The Living World: Biodiversity

Biodiversity 生物多样性 is the variety of life, measured at three levels:

• Genetic diversity – variety of genes within a species. More genetic variety means a population can better respond to environmental stressors 环境压力; a population bottleneck 种群瓶颈 (a sharp drop in numbers) throws that variety away and leaves the survivors vulnerable.
• Species diversity – the number of species (species richness 物种丰富度) and how evenly individuals are spread among them.
• Habitat diversity – the variety of habitats in a region.

More species-rich ecosystems are more resilient 有韧性的: they recover from disruption more readily, because varied species can fill roles when conditions change. Habitat loss removes species in a predictable order – specialist 特化 species (narrow needs) go first, then the hardier generalist 广适性 species, and species that need large territories decline as their range shrinks.

Biodiversity has three levels: genetic, species, and habitat

Ecosystem services 生态系统服务 are the free benefits nature gives people, in four categories:

• Provisioning – products: food, fresh water, timber, fibre.
• Regulating – processes that keep conditions stable: climate regulation, water filtration, pollination, flood control.
• Cultural – non-material value: recreation, beauty, spiritual and educational value.
• Supporting – the services that make the others possible: nutrient cycling, soil formation, photosynthesis.

Anthropogenic 人为的 (human-caused) activities can disrupt these services, with real economic and ecological costs – for example, losing pollinators forces farmers to pay for pollination that bees once did for free.

Island biogeography 岛屿生物地理学 studies how many species an island (or any isolated habitat patch) holds. The number settles at a balance between two rates:

• Immigration of new species – higher for islands closer to the mainland.
• Extinction of resident species – lower on larger islands.

So big, near islands hold the most species; small, remote ones the fewest. Isolation also drives evolution: island species often become specialists because resources are limited, and many are endemic 特有 – found nowhere else. That makes them fragile. When an invasive species 入侵物种 (usually a tough generalist) arrives, it can outcompete the specialists, which have nowhere else to go.

Larger and nearer islands hold more species, balancing immigration against extinction

Every species has a range of tolerance 耐受范围 – the span of conditions (temperature, salinity, pH, sunlight) it can survive. Near the middle is an optimal range 最适范围 where individuals thrive; toward the edges are zones of stress 胁迫区 where they survive but struggle to grow or reproduce; beyond the limits they die. Ecological tolerance 生态耐受性 applies both to a single individual and to a whole species, and it decides where an organism can live and how it copes with change.

Within its optimal range a species thrives; toward the limits it is stressed, then cannot survive

Natural events disrupt ecosystems across every timescale, and a single natural event can rival or exceed a human-made one. Earth's processes come in three timing patterns:

• Periodic 周期性 – regular cycles (seasons, tides).
• Episodic 偶发性 – occasional, irregular events (fires, floods, volcanic eruptions).
• Random 随机 – no pattern (a meteor strike).

Over geological time, Earth's climate has shifted for many reasons, and sea level has risen and fallen with the amount of glacial ice – each change reshaping large swaths of habitat. Wildlife responds by migration 迁徙, moving short or long distances to follow conditions.

A crown fire sweeping through a forest. Fire is a natural episodic disturbance, and many species are adapted to it

An adaptation 适应 is an inherited trait that improves survival and reproduction in an environment. Adaptations arise through natural selection 自然选择 – incremental genetic change accumulating over generations, not within a single lifetime. When the environment changes, individuals must alter their behaviour, move, or die; if change comes faster than a population can adapt, it declines. A periodic disturbance such as fire is often part of a healthy cycle, and many species are adapted to it (some pine cones open only in a fire's heat).

Ecological succession 生态演替 is the gradual, orderly change in a community over time:

• Primary succession 初级演替 starts on bare rock with no soil – after a glacier retreats or lava cools. Pioneer species 先锋物种 such as lichens 地衣 arrive first and slowly build soil. It is slow.

Lichens growing straight on bare rock. As pioneer species they slowly break the rock down and build the first thin soil

• Secondary succession 次级演替 follows a disturbance that leaves the soil intact – after a fire or abandoned farming – so it is much faster.

Succession tends toward a relatively stable climax community 顶极群落. Two special roles shape a community out of proportion to their numbers: a keystone species 关键种, whose activities hold the community structure together (remove it and the community collapses – sea otters keeping urchins in check), and an indicator species 指示物种, whose presence or health signals the condition of the whole ecosystem (lichens vanish where the air is polluted).

Primary succession builds from bare rock; secondary succession regrows from surviving soil

Worked example. A pond holds 3 fish species with 5, 3, and 2 individuals ($N=10$). Simpson's Diversity Index $=1-\sum\left(\tfrac{n}{N}\right)^2 = 1-\left[(0.5)^2+(0.3)^2+(0.2)^2\right] = 1-0.38 = 0.62$. A value near 1 means high diversity; near 0 means one species dominates. Adding a fourth, evenly-sized species would push the index higher.

• Describe biodiversity at three levels: genetic, species, and habitat, and link higher diversity to greater resilience.
• Specialist species (narrow needs) are more vulnerable than generalists when a habitat changes.
• Distinguish primary succession (bare rock, no soil, slow) from secondary (soil intact, faster).
• Explain a keystone species (outsized effect) and an indicator species (signals ecosystem health).
• Name the four ecosystem services (provisioning, regulating, cultural, supporting) with examples.