Biology: Ecosystems

To understand ecosystems, we look at them in a structured way, from the smallest unit to the whole system. An 'organism' is a single living being. A 'population' is a group of organisms of the same species living in the same area. A 'community' includes all the different populations of different species living and interacting in an area. Finally, an 'ecosystem' is the community of living organisms (biotic factors) interacting with their non-living environment (abiotic factors), such as soil, water, and sunlight.

Energy flows through an ecosystem from one organism to another. A food chain shows a simple, linear pathway of this energy flow. It always starts with a 'producer', an organism (usually a plant) that makes its own food using sunlight through photosynthesis. Organisms that eat producers are 'primary consumers' (herbivores). Organisms that eat primary consumers are 'secondary consumers' (carnivores or omnivores), and so on. Each stage is called a 'trophic level'. In reality, most ecosystems have complex, interconnected feeding relationships, which are better represented by a 'food web'. Arrows in a food chain or web show the direction of energy transfer.

When an organism is eaten, not all the energy it contains is passed on to the next trophic level. A large amount, typically around 90%, is lost. This energy is lost as heat during respiration, is used for life processes like movement, or is unavailable because not all parts of the organism are eaten. This means only about 10% of the energy is converted into new biomass in the next level. This energy loss limits the length of food chains. We can represent this using ecological pyramids. A 'pyramid of numbers' shows the total number of individual organisms at each trophic level. A 'pyramid of biomass' shows the total dry mass of organisms at each level. A 'pyramid of energy' shows the total energy content at each level. Pyramids of biomass and energy are always pyramid-shaped, but a pyramid of numbers can be inverted (e.g., one large tree supporting thousands of insects).

Carbon is an essential element for life, forming the backbone of all organic molecules. The carbon cycle describes how carbon atoms move between the atmosphere, oceans, land, and living organisms. Key processes include: 'Photosynthesis', where plants take in carbon dioxide (CO2) from the atmosphere to make glucose. 'Respiration', where all living organisms (including plants) release CO2 back into the atmosphere. 'Decomposition', where bacteria and fungi break down dead organisms and waste, releasing CO2 through respiration. 'Combustion', where burning fossil fuels (coal, oil, gas) or wood releases large amounts of stored carbon into the atmosphere as CO2. Human activities, particularly combustion and deforestation (which reduces photosynthesis), are significantly increasing atmospheric CO2 levels.

Nitrogen is crucial for making proteins and DNA. Although the atmosphere is 78% nitrogen gas (N2), plants and animals cannot use it in this form. The nitrogen cycle makes nitrogen available. Key stages involve specialized bacteria: 1. 'Nitrogen Fixation': Nitrogen-fixing bacteria (in soil or root nodules of legume plants) convert atmospheric N2 into ammonia (NH3), which then forms ammonium ions (NH4+). Lightning also fixes a small amount of nitrogen. 2. 'Nitrification': Nitrifying bacteria convert ammonium ions into nitrites (NO2-), and then into nitrates (NO3-). 3. 'Assimilation': Plants absorb nitrates from the soil through their roots to make proteins. Animals then get nitrogen by eating plants. 4. 'Ammonification': Decomposers (bacteria and fungi) break down dead organisms and waste products (like urea), converting the nitrogen in them back into ammonium ions. 5. 'Denitrification': Denitrifying bacteria, found in waterlogged or anaerobic soil, convert nitrates back into atmospheric nitrogen gas (N2), completing the cycle.