平方和为 0.04 + 0.09 + 0.25 = 0.38，于是 D = 1 - 0.38 = 0.62。若另一块样地同样是 100 个个体但分布为 90、5、5，则平方和为 0.81 + 0.0025 + 0.0025 = 0.815，D = 0.185，明显更低。答题时必须写单位与比较结论，例如 "D 值从 0.62 降至 0.185，说明该地物种多样性下降，可能是单一作物种植导致均匀度降低"，只写数字不给结论会丢分析分。
The Simpson's Index of Diversity is calculated as D = 1 - sum of (n/N)^2, where n is the number of individuals of one species, N is the total number of individuals of all species, and the sum runs over every species present. The index ranges from 0 to 1, and values closer to 1 indicate greater diversity. The calculation takes four steps: find the total N, calculate n/N for each species, square those values and add them, then subtract the total from 1. For example, a habitat contains three species with 20, 30 and 50 individuals. Then N = 100 and the fractions are 0.2, 0.3 and 0.5. Their squares add to 0.04 + 0.09 + 0.25 = 0.38, so D = 1 - 0.38 = 0.62. A second site with the same 100 individuals split 90, 5 and 5 gives 0.81 + 0.0025 + 0.0025 = 0.815, so D = 0.185. Always finish with an interpretation: a fall from 0.62 to 0.185 means diversity has decreased, which may be caused by monoculture reducing evenness. A raw number without a conclusion usually earns no analysis mark.
4. 采样方法:样方与样带
Required Practical 里考查的采样方法主要有两种。随机取样 (random sampling) 使用随机数生成器产生坐标，把样方 (quadrat) 放在该位置，用于比较两个区域的平均物种数或百分比覆盖度。系统性取样 (systematic sampling) 沿着一条样带 (belt transect) 每隔固定距离放置一个样方，用于观察环境梯度，例如从海滩高潮线到低潮线的物种变化。随机取样必须真正做到随机，允许样方落在地面上，不能刻意避开空地，否则会引入主观偏差 (bias)，使结果失去代表性。评价采样方案时通常可以提出三点改进：增加样方数量以提高可靠性；使用更大或更多的样方以减少偶然误差；采用随机而不是有选择性的位置以避免人为偏差。计算平均值时要注意，比较的是每个样方的平均数，因为样方大小相同，所以平均值可以直接比较不同区域的物种丰富度。
The required practical examines two sampling approaches. Random sampling uses a random number generator to produce coordinates and places a quadrat at that position; it is used to compare mean species number or percentage cover between two areas. Systematic sampling places quadrats at fixed intervals along a belt transect; it is used to follow an environmental gradient, for example the change in species from the high-water mark to the low-water mark on a shore. Random sampling must be genuinely random, and quadrats should be allowed to land on bare ground. Deliberately avoiding bare patches introduces bias and makes the estimate unrepresentative. When you are asked to evaluate a sampling method, three improvements are usually expected: increase the number of quadrats to improve reliability, use a larger quadrat or more replicates to reduce the effect of chance, and select positions randomly rather than selectively to avoid bias. Compare mean values per quadrat, because equal quadrat sizes make means directly comparable between areas.
5. 影响生物多样性的因素
人类活动是当前生物多样性下降的主要原因，OCR 常考的条目可以归为四类。第一是栖息地破坏与碎片化，例如森林砍伐、湿地排干或城市化，直接减少可用面积并隔离种群，使基因流动受限。第二是农业实践，单一作物种植 (monoculture) 与广谱除草剂、杀虫剂的使用直接降低植物与昆虫的物种丰富度与均匀度。第三是过度开发，例如过度捕捞与非法采伐，使目标物种的种群数量低于可恢复水平。第四是外来物种入侵与气候变化，外来物种通过竞争或捕食排挤本地物种，而气温与降水格局改变会使部分物种的适宜分布区向高纬度移动。答题时要注意因果链的完整性：人类活动导致栖息地减少，进而导致种群变小，进而导致遗传多样性下降，最终使该物种面对疾病或气候波动时更容易灭绝。
Human activity is the main cause of the current decline in biodiversity, and the OCR syllabus organises the factors into four groups. The first is habitat destruction and fragmentation, such as deforestation, drainage of wetlands or urbanisation, which reduces the available area, isolates populations and limits gene flow. The second is agricultural practice: monoculture, broad-spectrum herbicides and insecticides reduce both the richness and the evenness of plants and insects. The third is overexploitation, including overfishing and illegal logging, which pushes populations of the target species below a sustainable level. The fourth is invasive species and climate change. Invasive species outcompete or prey on native species, while shifts in temperature and rainfall move the suitable range of many species towards higher latitudes. In written answers, show the complete causal chain: human activity reduces habitat, populations become smaller, genetic diversity falls, and the species becomes more vulnerable to disease or climate fluctuation. Full chains score better than isolated statements.
