Ecology and Interdependence of Organisms
What to know
Ecology studies interactions among organisms and their environment across levels: organism, population, community, ecosystem, and biosphere. Understand population concepts - size, density, distribution, carrying capacity, and limiting factors (density-dependent such as food and disease; density-independent such as weather). Recognize growth patterns: exponential (unlimited resources) and logistic (leveling at carrying capacity).
Know community interactions: competition, predation, mutualism (both benefit), commensalism (one benefits, other unaffected), parasitism (one benefits, other harmed), and how these shape populations. Understand food chains and webs, trophic levels (producer, primary consumer, secondary consumer, decomposer), and the rule that energy decreases at each level (roughly 10 percent transferred), so food chains are short and top predators are few. Trace biogeochemical cycles - carbon, nitrogen, water - and the roles of photosynthesis, respiration, decomposition, and nitrogen fixation. Understand succession (primary on bare rock, secondary after disturbance) and biodiversity's role in ecosystem stability. Recognize human impacts: habitat loss, pollution, invasive species, and climate change.
Traps
- Energy vs. nutrients. Energy flows one way and is lost as heat; nutrients cycle.
- 10 percent rule. Only about a tenth of energy passes to the next level, so higher levels support fewer organisms.
- Symbiosis labels. Match who benefits and who is harmed; parasitism harms the host.
- Carrying capacity. Logistic growth levels off because resources are finite, not because reproduction stops entirely.
Rule of thumb
When asked why a population declines or an ecosystem destabilizes, trace the food web and identify the limiting factor or the disrupted interaction.
Worked example
If producers hold 10,000 units of energy, primary consumers receive about 1,000, secondary consumers about 100, and tertiary consumers about 10. A top predator therefore needs a large producer base to persist.