Ecosystems: Interactions, Energy, and Dynamics
What you must know
Ecosystems are communities of organisms interacting with each other and with the nonliving environment. Understand roles: producers capture energy (usually from sunlight), consumers eat other organisms, and decomposers break down dead material and recycle nutrients. Trace food chains and food webs, and know that arrows show the direction of energy flow - from the organism eaten to the eater.
Energy decreases at each trophic level because most is lost as heat and used for life processes; only a small fraction passes on. This is why food pyramids narrow upward and why top predators are fewer. Matter, unlike energy, cycles through ecosystems: carbon, nitrogen, and water move between living things and the environment.
Understand carrying capacity and limiting factors (food, water, shelter, disease, predators). Populations rise when resources are abundant and fall when they run short. Recognize symbiotic relationships: mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed). Changes such as removing a species or adding an invasive one ripple through the web.
Traps to watch
- Energy flows one way and is lost as heat; matter cycles.
- Arrows in food webs show energy flow, not "who eats whom" in reverse.
- Removing a predator can cause prey to boom, then crash.
- Decomposers are not just "cleanup"; they return nutrients producers need.
Rule of thumb
For any change, trace two steps: what happens to the population directly affected, then to what eats it or what it eats.
Worked example
If a drought reduces grass, herbivores have less food, so their numbers fall; predators that eat them also decline. Grass regrows if rain returns, but recovery lags behind the producer level.