Ecosystems: Interactions, Energy, and Dynamics
Core concepts
Ecosystems include living communities and the abiotic environment. Energy flows in one direction, starting with sunlight captured by producers (autotrophs). Consumers occupy trophic levels; decomposers break down dead matter and recycle nutrients. Only about 10% of energy passes to the next trophic level, so food chains are short and top predators are few. Nutrients such as carbon, nitrogen, and water cycle through biotic and abiotic reservoirs. Population size is limited by carrying capacity, competition, predation, disease, and resources. Species interactions include competition, predation, parasitism, mutualism, and commensalism. Ecological succession changes communities over time after disturbance. Biodiversity supports ecosystem stability and resilience. Human activities can alter nutrient cycles, habitat, and biodiversity.
Traps to watch
- Energy flows and is lost as heat; matter cycles.
- Decomposers are not a trophic level in the same way as producers and consumers; they recycle nutrients from all levels.
- A food web is more realistic than a single food chain because organisms eat multiple things.
- Competition is between organisms; predation is one organism eating another.
- Primary succession starts on bare rock with no soil; secondary succession starts where soil remains.
Worked example
If producers fix 10,000 kcal of energy, about 1,000 kcal is available to primary consumers, 100 kcal to secondary consumers, and 10 kcal to tertiary consumers. Rule of thumb: multiply by 0.10 per trophic level.