Ecology and the Environment
Ecosystems and food chains
An ecosystem is a community of organisms interacting with their environment. Know the terms: habitat, population, community, producer, consumer, decomposer. Food chains show energy flow, always starting with a producer. Arrows show the direction of energy transfer. A common trap is reversing the arrow direction or forgetting that energy is lost as heat at each level, so fewer organisms can be supported at higher trophic levels.
Cycles and interactions
Learn the carbon cycle: photosynthesis removes carbon dioxide, respiration and combustion release it, decomposers return carbon to the soil and air. The nitrogen cycle involves nitrogen-fixing bacteria, decomposers and nitrifying bacteria. Predator-prey graphs show cyclical changes: an increase in prey is followed by an increase in predators, then a fall in prey.
Human impact
Deforestation, pollution, eutrophication, greenhouse gases and habitat destruction affect ecosystems. Eutrophication occurs when fertiliser runoff causes algal growth, then decay, then oxygen depletion and death of fish. A frequent trap is blaming the algae directly for killing fish; it is the lack of oxygen after decay.
Worked example
If a food chain is grass, rabbit, fox, and foxes are removed, rabbits increase, then food and space run out and numbers fall. Rule of thumb: populations are limited by food, space, disease and predators.
Practical trap
When sampling with quadrats, use random placement and repeat to make results representative.