2.2 Energy and biomass in ecosystemsIB Environmental Systems and Societies HL: Revision notes

Ecosystems are sustained by supplies of energy and matter. The first law of thermodynamics states that energy can be transformed from one form to another but cannot be created or destroyed. The second law states that energy transformations are inefficient: some energy is always lost, mostly as heat.

Photosynthesis converts light energy to chemical energy in glucose, some of which is stored as biomass by autotrophs. Cellular respiration releases energy from glucose in a form (ATP) that can easily be used for active processes in cells; some of the energy is transformed into heat.

Producers make their own carbon compounds by photosynthesis, so they form the first trophic level. Consumers obtain carbon compounds, and the energy in them, from other organisms by a variety of feeding strategies. Carbon compounds and energy pass from one organism to the next in a food chain; each stage is a trophic level. Food webs show the complexity of the trophic relationships in a community.

Energy and organic matter are lost at each transfer (respiration, uneaten parts, egestion, death), so the number of trophic levels is limited.

Gross productivity (GP) is the total gain in biomass by an organism. Net productivity (NP) is what remains after losses due to respiration: NP = GP minus respiration.

Biomass of a trophic level can be measured by collecting and drying samples. Ecological pyramids represent the relative numbers, biomass or energy of trophic levels.

Non-biodegradable pollutants such as PCBs, DDT and mercury are not broken down or excreted. Bioaccumulation is their build-up in an organism's tissues; biomagnification is the increase in concentration at each trophic level, so top predators are most affected (for example, DDT thinned the eggshells of fish-eating birds). Pollutants absorbed within microplastics are transmitted more readily along food chains.

Human activities, including burning fossil fuels, deforestation, urbanization and agriculture, change energy flows and matter transfers by removing producers, releasing stored carbon and adding extra inputs.

Autotrophs synthesise carbon compounds from inorganic sources of carbon and other elements. Heterotrophs obtain carbon compounds from other organisms.

Photoautotrophs use light as the external energy source (photosynthesis). Chemoautotrophs use energy from exothermic inorganic chemical reactions (chemosynthesis). Example: bacteria in the giant tube worms (Riftia pachyptila) at deep-sea hydrothermal vents oxidise hydrogen sulphide, so those communities do not depend on sunlight.

Primary productivity is the rate of biomass production using an external energy source and inorganic sources of carbon. Secondary productivity is the gain in biomass by consumers using carbon compounds absorbed and assimilated from ingested food. Net primary productivity is the basis of food chains because it is the quantity of carbon compounds sustainably available to primary consumers.

Maximum sustainable yield (MSY) is the net primary or net secondary productivity of a system. Sustainable yields are higher for lower trophic levels.

Ecological efficiency = (energy received by the next level ÷ energy received by this level) × 100. Typically about 10 %.

The second law also shows that entropy (disorder) increases as biomass passes through ecosystems.