Sort ecosystem components into biotic and abiotic factors, place organisms within the hierarchy from organism to biosphere, and distinguish habitat from niche.
Ecologists organize the living world into a hierarchy: an individual organism belongs to a population (all members of one species in an area), which is part of a community (all the populations interacting in an area), which together with the nonliving environment forms an ecosystem, which is part of a larger biome (a major regional ecosystem type, like temperate forest or grassland), which is part of the whole biosphere. Every ecosystem is made of biotic factors (living things -- plants, animals, fungi, bacteria) and abiotic factors (nonliving things -- temperature, sunlight, water, soil chemistry, pH).
Two more terms are easy to confuse but distinct: habitat is WHERE an organism lives (its address), while niche is its full role in the ecosystem -- what it eats, what eats it, when it is active, and how it affects its surroundings (more like its occupation). Two species can share the exact same habitat but have different niches, which is how they avoid directly competing for the same resources.
Mastery looks like: They can correctly sort a mixed list into biotic/abiotic, identify the matching level of organization for a scenario, and explain the habitat-versus-niche distinction with an original example.
Common stumbles: Calling a species' niche "the forest" (that is its habitat); assuming abiotic factors do not matter as much as biotic ones to an ecosystem's function.