Population Ecology and Biogeochemical Cycles

Population ecology tracks population size, density (individuals per unit area), and distribution pattern (clumped, uniform, or random spacing). Two growth models describe how populations change over time: exponential growth assumes unlimited resources and produces a sharply upward, J-shaped curve, rare in nature and typically brief, such as right after a species is introduced to a new, resource-rich environment; logistic growth is the realistic long-term pattern, in which growth slows as a population approaches carrying capacity, the maximum population size a given environment can sustainably support, producing an S-shaped curve. Limiting factors set that carrying capacity: density-dependent factors, such as food availability, disease spread, and competition, intensify specifically as a population grows larger and more crowded, while density-independent factors, such as a wildfire, a severe drought, or an unusually harsh winter, affect a population's size regardless of how large or small it currently is.

Unlike energy, which flows through an ecosystem once and is ultimately lost as heat, matter is recycled repeatedly through biogeochemical cycles. The water cycle moves water through evaporation, condensation, precipitation, and transpiration, water release from plants. The carbon cycle links directly back to Unit 4's chemical equations at planetary scale: photosynthesis pulls carbon dioxide out of the atmosphere into living matter, while respiration, decomposition, and combustion return it, and, stated factually, human combustion of fossil fuels has measurably increased the concentration of carbon dioxide in the atmosphere over the past two centuries. The nitrogen cycle depends on a small group of specialized nitrogen-fixing bacteria, living freely in soil or in a mutualistic partnership inside the root nodules of certain plants, to convert nitrogen gas, which makes up 78 percent of the atmosphere but which most organisms cannot use directly, into a chemical form plants and animals can actually use -- a striking real-world dependency of the entire living world on a small and easily overlooked group of microorganisms.