The Living World: Ecosystems

An ecosystem 生态系统 is all the living things (biotic) and non-living things (abiotic) in an area, interacting together. Organisms depend on each other and on their physical environment, and interactions like predation 捕食, competition 竞争, and symbiosis 共生 (mutualism, commensalism, parasitism) shape the community.

A biome 生物群系 is a large region defined by its climate and characteristic life. Terrestrial biomes (deserts, grasslands, tropical rainforest, temperate forest, taiga, tundra) are determined mainly by temperature and precipitation 降水. Each has plants and animals adapted to those conditions.

The two images below sit at opposite ends of the precipitation scale, which is why their life looks so different:

A tropical rainforest 热带雨林: hot and wet all year, giving dense layered plant life and the richest biodiversity of any land biome

A desert 沙漠: very low precipitation, so only sparse drought-adapted life survives

Aquatic biomes cover more of Earth than land. Freshwater (streams, lakes, wetlands) and marine (open ocean, coral reefs, estuaries) systems differ in salinity 盐度, sunlight, and nutrients. Estuaries 河口 (where rivers meet the sea) and coral reefs are especially productive and biodiverse.

A coral reef 珊瑚礁: warm, sunlit, shallow water makes it one of the most productive and biodiverse of all biomes

The carbon cycle 碳循环 moves carbon between the atmosphere, organisms, oceans, and rocks. Photosynthesis removes $\text{CO}_2$; respiration, decomposition, and burning fossil fuels 化石燃料 return it. Burning fossil fuels adds carbon far faster than natural processes remove it.

The carbon cycle moves carbon among the atmosphere, living things, and fossil fuels

The nitrogen cycle 氮循环 makes nitrogen usable for life. Nitrogen fixation 固氮 (by bacteria) converts atmospheric $\text{N}_2$ to ammonia; nitrification and assimilation pass it through organisms; denitrification returns $\text{N}_2$ to the air. Fertilizer adds reactive nitrogen, which can pollute water.

The nitrogen cycle: N$_2$ is fixed into soil nitrates, passed through food, and returned to the air by denitrification

The phosphorus cycle 磷循环 has no atmospheric (gas) stage – phosphorus moves through rock, soil, water, and organisms. It is slow and often the limiting nutrient 限制性营养 for growth, so added phosphorus (fertilizer, detergents) can trigger algal blooms.

The phosphorus cycle: rock weathers to soil phosphate and slowly sediments back, with no gas phase

The water cycle 水循环 moves water by evaporation 蒸发, transpiration 蒸腾 (from plants), condensation, precipitation, and runoff. It distributes fresh water and shapes climate; human use and land changes can alter its flow.

The water cycle: evaporation and transpiration lift water to the air; precipitation and runoff return it

Primary productivity 初级生产力 is the rate at which producers capture energy (usually by photosynthesis). Gross primary productivity (GPP) is the total captured; net primary productivity (NPP) is what remains after the producers' own respiration – the energy available to the rest of the ecosystem. Warm, wet, sunlit systems (rainforests, reefs) are most productive.

Worked example. If producers in a grassland capture $20{,}000\ \text{kcal/m}^2/\text{yr}$ of energy (GPP) and burn $8{,}000\ \text{kcal/m}^2/\text{yr}$ in their own respiration, then $\text{NPP}=\text{GPP}-R=20{,}000-8{,}000=12{,}000\ \text{kcal/m}^2/\text{yr}$. Only that NPP – not the full GPP – is available to the consumers above.

Organisms occupy trophic levels 营养级: producers 生产者 (make their own food), then primary, secondary, and tertiary consumers 消费者, with decomposers 分解者 recycling nutrients at every level. Each level depends on the energy captured below it.

Energy flows one way through an ecosystem and is lost as heat at each transfer. The 10% rule: only about 10% of the energy at one trophic level passes to the next; the other 90% powers life processes and escapes as heat. This is why food chains are short and top predators are few.

Worked example. Continuing from $12{,}000\ \text{kcal/m}^2/\text{yr}$ of NPP at the producer level, the primary consumers receive about $10\%=1{,}200\ \text{kcal}$, secondary consumers $\approx120\ \text{kcal}$, and tertiary consumers only $\approx12\ \text{kcal}$. After three transfers the energy has shrunk a thousand-fold – the arithmetic reason a fourth or fifth trophic level rarely has enough to support it.

An energy pyramid: only about 10% of energy passes to the next trophic level

A food chain 食物链 is a single line of who-eats-whom; a food web 食物网 links many chains into a realistic picture of feeding relationships. Because species are interconnected, removing one (especially a keystone species 关键种) can disrupt the whole web.

A food web links many food chains; arrows point the way energy flows

• Remember energy flows and is lost (the 10% rule) while matter cycles (carbon, nitrogen, phosphorus, water).
• Apply the 10% rule down a food chain and know NPP = GPP − respiration (only NPP is available to consumers).
• Learn each biogeochemical cycle's stores and how humans disturb it (burning fossil fuels adds CO₂; fertiliser adds nitrogen/phosphorus).
• Producers are most abundant because so little energy reaches the top.
• Use correct units and show your arithmetic on the free-response math questions (no calculator — keep numbers round).