📚 Biodiversity: Measuring, Threats and Conservation | 生物多样性：测量、威胁与保护
Biodiversity describes the variety of life at every level, from the alleles within a single gene pool to the ecosystems that cover the planet. In Biology 4.2, the focus is on defining biodiversity, measuring species and genetic diversity, and evaluating the human activities that reduce it.
生物多样性描述从单一基因库中的等位基因到覆盖整个地球的生态系统等各个层次的生命多样性。在生物 4.2 中，重点是定义生物多样性、测量物种多样性和遗传多样性，并评估人类活动如何使其降低。
1. Defining Biodiversity | 生物多样性的定义
Biodiversity is the total variety of living organisms in a given area. It includes three main levels: species diversity, genetic diversity, and ecosystem diversity.
生物多样性是某一特定区域内生物的总多样性。它包括三个主要层次：物种多样性、遗传多样性和生态系统多样性。
Species diversity refers to the number of different species and their relative abundance in a community. Genetic diversity is the variety of alleles within a species or population. Ecosystem diversity refers to the range of different habitats and ecological communities.
物种多样性指群落中不同物种的数量及其相对丰度。遗传多样性是物种或种群内等位基因的多样性。生态系统多样性指不同栖息地和生态群落的范围。
2. Species Diversity and Species Richness | 物种多样性与物种丰富度
Species richness is simply the number of different species present in a habitat. It is quick to record but can be misleading because it does not account for how many individuals belong to each species.
物种丰富度仅仅是栖息地中存在的不同物种数。记录快捷，但可能产生误导，因为它没有考虑每个物种分别有多少个体。
A community with two species may have 50 individuals of each, while another has 99 of one and 1 of the other. Both have the same richness, but the first is more even and therefore more diverse.
一个群落可能有两个物种，各有 50 个个体；另一个群落有 99 个个体属于一种，1 个属于另一种。两者的丰富度相同，但第一个群落的均匀度更高，因此多样性更高。
Species diversity combines richness with evenness. Ecologists use an index of diversity to quantify this in a single value.
物种多样性将丰富度与均匀度结合起来。生态学家使用多样性指数将其量化为一个数值。
3. Index of Diversity (Simpson’s Index) | 多样性指数（辛普森指数）
One common measure is Simpson’s index of diversity. A widely used form is:
常用的测量方法是辛普森多样性指数。广泛使用的公式为：
D = 1 − Σ(n/N)²
where N is the total number of organisms of all species, n is the number of organisms of a particular species, and Σ means the sum across all species.
其中 N 是所有物种的个体总数，n 是某一特定物种的个体数，Σ 表示对所有物种求和。
The value of D ranges from 0 to 1. A value close to 1 indicates high diversity, while a value close to 0 indicates low diversity because one or a few species dominate.
D 值范围在 0 到 1 之间。接近 1 表示多样性高；接近 0 表示多样性低，因为一个或少数几个物种占绝对优势。
Some exam specifications use the alternative form D = N(N − 1) / Σ n(n − 1). Always use the formula supplied in the question or your specification.
一些考试大纲使用另一种形式 D = N(N − 1) / Σ n(n − 1)。始终使用题目或大纲中给出的公式。
For example, a sample contains 20 daisies, 10 clover plants and 20 grasses. Here N = 50. D = 1 − [(20/50)² + (10/50)² + (20/50)²] = 1 − (0.16 + 0.04 + 0.16) = 0.64.
例如，一个样本包含 20 株雏菊、10 株三叶草和 20 株禾草。此时 N = 50。D = 1 − [(20/50)² + (10/50)² + (20/50)²] = 1 − (0.16 + 0.04 + 0.16) = 0.64。
If 48 of the 50 plants were one species, D would be very low, reflecting low diversity.
如果 50 株植物中有 48 株属于同一物种，D 值会非常低，反映出低多样性。
4. Genetic Diversity and Heterozygosity | 遗传多样性与杂合度
Genetic diversity is the variety of alleles within a population. A high level of genetic diversity allows a species to adapt to environmental change and reduces the risk of inherited diseases.
遗传多样性是种群内等位基因的多样性。高水平的遗传多样性使物种能够适应环境变化，并降低遗传疾病的风险。
Genetic diversity can be estimated using the heterozygosity index:
遗传多样性可以用杂合度指数来估算：
Heterozygosity index = number of heterozygotes / total number of individuals
For a gene with two alleles, heterozygotes have two different alleles, so a higher proportion of heterozygotes indicates a more genetically diverse population.
对于一个具有两个等位基因的基因，杂合子拥有两个不同的等位基因，因此杂合子比例越高，说明种群遗传多样性越高。
Loss of genetic diversity often occurs when populations become small, for example after a severe bottleneck or habitat fragmentation. Small populations may experience inbreeding, which reduces fitness.
当种群变得很小时，遗传多样性往往会丧失，例如在严重的瓶颈效应或栖息地破碎化之后。小种群可能发生近亲繁殖，从而降低适合度。
5. Ecosystem Diversity | 生态系统多样性
Ecosystem diversity refers to the variety of habitats, communities and ecological processes in an area. It includes terrestrial habitats such as woodland, grassland and wetland, as well as aquatic habitats such as lakes, rivers and coral reefs.
生态系统多样性指一个区域内栖息地、群落和生态过程的多样性。它包括林地、草地和湿地等陆地栖息地，以及湖泊、河流和珊瑚礁等水生栖息地。
A landscape with many different habitats usually supports more species because it provides a wider range of niches, food sources and breeding sites.
具有许多不同栖息地的景观通常能支持更多物种，因为它提供了更多样的生态位、食物来源和繁殖场所。
Ecosystem diversity also includes interactions among species, such as pollination, predation, decomposition and nutrient cycling. These processes are often disrupted when habitat structure is simplified.
生态系统多样性还包括物种之间的相互作用，如传粉、捕食、分解和养分循环。当栖息地结构被简化时，这些过程往往会受到干扰。
6. Sampling Biodiversity in the Field | 野外生物多样性取样
Ecologists cannot usually count every organism in a habitat, so they sample a representative portion. Random sampling using quadrats is common for plants and slow-moving organisms.
生态学家通常无法清点栖息地中的每一个生物，因此他们取样一个有代表性的部分。使用样方进行随机取样常用于植物和移动缓慢的生物。
To avoid bias, sample sites should be chosen randomly, for example using random number coordinates. A quadrat is placed at each site, and the abundance of each species is recorded.
为了避免偏差，取样点应随机选择，例如使用随机数坐标。将样方放置在每个取样点，并记录每个物种的丰度。
For organisms distributed along an environmental gradient, a belt transect or line transect can be used. Quadrats are placed at regular intervals along the line to show how species composition changes.
对于沿环境梯度分布的生物，可使用样带（belt transect）或线状样带（line transect）。沿线每隔一定距离放置样方，以显示物种组成如何变化。
Abundance can be recorded as frequency, percentage cover, or a scale such as ACFOR. Percentage cover is useful for plants but may exceed 100% when layers overlap.
丰度可以记录为频度、覆盖百分比或 ACFOR 等级。覆盖百分比对植物很有用，但当层次重叠时可能超过 100%。
7. Estimating Population Size: Mark-Release-Recapture | 种群数量估计：标记重捕法
For mobile animals, the mark-release-recapture method estimates population size. A sample of animals is caught, marked harmlessly and released. Later, a second
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