生态学

Organisms sense and respond to their surroundings in ways that aid survival and reproduction. Behaviors may be innate (inherited, like reflexes and instincts) or learned. Responses such as migration, hibernation, and phototropism, and signals between organisms, are shaped by natural selection because they improve fitness.

The behaviours the CED lists by name

These are the illustrative behaviours an exam question builds its scenario from, so learn what each one is and what it buys the organism:

⚠️ Taxis and kinesis are the pair most often confused. Taxis has a direction: the woodlouse walks towards the damp. Kinesis has only a rate: the woodlouse moves quickly and turns often in dry air and slows down in damp air, so it ends up in the damp without ever steering there.

Life-history strategy 生活史策略 is the pattern of when an organism grows, reproduces and dies, and it too is shaped by selection: a biennial plant stores resources in its first year and flowers in its second; reproductive diapause pauses development until conditions improve, so offspring arrive when they can survive. Both trade the timing of reproduction against the chance that it succeeds.

Energy enters most ecosystems 生态系统 as sunlight, is captured by producers 生产者 (photosynthesizers), and passes to consumers 消费者 along a food chain 食物链. Each level is a trophic level 营养级. Only about 10% of energy transfers up each level (the rest is lost as heat), so food chains are short and producers are the most abundant. Energy flows through and is lost, while matter (carbon, nitrogen) cycles.

Worked example. Suppose producers capture $10{,}000\ \text{kcal}$. Applying the $10\%$ rule, primary consumers receive about $1{,}000\ \text{kcal}$, secondary consumers $100\ \text{kcal}$, and tertiary consumers only $10\ \text{kcal}$. Losing $90\%$ as heat at every step is exactly why food chains rarely exceed four or five levels – there is too little energy left to support another.

A population 种群 is the individuals of one species in an area. Its growth depends on birth, death, immigration, and emigration. Exponential growth 指数增长 ($J$-shaped) happens with unlimited resources; logistic growth 逻辑斯蒂增长 ($S$-shaped) levels off at the carrying capacity 环境容纳量 $K$ – the maximum the environment can sustain – following $\dfrac{dN}{dt}=r_{\max}N\dfrac{K-N}{K}$.

Worked example. A population has $r_{\max}=0.5\ \text{yr}^{-1}$, carrying capacity $K=1000$, and current size $N=400$. Then $\dfrac{dN}{dt}=0.5\times400\times\dfrac{1000-400}{1000}=0.5\times400\times0.6=120$ individuals per year. The $\frac{K-N}{K}$ term is why growth is fastest near $N=K/2$ and slows toward zero as $N$ approaches $K$.

Some factors depend on crowding, others do not:

• Density-dependent 密度制约 factors intensify as a population grows – competition, predation, disease.
• Density-independent 非密度制约 factors act regardless of density – weather, natural disasters.

These factors regulate population size around the carrying capacity.

A community 群落 is all the interacting populations in an area. Key interactions: competition 竞争 (for shared resources), predation 捕食, symbiosis 共生 – mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed). These relationships shape which species coexist.

Biodiversity 生物多样性 is the variety of life – genes, species, and ecosystems. Higher diversity generally makes a community more resilient 有韧性, better able to withstand and recover from disturbance. A keystone species 关键种 has an outsized effect, so losing it can collapse the community.

Ecosystems change from natural and human causes – climate shifts, invasive species, habitat loss, and pollution. A disturbance can trigger ecological succession 生态演替 (the community rebuilds over time). Because species are interconnected, a change to one – especially a keystone or a trophic level – can ripple through the whole ecosystem.

Common mistakes.

• Saying energy is recycled in an ecosystem. Matter cycles; energy flows one way and leaves as heat.
• Saying the 90% "lost" energy is destroyed. It is still energy, mostly released as heat in respiration, or left in parts that are not eaten or digested.
• Placing the fastest logistic growth at the carrying capacity. Growth is fastest near $N=K/2$; at $N=K$ the term $\frac{K-N}{K}$ is zero, so $\frac{dN}{dt}=0$.
• Calling a flood or a frost density-dependent because it kills many individuals. A factor is density-dependent only if its effect per individual grows as the population becomes more crowded.
• Mixing up taxis and kinesis. Taxis is directed movement towards or away from a stimulus; kinesis changes only the rate of random movement.
• Calling a keystone species "the most common species". A keystone species has an effect far larger than its numbers suggest; a sea star or a wolf can be rare and still hold the community together.
• Calling every symbiosis mutualism. Symbiosis includes mutualism (+/+), commensalism (+/0) and parasitism (+/−).

• Apply the 10% rule: about 90% of energy is lost at each trophic level, so food chains are short.
• Distinguish exponential ($J$) from logistic ($S$) growth; the latter levels off at the carrying capacity ($\frac{dN}{dt}=r_{\max}N\frac{K-N}{K}$).
• Separate density-dependent (competition, disease, predation) from density-independent (weather) limiting factors.
• Name community interactions (competition, predation, symbiosis) and the effect of a keystone species.
• Explain how disturbing one species — especially a keystone or a whole trophic level — ripples through the ecosystem.