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

Ecosystems are sustained by supplies of energy and matter. Energy flows through ecosystems; matter is recycled. The first law of thermodynamics says energy can be transformed from one form to another but cannot be created or destroyed. The second law says energy transformations are inefficient: some energy is always dispersed as heat, so a continual supply of energy (from the Sun) is needed.

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

Consumers gain chemical energy from carbon compounds in other organisms and have diverse feeding strategies. Carbon compounds and the energy they contain pass along a food chain through trophic levels.

Gross productivity (GP) is the total gain in biomass by an organism; net productivity (NP) is what remains after losses to respiration: NP = GP − R. Energy and organic matter are lost along a food chain (uneaten parts, egestion, respiration), so the number of trophic levels is limited. Food webs show the complexity of feeding relationships in communities.

Biomass of a trophic level is measured by collecting and drying samples. Ecological pyramids represent numbers, biomass or energy of trophic levels; energy pyramids per unit area per year are never inverted.

Pollutants that are non-biodegradable (PCBs, DDT, mercury) are absorbed and stored, often in fat, and not excreted. Bioaccumulation is build-up in one organism's tissues over its life. Biomagnification is the increase in concentration at successive trophic levels, so top consumers are most at risk. Pollutants absorbed within microplastics can enter the food chain and increase their transmission.

Case study: DDD in Clear Lake, California, which reached more than 1,000 ppm in fish-eating grebes from about 0.02 ppm in the water.

Human activities change flows of energy and transfers of matter. Burning fossil fuels releases carbon stored for millions of years. Deforestation reduces producers and releases stored carbon. Urbanisation replaces productive land with surfaces that support little biomass. Agriculture simplifies food webs, removes biomass at harvest and adds fertiliser and fossil-fuel energy inputs.