分类、生物多样性与保护

A species 物种 can be defined in more than one way:

• the biological species concept — a group whose members can breed together to produce fertile offspring.
• the morphological species concept — a group whose members look alike.
• the ecological species concept — a group that fills the same role in its surroundings.

The three domains

The largest groups in classification 分类 are three domains 域:

• Archaea 古菌 and Bacteria 细菌 — both are prokaryotes 原核生物 (no nucleus). They look similar but differ in their membrane lipids, their ribosomal RNA, and the make-up of their cell walls 细胞壁.
• Eukarya 真核生物 — all organisms whose cells have a nucleus.

The taxonomic hierarchy

Inside the Eukarya domain, organisms are sorted into a taxonomic hierarchy 分类层级 — a set of smaller and smaller groups: kingdom 界, phylum 门, class 纲, order 目, family 科, genus 属 and species.

The Eukarya are split into four kingdoms:

• Protoctista 原生生物界 — mostly single-celled (such as Amoeba).
• Fungi 真菌界 — feed by absorbing food; have cell walls of chitin.
• Plantae 植物界 — make their own food by photosynthesis.
• Animalia 动物界 — feed on other organisms.

Viruses are not placed in these groups. They are classified by the type of nucleic acid 核酸 they contain (DNA or RNA) and whether it is single-stranded 单链 or double-stranded 双链.

An ecosystem 生态系统 is all the living things in an area together with their non-living surroundings. A niche 生态位 is the particular role and place of a species within it.

A coral reef is a good picture of high biodiversity — many different corals, fish and other species living together:

Biodiversity 生物多样性 can be measured at three levels:

• the number and range of different ecosystems and habitats 栖息地.
• the number of species and their relative abundance 丰度 (how common each one is).
• the genetic variation within each species.

Sampling an area

You cannot count every organism, so you take samples. Random sampling 随机取样 (choosing positions by chance) avoids bias and gives a fair picture. Useful methods are:

• quadrats 样方 — square frames placed to count or estimate the species inside them.
• transects 样带 — counting along a line across the area (a line transect records what touches the line; a belt transect counts within a strip).
• mark-release-recapture 标志重捕法 — for moving animals: catch, mark and release some, then later see what fraction of a new catch is marked (the Lincoln index).

To link the spread of a species to biotic factors 生物因素 (living) or abiotic factors 非生物因素 (non-living), you can use Spearman's rank or Pearson's correlation 相关性. To measure the diversity of an area as a single number, you use Simpson's index of diversity 辛普森多样性指数: a higher value means more diverse and usually more stable.

Worked example. 60 snails are caught, marked and released. Later, 80 snails are caught, of which 15 are marked. Estimate the population. The Lincoln index assumes the marked animals have mixed back in evenly, so the fraction marked in the second sample equals the fraction marked in the whole population:

So there are about 320 snails. The estimate is only as good as its assumptions, so state them: no births, deaths or migration between the two catches; the marks neither rub off nor make the animal easier for a predator to spot; and enough time is allowed for mixing. Break one and the estimate breaks with it - a mark that attracts predators lowers the recapture count and therefore overestimates the population.

A species can become extinct 灭绝 (die out completely) because of climate change 气候变化, competition (often from new species), hunting by humans, or the damage and loss of its habitats.

We try to protect biodiversity because other species give us food, medicines and materials, help keep ecosystems stable, and have value in themselves.

Ways to conserve species

• zoos 动物园 and botanic gardens 植物园 keep and breed endangered species 濒危物种.
• protected conservation 保护 areas, such as national parks and marine parks, keep habitats safe.
• 'frozen zoos' store frozen cells, eggs and sperm, and seed banks 种子库 store seeds for the future.

For rare mammals, assisted reproduction can boost numbers: in vitro fertilisation 体外受精 (IVF), embryo transfer 胚胎移植, and surrogacy 代孕 (another female carries the young).

Conservationists also control invasive species 入侵物种, which are brought in from elsewhere and out-compete native species.

Two organisations help worldwide: the IUCN, which lists how threatened each species is, and CITES, which controls the international trade in endangered animals and plants.

Common mistakes.

• Saying biodiversity is just the number of species. That is species richness 物种丰富度; biodiversity also counts how evenly the individuals are spread among the species, the variety of habitats, and the genetic variation within each species.
• Reading a high Simpson's index as low diversity. In the syllabus form $D = 1 - \sum (n/N)^2$, a value near 1 means high diversity and a value near 0 means low diversity.
• Placing quadrats where the plants look interesting. Choose positions at random, for example from random-number coordinates, so the sample is not biased.
• Using a random quadrat to show how plants change across a shore or from a path. Where a factor changes along a line, use a belt or line transect.
• Saying a correlation proves that one factor causes the change. A significant correlation shows only a link; something else may cause both.
• Listing bacteria and archaea as one kingdom. They are two separate domains; both are prokaryotes but they differ in membrane lipids, ribosomal RNA and cell walls.
• Putting viruses in a kingdom. Viruses are not cells, so they sit outside the three domains and are classified by their nucleic acid.

• Learn the taxonomic hierarchy (domain → kingdom → … → species) and the three domains; a species interbreeds to give fertile offspring.
• Measure biodiversity with Simpson's index (a higher value = more diverse) and explain why high biodiversity aids stability.
• Give in-situ vs ex-situ conservation methods with the advantage of each.