Ecosystems, Energy and Nutrient Cycles

Producers capture light energy: gross primary production (GPP) is the total chemical energy fixed; net primary production $\text{NPP}=\text{GPP}-R$ (respiratory losses) is available to consumers. For consumers, $N=I-(F+R)$ (ingested minus faeces/urine and respiration). Only about 10% passes between trophic levels: energy is lost as heat in respiration, in uneaten parts and in faeces. Farming increases efficiency by reducing respiratory losses (warm, restricted animals) and removing competitors and pests. Nitrogen cycle: nitrogen fixation (Rhizobium in root nodules; lightning), ammonification (saprobionts), nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻, aerobic bacteria), denitrification (anaerobic bacteria → N₂). Phosphorus cycles through rocks, soil and organisms; mycorrhizae aid uptake. Fertilisers that leach cause eutrophication: algal bloom → light blocked → plants die → decomposers use O₂ → fish die. Succession: pioneer species → climax community. Rising CO₂ and methane drive climate change.