生物与环境

The Sun is the main source of energy 能量 for almost all life. Plants capture light energy by photosynthesis 光合作用 and store it as chemical energy in food. This energy then passes from organism to organism as they feed. At every step, some energy is lost (mostly as heat from respiration 呼吸作用) to the environment 环境. Energy flows one way — it is not recycled.

A food chain 食物链 shows the transfer of energy from one organism to the next, starting with a producer. Each arrow points in the direction the energy flows:

grass → rabbit → fox

A food web 食物网 is many food chains linked together.

Producers, consumers and decomposers

Consumers are named by their position: a primary consumer eats the producer; a secondary consumer eats the primary consumer; then come tertiary and quaternary consumers.

Trophic levels and pyramids

A trophic level 营养级 is the position of an organism in a food chain (producers first, then primary consumers, and so on). You can show a food chain as a pyramid 金字塔:

• a pyramid of numbers counts the organisms at each level.
• a pyramid of biomass 生物量 shows the total mass at each level — usually a better picture, because it does not depend on how big the organisms are.
• (Supplement) a pyramid of energy shows the energy at each level — the most useful picture of all.

Energy loss along a chain (Supplement)

Only about 10% of the energy at one trophic level passes to the next. The rest is lost as heat (from respiration), in movement, and in waste. Because so much energy is lost, food chains usually have fewer than five trophic levels. It is more energy-efficient for people to eat crop plants 农作物 directly than to eat animals that were fed on those crops, because each extra level wastes energy.

Worked example. The producers in a field trap 40 000 kJ of energy. Roughly how much reaches a secondary consumer, and why do food chains stay short? About 10% passes on at each step. Primary consumers receive 10% of 40 000 = 4000 kJ. Secondary consumers receive 10% of 4000 = 400 kJ - only 1% of what the plants trapped. After a few levels too little energy is left to support another one, which is why chains rarely pass five levels. Apply the 10% again at every step, not once for the whole chain.

Human impact

Humans can damage food webs by:

• overharvesting 过度捕捞 — taking too many of one species (such as overfishing), so its numbers crash.
• introducing a foreign species 外来物种 — a new species may have no predators and may crowd out native species.

Unlike energy, nutrients are recycled — used again and again.

The carbon cycle

In the carbon cycle 碳循环:

• photosynthesis removes carbon dioxide 二氧化碳 from the air.
• respiration and combustion 燃烧 (burning) return carbon dioxide to the air.
• feeding passes carbon from one organism to the next.
• decomposition returns carbon to the air and soil.
• over millions of years, dead organisms can form fossil fuels 化石燃料 (coal and oil), which release carbon dioxide when they are burned.

The nitrogen cycle (Supplement)

The nitrogen cycle 氮循环 recycles nitrogen for making proteins 蛋白质. Microorganisms 微生物 do most of the work:

• decomposition: decomposers break dead protein down into ammonium ions 铵离子.
• nitrification 硝化作用: bacteria change ammonium ions into nitrate ions 硝酸根离子.
• nitrogen fixation 固氮作用: lightning and some bacteria 细菌 turn nitrogen gas into compounds that plants can use.
• plants absorb nitrate ions to make amino acids 氨基酸 and proteins; animals get them by feeding and digestion 消化.
• deamination 脱氨基作用 (in the liver) and decomposition release nitrogen compounds again.
• denitrification 反硝化作用: some bacteria turn nitrate ions back into nitrogen gas.

• a population 种群 is a group of organisms of one species 物种, living in the same area at the same time.
• a community 群落 is all the populations of different species in one place.
• an ecosystem 生态系统 is the community together with its environment, all interacting.

Population growth

The growth of a population depends on the food supply, competition 竞争, predation 捕食 (being eaten) and disease.

When a population grows in a place with limited resources 资源, its growth follows an S-shaped curve with four phases:

Common mistakes.

• Drawing food-chain arrows from the eater to the food. The arrow shows the direction energy flows, so it points into the organism that eats: grass → rabbit → fox.
• Saying energy is recycled. Energy flows one way and leaves as heat; only nutrients such as carbon and nitrogen go round a cycle.
• Saying the lost energy is "used up" or "destroyed". It is lost to the surroundings as heat from respiration, and in waste and uneaten parts.
• Applying the 10% once for the whole chain. Take 10% again at each step: 40 000 kJ → 4000 kJ → 400 kJ.
• Leaving decomposers out of the carbon cycle. Decomposers respire, and that respiration returns the carbon in dead matter to the air as carbon dioxide.
• Saying plants absorb nitrogen gas from the air. Plants absorb nitrate ions from the soil through their roots; only nitrogen-fixing bacteria (and lightning) can use nitrogen gas.
• Mixing up the bacteria. Nitrogen-fixing bacteria turn nitrogen gas into nitrogen compounds, nitrifying bacteria turn ammonium ions into nitrates, and denitrifying bacteria turn nitrates back into nitrogen gas.
• Calling the flat part of the population curve "no births". In the stationary phase births still happen; they are simply equal to deaths.

• Energy enters from the Sun, flows one way along the chain, and is lost as heat at each step. Nutrients are recycled.
• Food chain starts with a producer; arrows show energy flow. Learn producer, consumer, herbivore, carnivore, decomposer.
• Only ~10% of energy passes up each trophic level, so chains are short and eating plants is more efficient.
• Carbon cycle: photosynthesis ↔ respiration and combustion; nitrogen cycle (Supplement): fixation, nitrification, denitrification by microorganisms.
• Population: one species, one area, one time. Learn the four phases of the S-shaped growth curve.