Ecology, Environment and Human Influence
Ecosystems and energy flow
A habitat is where an organism lives; a population is all organisms of one species in an area; a community is all populations together; an ecosystem includes the community and its physical environment. Food chains begin with a producer, and arrows show the direction of energy flow. Only about 10% of energy passes to the next trophic level; the rest is lost through respiration, heat, movement and undigested material. Pyramids of numbers, biomass and energy represent these levels.
Cycles and interactions
In the carbon cycle, photosynthesis removes carbon dioxide and respiration, combustion and decomposition release it. The nitrogen cycle includes nitrogen fixation, nitrification, assimilation and denitrification. Organisms compete for resources, and adaptations help survival in particular habitats.
Human influence
Deforestation reduces carbon dioxide absorption and destroys habitats. Burning fossil fuels releases carbon dioxide, a greenhouse gas. Sewage and fertiliser cause eutrophication: nutrients increase algal growth, decomposition uses oxygen, and fish die. Conservation protects habitats and endangered species.
Worked example
If producers contain 10 000 kJ of energy, the primary consumers receive about 1 000 kJ, secondary consumers about 100 kJ and tertiary consumers about 10 kJ. The top predator has the least energy and therefore usually the smallest population.
Traps that cost marks
Arrows in food chains show energy flow, not what eats what in reverse. Decomposers are not always drawn but recycle nutrients. Do not describe the 10% rule as exact; it is an approximate transfer. Eutrophication is caused by excess nutrients, not directly by oxygen.