种群

A generalist 广适性物种 can live in many habitats 栖息地 and eat many foods (raccoon, cockroach) – flexible, so it copes well with change. A specialist 狭适性物种 needs specific conditions or foods (panda, koala) – efficient in a stable habitat but very vulnerable 脆弱的 if that habitat changes.

Two reproductive strategies 繁殖对策:

• r-selected r选择 species produce many offspring 后代 with little care, mature 成熟 fast, and have short lives (insects, weeds). They colonize 定殖 quickly and boom-and-bust.
• K-selected K选择 species produce few offspring with much care, mature slowly, and live long (elephants, humans). They do well near carrying capacity 环境容纳量 in stable environments but recover slowly from losses.

A survivorship curve 存活曲线 plots how many individuals survive at each age (on a log scale 对数刻度). Type I (humans): most survive to old age, then die – a convex 凸的 curve. Type II (many birds): a constant death rate 死亡率 at all ages – a straight line. Type III ($r$-selected: fish, insects): most die young, a few survive – a concave 凹的 curve.

The carrying capacity ($K$) is the maximum population an environment can support long-term. A population may overshoot 超出 $K$ when resources 资源 are temporarily plentiful, then crash (dieback 种群骤减) as resources run out. Populations tend to fluctuate 波动 around $K$.

With abundant resources a population grows exponentially 指数增长 ($J$-curve). As resources become limited, growth slows to logistic 逻辑斯蒂增长 ($S$-curve), leveling at $K$. Limiting factors 限制因素 (food, water, space, disease) set the ceiling.

An age-structure diagram 年龄结构图 shows the proportion 比例 of a population in each age group. A wide base (many young) predicts rapid growth; even bars predict a stable population; a narrow base predicts decline 衰退. They forecast a country's future population.

The total fertility rate 总和生育率 (TFR) is the average number of children per woman. A replacement level 更替水平 of about 2.1 keeps a population steady; above it grows, below it shrinks. TFR falls with education, access to family planning 计划生育, and lower infant mortality 婴儿死亡率.

Human population growth depends on birth rate 出生率, death rate, and migration 迁移. Rapid growth strains resources and increases the ecological footprint 生态足迹. Factors lowering birth rates include education (especially of women), urbanization 城市化, and economic development 经济发展.

Worked example. A country has a birth rate of $30$ per $1000$ and a death rate of $10$ per $1000$. The natural growth rate is $\dfrac{30-10}{10}=2.0\%$ per year (dividing the per-thousand difference by $10$ converts it to a percent). By the rule of 70, the population doubles in about $\dfrac{70}{2.0}=35$ years. If development cut the growth rate to $1.0\%$, the doubling time would stretch to $\dfrac{70}{1.0}=70$ years.

The demographic transition 人口转变 describes how a country's growth changes as it develops, in stages: (1) high birth and death rates (slow growth); (2) death rate falls (rapid growth); (3) birth rate falls (slowing growth); (4) both low (stable). Developed nations are in later stages; many developing nations are in the rapid-growth stages.

The population grows fastest in stages 2 and 3 – the gap between a still-high birth rate and an already-fallen death rate. That lag is why a country's population keeps climbing long after its death rate drops.

• Use the rule of 70 七十法则: doubling time 倍增时间 ≈ 70 ÷ growth-rate(%); a growth rate is (births − deaths) per 1000 ÷ 10.
• Read age-structure diagrams: a wide base predicts rapid growth, even bars a stable population.
• Contrast r-selected (many offspring, little care) and K-selected (few, much care) strategies.
• Know the demographic transition stages and what lowers birth rates (education, family planning, development).
• A falling growth rate 增长率 still means growth — the population only shrinks when deaths exceed births.