Ecosystems, Food Chains and the Nitrogen Cycle

<h3>Ecosystems and energy flow</h3><p>An <strong>ecosystem</strong> is made of living organisms (biotic factors) interacting with their non-living surroundings (abiotic factors) such as water, light and temperature.</p><p>Energy flows in one direction. <strong>Producers</strong> (green plants) trap sunlight; <strong>primary consumers</strong> (herbivores) eat plants; <strong>secondary and tertiary consumers</strong> (carnivores) eat other animals. At each step, energy is lost as heat, so the amount of energy <strong>decreases</strong> along the chain.</p><h3>Food chains, webs and decomposers</h3><p>A <strong>food chain</strong> shows a single feeding path, for example: grass &rarr; grasshopper &rarr; lizard &rarr; hawk. A <strong>food web</strong> is many food chains joined together. <strong>Decomposers</strong> like bacteria and fungi rot dead plants and animals, releasing nutrients back into the soil.</p><h3>Nitrogen cycle and human impact</h3><p>Plants cannot use nitrogen gas directly. Bacteria in the soil and in the roots of legumes such as beans <strong>fix</strong> nitrogen into forms plants can absorb. Human activities like <strong>deforestation, pollution, mining and over-population</strong> degrade the environment, but conservation, tree planting and reduced waste help protect it.</p><div class="callout"><strong>Kenyan context:</strong> When farmers in Kenya plant beans together with maize, the bean roots host nitrogen-fixing bacteria that enrich the soil naturally. This traditional intercropping reduces the need for expensive fertiliser and keeps the land productive.</div>