6. 保护策略与考试技巧
保护策略分为就地保护与迁地保护。就地保护 (in-situ conservation) 在原有栖息地内建立保护区、国家公园与海洋保护区，优点是维持生态过程与物种间关系，种群可自然演化；缺点是保护区可能被分割、易受气候与人类活动影响。迁地保护 (ex-situ conservation) 包括动物园、植物园与种子库 (seed bank)，优点是能为濒危物种提供安全的繁殖环境并进行人工繁育；缺点是样本量小、适应野外环境的能力可能下降。国际协议如生物多样性公约 (CBD) 与 CITES 通过限制贸易与制定国家目标起作用。答题技巧上有三点：定义题写全三层含义；计算题保留步骤并写出结论；评价题从 "可靠性、代表性、偏差、样本量" 四个词入手。另外，绘图题必须标注坐标轴名称与单位，并画出平滑曲线而不是折线。
Conservation strategies fall into two categories. In-situ conservation protects species inside their original habitat through reserves, national parks and marine protected areas. Its strength is that ecological processes and species interactions continue, and populations can keep evolving. Its weakness is that reserves may be fragmented and remain exposed to climate change and human pressure. Ex-situ conservation includes zoos, botanic gardens and seed banks. It provides a safe breeding environment and enables captive breeding programmes, but the population is small and may lose the ability to survive in the wild. International agreements such as the Convention on Biological Diversity and CITES work by restricting trade and setting national targets. For exam technique, remember three points. Definition questions need all three components of biodiversity. Calculation questions need visible steps plus a written interpretation. Evaluation questions are best answered using four keywords: reliability, representativeness, bias and sample size. On graph questions, label both axes with units and draw a smooth curve rather than straight lines between points.
7. 高频考点速记与常见错误
把这一章的考点压缩成五条速记。第一，生物多样性的定义必须包含遗传、物种、生态系统三个层次。第二，物种多样性由丰富度与均匀度共同决定，丰富度高不代表多样性高。第三，辛普森指数的公式是 D = 1 - Σ(n/N)²，计算后必须写解释。第四，采样要随机、要重复、要足够多的样方，评价时用偏差与可靠性两个词。第五，保护策略分就地与迁地，答案中要分别写出优点与局限。常见错误有四个：把物种数目直接当作多样性；计算时忘记平方或忘记用 1 去减；把样方放在 "看起来物种更多" 的位置导致偏差；只列保护措施而不解释其作用机制。若题目要求比较两个生境，答案结构应是先给出两组数据，再给出计算值，最后给出结论并解释原因，三步缺一都会扣分。
Here is a five-point summary of the topic. First, a definition of biodiversity must cover genetic, species and ecosystem components. Second, species diversity depends on both richness and evenness, so high richness alone does not guarantee high diversity. Third, the Simpson's Index formula is D = 1 - sum of (n/N)^2, and the calculated value must be followed by an interpretation. Fourth, sampling must be random, replicated and based on enough quadrats; use the words bias and reliability when evaluating a method. Fifth, conservation strategies are either in-situ or ex-situ, and answers should state both the advantage and the limitation of each. Four mistakes appear again and again. Students treat the number of species as if it were diversity itself. They forget to square the fractions or forget to subtract the total from 1. They place quadrats where the vegetation looks richer, which introduces bias. They list conservation measures without explaining how those measures work. When a question asks you to compare two habitats, the expected structure is: present both data sets, show the calculated index values, then state a conclusion and explain the reason behind it.
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