Life Science: Organisms and Ecosystems
What this domain tests
An ecosystem is all the living and nonliving things in an area interacting together. Know the roles: producers make their own food using sunlight, consumers eat other organisms, and decomposers break down dead material and return nutrients to the soil. Understand food chains and food webs, and read the arrows as the direction energy flows, from the organism being eaten to the eater.
Energy and matter
Energy enters most ecosystems as sunlight and moves through organisms, but much of it is lost as heat at each step, so food chains rarely have more than four or five links. Matter, unlike energy, cycles: nutrients from decomposers are reused by producers. Know the terms habitat (where an organism lives), population (one species in an area), community (all populations), and ecosystem (the community plus the nonliving environment).
Traps to watch for
Many questions reverse the arrows in a food chain or label a decomposer as a producer. Another trap is confusing a population with a community, or assuming energy is recycled. Watch for claims that removing one organism affects only its predator; in a web, effects spread.
Rule of thumb
Follow the arrows: energy goes toward the eater, and the Sun is the ultimate source. For example, in grass to rabbit to fox, the grass is the producer, the rabbit a primary consumer, and the fox a secondary consumer. If rabbits were removed, grass would increase and foxes would have less food, showing that a web connects many paths.