Systematic Sampling | 系统抽样

📚 Systematic Sampling | 系统抽样

In ecological fieldwork, reliable estimates of the distribution and abundance of organisms depend on choosing the right sampling method. Systematic sampling is a structured technique in which samples are taken at regular, predetermined intervals along a line or across an area. It is particularly useful for detecting changes in species composition along an environmental gradient.

在生态学野外调查中，对生物分布与丰度的可靠估计依赖于选择合适的采样方法。系统抽样是一种结构化技术，按照预先设定的固定间隔沿线或跨区域采集样本。它特别适合探测物种组成沿环境梯度的变化。

1. What Is Systematic Sampling? | 什么是系统抽样？

Systematic sampling involves placing quadrats or recording points at equal intervals rather than at random positions. The start point may be chosen randomly to reduce bias, but after that every nth position is sampled. This creates regularity and allows ecologists to describe how abundance changes with distance.

系统抽样是指以相等间隔放置样方或记录点，而不是随机位置。起点可以随机选择以减少偏差，但之后每隔第 n 个位置采样。这产生了规律性，使生态学家能够描述丰度如何随距离变化。

• Line transect: a tape or rope is laid along the gradient; organisms touching the line are recorded. 样线法：沿梯度铺设卷尺或绳子；记录与线接触的生物。
• Belt transect: two parallel lines create a strip; quadrats are placed at fixed intervals within the strip. 样带法：两条平行线形成带状；在带内以固定间隔放置样方。
• Point quadrat: a vertical pin is lowered at fixed points; species touching the pin are recorded. 点样方：在固定点垂直放下细针；记录接触细针的物种。

2. When Is Systematic Sampling Used? | 何时使用系统抽样？

Systematic sampling is the method of choice when the habitat shows a clear environmental gradient, such as light, moisture, salinity, altitude, or soil pH. It allows a regular description of zonation, for example from the seashore high-water mark down to the low-water mark, or from a hedge into an open field.

当生境表现出明显的环境梯度（如光照、湿度、盐度、海拔或土壤 pH）时，首选系统抽样。它可以规律地描述带状分布，例如从海岸高潮线向下至低潮线，或从树篱进入开阔田地。

Random sampling is better for uniform habitats where an unbiased estimate of overall density is needed. Systematic sampling is better for gradient habitats where the aim is to reveal change, not just an average.

随机抽样更适合均匀生境，在需要无偏估计总体密度时使用。系统抽样更适合梯度生境，其目的是揭示变化，而不仅仅是平均值。

3. Line Transects | 样线法

A line transect is a long, straight string or measuring tape stretched across the habitat. The observer walks along the line and records each organism that touches or crosses the line. It is quick and ideal for conspicuous plants or sessile animals; however, it gives presence/absence or frequency along the line, not true density per unit area.

样线法是横跨生境拉一根长直的绳子或卷尺。观察者沿线行走，记录每一个接触或穿过线的生物。它快速，适合显眼的植物或固着动物；但它给出的是沿线的存在/缺失或频度，而不是单位面积真实密度。

Typical use includes recording zonation of seaweeds on a rocky shore or changes in plant species from a path into woodland. The line should be laid at right angles to the gradient to capture the maximum change in the shortest distance.

典型用途包括记录岩石海岸上大型海藻的带状分布，或从小路进入林地时植物种类沿环境梯度的变化。样线应与梯度方向成直角铺设，以便在最短距离内捕捉到最大变化。

4. Belt Transects | 样带法

A belt transect is a widened strip of habitat, often created by laying two parallel lines a set distance apart, such as 1 m. Quadrats are placed at fixed intervals along the strip. This gives quantitative data on density, frequency, and percentage cover within a known area, so it is better for estimating abundance than a line transect.

样带法是一条加宽的栖息地带，通常通过铺设两条相距一定距离（如 1 m）的平行线形成。沿线带以固定间隔放置样方。这样可以获得已知面积内的密度、频度和百分比盖度等定量数据，因此比样线法更适合估计丰度。

Because the belt has a defined width and length, the area of each sampling position is known. This allows the number of individuals to be converted into a density per square metre, making results comparable between sites or repeat surveys.

由于样带具有明确的宽度和长度，每个采样位置的面积是已知的。这样可以将个体数转换为每平方米的密度，使结果可在不同地点或重复调查之间进行比较。

5. Quadrats Along a Transect | 沿样带设置样方

A quadrat is a square frame, often 0.25 m² or 1 m², used to define a sampling area. In systematic sampling, quadrats are placed at regular intervals, e.g., every 2 m or every 5 m, along the transect line. The number of individuals of each species, or percentage cover, is recorded in each quadrat. The regular spacing ensures the entire gradient is sampled evenly.

样方是一个方形框，通常为 0.25 m² 或 1 m²，用于界定采样面积。在系统抽样中，样方沿样带线以固定间隔（如每 2 m 或每 5 m）放置，并记录每个样方中各物种的个体数或百分比盖度

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