Ecology, Environment, and Natural Resources
What to know
Ecology studies interactions among organisms and their environment. Levels of organization: organism, population, community, ecosystem, biome, biosphere. Biotic factors are living; abiotic factors are nonliving (sunlight, water, temperature, soil). Producers (autotrophs) capture energy by photosynthesis; consumers (heterotrophs) eat others; decomposers recycle nutrients.
Food chains and webs show energy flow. Energy decreases at each trophic level (roughly 10% transferred), so food chains rarely exceed four or five levels. Trophic levels: producers, primary consumers, secondary consumers, tertiary consumers. The carbon, nitrogen, water, and phosphorus cycles move matter through ecosystems. Symbiosis includes mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed).
Population dynamics: carrying capacity is the maximum sustainable population; limiting factors include food, space, and disease. Exponential growth is rapid and unchecked; logistic growth levels off at carrying capacity. Biodiversity supports ecosystem stability. Human impacts include habitat destruction, pollution, overexploitation, invasive species, and climate change. Renewable resources (solar, wind, water) replenish; nonrenewable resources (fossil fuels, minerals) do not. Conservation and sustainable use protect resources.
Traps
Questions reverse the 10% rule, confuse mutualism with parasitism, or claim energy cycles (matter cycles; energy flows). Watch for invasive species described as beneficial without evidence.
Rule of thumb and worked example
If producers hold 10,000 kcal of energy, primary consumers receive about 1,000 kcal, secondary consumers about 100 kcal, and tertiary consumers about 10 kcal. The 10% rule explains why top predators are few and food chains are short.