Ecosystems, cycles and sustainability

Energy flows, matter cycles

Energy enters an ecosystem as sunlight, passes along the food chain and is lost as heat at every step — it never comes back. Matter (carbon, water, nitrogen) is different: it goes round and round. Getting this distinction right is the backbone of the whole topic.

Why food chains are short

Only about a tenth of the energy at one level reaches the next; the rest is used for movement and respiration or lost as heat. After four or five steps there is too little energy left to support another level.

The carbon cycle in four arrows

Photosynthesis takes carbon dioxide out of the air; respiration by plants, animals and decomposers puts it back; combustion of fuels puts it back much faster; and fossil fuels are carbon that was locked away for millions of years.

Sustainability

A sustainable practice meets needs now without stopping future generations meeting theirs: replanting trees, fishing quotas, recycling. An answer that names the practice and the reason it helps scores full marks.