10–15% of the AP exam. Key topics: Community ecology: species interactions — predation, interspecific competition, mutualism, commensalism, parasitism, Population ecology: exponential growth model (dN/dt = r_max N), logistic growth model (dN/dt = r_max N × (K−N)/K), carrying capacity (K), Limiting factors: density-dependent vs. density-independent, Energy flow through ecosystems: trophic levels, food webs, ~10% energy transfer efficiency rule, Primary productivity: gross primary productivity (GPP) vs. net primary productivity (NPP), Biogeochemical cycles: carbon cycle, nitrogen cycle (fixation, nitrification, denitrification), phosphorus cycle, water cycle, Ecosystem stability: keystone species, biodiversity, ecological succession (primary and secondary), Human impacts: habitat fragmentation, climate change effects, invasive species, conservation biology.
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