Construct a food web from a list of organisms and feeding relationships, and explain why energy pyramids narrow sharply at higher trophic levels.
Ecological organization builds up in levels: an organism, a population (the same species in one area), a community (all populations interacting in an area), an ecosystem (a community plus its physical environment), a biome, and the biosphere. Producers (autotrophs, capturing energy from sunlight through photosynthesis, a direct callback to Unit 4) differ from consumers (heterotrophs: herbivores eat producers, carnivores eat other consumers, omnivores eat both) and from decomposers, which break down dead organic material and recycle its nutrients back to producers -- an unglamorous but essential role, since without decomposers, nutrients would stay locked in dead material indefinitely.
A food web, a realistic network of feeding relationships, is more accurate than any single food chain, since most consumers eat, and are eaten by, more than one other kind of organism. Energy moves through trophic levels -- producer, primary consumer, secondary consumer, and so on -- but only roughly 10 percent of the energy at one trophic level typically transfers to the next, with the rest lost as heat through ordinary metabolism, a direct consequence of the thermodynamics discussed in Unit 4. This "10 percent rule" is exactly why food chains rarely extend past four or five levels, and why top predators are always dramatically less numerous than the producers supporting the whole system below them, a pattern usually drawn as a narrowing energy pyramid.
Mastery looks like: They can build a food web from a list of organisms and feeding relationships, correctly identify trophic levels, and explain why energy sharply decreases at each higher trophic level.
Common stumbles: Confusing a food web with a single food chain and missing most of the real feeding connections; forgetting that decomposers, not just producers and consumers, are a required part of a functioning ecosystem.