人类对生态系统的影响

To grow more food, humans use:

• agricultural machinery to farm larger areas of land quickly.
• chemical fertilisers 化肥 to improve yields 产量.
• insecticides 杀虫剂 to kill insect pests.
• herbicides 除草剂 to kill weeds 杂草 that compete with the crops.
• selective breeding 选择育种 to improve crop plants 农作物 and livestock 牲畜.

A monoculture 单一栽培 (growing one crop over a large area) gives a big, easy harvest, but one pest or disease can destroy the whole crop, and it lowers biodiversity 生物多样性. Intensive livestock farming (many animals in a small space) gives cheap meat, but raises worries about animal welfare and the spread of disease.

Biodiversity is the number of different species 物种 living in an area. Humans destroy habitats 栖息地 (the places where organisms live) by:

• clearing land for housing, crops and livestock.
• extracting natural resources 资源, such as by mining.
• pollution 污染 of fresh water and the sea.

By changing food webs 食物网 and food chains 食物链, humans harm habitats.

Deforestation 森林砍伐 (cutting down forests) is a major example. It causes:

• lower biodiversity and the extinction 灭绝 of species.
• loss of soil (no roots to hold it, so it washes away).
• more flooding.
• more carbon dioxide 二氧化碳 in the air, because there are fewer trees to take it in.

Water pollution

Untreated sewage 污水 and excess fertiliser washed into rivers and lakes damage water ecosystems 生态系统.

(Supplement) Excess fertiliser causes eutrophication 富营养化:

1. the fertiliser adds nitrate ions 硝酸根离子 and other ions to the water.
2. producers 生产者 such as algae grow very fast.
3. they soon die, and decomposition 分解 increases.
4. decomposers 分解者 use up the oxygen 氧气 in aerobic respiration 有氧呼吸.
5. the dissolved oxygen falls, so fish and other organisms die.

Worked example. A farmer spreads fertiliser on a field; weeks later the fish in a nearby river die. Put the steps in order and explain. Rain washes nitrate ions off the field into the river. The extra nitrate makes algae grow very fast and cover the surface. The algae block the light, so the plants below die. Decomposers multiply as they feed on the dead algae and plants, and their aerobic respiration uses up the oxygen dissolved in the water. With too little oxygen left, the fish die. The fish are not poisoned by the fertiliser - they suffocate, and it is the decomposers that actually remove the oxygen.

Plastics and air pollution

Non-biodegradable 不可降解 plastics 塑料 do not rot away. They build up in the sea and on land, harming wildlife — animals may eat them or get trapped.

Methane 甲烷 and carbon dioxide are greenhouse gases. Too much of them strengthens the greenhouse effect 温室效应 (the enhanced greenhouse effect), which causes climate change 气候变化.

Sustainable resources

A sustainable 可持续 resource is produced as fast as it is used up, so it never runs out. Forests and fish 鱼类 stocks can both be managed in this way.

(Supplement) Forests are conserved 保护 by education, protected areas, quotas 配额 (limits on how much is taken) and replanting. Fish stocks are conserved by closed seasons 禁渔期, protected areas, controlled net mesh sizes (so young fish can escape), quotas and monitoring.

Saving endangered species

A species becomes endangered 濒危 or extinct when its numbers fall, because of climate change, habitat destruction, hunting 狩猎, overharvesting 过度捕捞, pollution and introduced species 外来物种.

Endangered species can be saved by:

• monitoring and protecting the species and its habitat.
• education.
• captive breeding 圈养繁殖 programmes, which breed animals in zoos. (Supplement) These may use artificial insemination 人工授精 (AI) or in vitro fertilisation 体外受精 (IVF).
• seed banks 种子库, which store seeds safely.

(Supplement) We conserve nature to keep up biodiversity, reduce extinction, protect ecosystems, and keep ecosystem functions going — such as recycling nutrients and providing food, drugs 药物, fuel and useful genes 基因. If a population becomes too small, it loses genetic variation 变异, which makes it harder for the species to survive future change.

Common mistakes.

• Saying the algae use up the oxygen in eutrophication. The algae die; the decomposers feeding on them use up the oxygen by aerobic respiration, and that is why the fish die.
• Saying fertiliser poisons the fish. Fertiliser is not toxic to fish; it kills them only indirectly, through the chain of extra algae, extra decomposers and less dissolved oxygen.
• Confusing the greenhouse effect with the hole in the ozone layer. Greenhouse gases trap heat leaving the Earth; the greenhouse effect itself is natural, and it is the enhanced effect from extra gases that causes climate change.
• Saying deforestation adds carbon dioxide only because the trees are burned. It also means fewer trees take in carbon dioxide by photosynthesis.
• Calling any resource that is reused "sustainable". A sustainable resource is produced as fast as it is used, so its stock does not fall.
• Saying captive breeding alone saves a species. The habitat must also be protected, or the animals have nowhere to be released.

• More food: machinery, fertilisers, insecticides, herbicides, selective breeding. Monocultures and intensive farming each have advantages and disadvantages.
• Deforestation: less biodiversity, extinction, soil loss, flooding, more carbon dioxide.
• Eutrophication (Supplement): fertiliser → more producers → more decomposers → less dissolved oxygen → organisms die.
• Greenhouse gases (carbon dioxide, methane) → enhanced greenhouse effect → climate change.
• Conservation: sustainable use, protected areas, quotas, captive breeding and seed banks. Small populations lose genetic variation.