Ecology and Environment
Core concepts
Ecology studies interactions among organisms and their environment. Energy flows through food chains from producers to consumers, while nutrients cycle through ecosystems. Only about 10 percent of energy transfers to the next trophic level, so food chains are short. Producers make food by photosynthesis; consumers eat other organisms; decomposers break down dead matter and recycle nutrients. Symbiosis includes mutualism (both benefit), commensalism (one benefits, other unaffected), and parasitism (one benefits, other harmed). Competition occurs when organisms need the same resources. Ecosystems include biotic factors (living) and abiotic factors (nonliving, such as temperature, water, and light). Biodiversity supports ecosystem stability, and human activities can disrupt habitats.
Common traps
Questions often confuse a food chain with a food web, or misidentify decomposers as producers. Do not assume all energy reaches the top predator; most is lost as heat. Distinguish parasitism from predation, and mutualism from commensalism. Remember that abiotic factors are nonliving.
Worked example
A grass-rabbit-fox chain starts with 10,000 units of energy in grass. At 10 percent transfer, rabbits receive 1,000 and foxes receive 100. Rule of thumb: each trophic level holds roughly one-tenth the energy of the level below, so top predators need large areas to find enough food.