Topic 4 examines interactions between organisms and their environment: energy flow through ecosystems, nutrient cycling, population dynamics, and the human impact on global systems.
Ecosystem: community of organisms + abiotic environment. Autotrophs (producers) convert light energy to chemical energy (photosynthesis). Heterotrophs (consumers) obtain energy by feeding. Food chain: producer → primary consumer → secondary consumer → tertiary consumer. Approximately 10% of energy is transferred between trophic levels (rest lost as heat through respiration).
Carbon enters biosphere via photosynthesis (CO₂ → organic compounds). Returns via respiration, decomposition, and combustion. Carbon pools: atmosphere (CO₂), oceans (dissolved CO₂, carbonate), biosphere (organic molecules), lithosphere (fossil fuels, limestone). Human impact: burning fossil fuels and deforestation increase atmospheric CO₂.
Enhanced greenhouse effect: increased CO₂, CH₄, N₂O trap more infrared radiation. Evidence: rising global temperatures, ice core data (CO₂ and temperature correlation), sea level rise, glacier retreat. Consequences: extreme weather, habitat loss, ocean acidification, species extinction. Mitigation: renewable energy, reforestation, carbon capture.
Exponential growth (J-curve) when resources are unlimited. Logistic growth (S-curve) with carrying capacity K. Factors: natality, mortality, immigration, emigration. Density-dependent (competition, disease, predation) and density-independent (natural disasters, climate) factors. Sampling techniques: quadrats (sessile organisms), capture-mark-recapture (mobile organisms).
Only about 10% of energy at each trophic level is passed to the next (the rest is lost as heat through respiration, in uneaten parts, or in undigested waste). This limits food chain length to typically 4–5 levels, because there is insufficient energy to support another level. It also explains why top predators are rare and why more people can be fed on a plant-based diet than a meat-based one.
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