Topic 5.1 / Option G.2 Ecosystem Ecology 1 Matter and Energy Flow in Ecosystems Assessment Statements: 5.1.9 - 5.1.14, G.1.9 - G.1.10, G.2.1 – G.2.5

Energy flow in ecosystems The initial source of energy for nearly all communities is the sun. The sun’s physical energy is converted into the chemical energy of glucose through the process of photosynthesis by the community’s autotrophs or producers. Only a small percentage of the total energy reaching the earth is available for photosynthesis. It is reflected or absorbed by other substances on earth or in the atmosphere It is of wavelengths not suitable for photosynthesis

Primary Production and Biomass Approximately 1 % of light reaching autotrophs is converted into chemical energy by photosynthesis. The actual amount depends on the type of organism, type of light and other environmental factors. The total energy converted per unit of time in a given ecosystem is called gross primary production (GPP). Plants use chemical energy for their own life processes and only some is converted into new biomass available for consumption by primary consumers. This amount is called net primary production (NPP). Net primary production is equal to gross primary production minus the amount used by plants through cellular respiration. NPP = GPP – R Since energy in living things is stored as molecules and all mass took energy to produce, biomass may be thought of as the matter form of energy.

Secondary production Secondary production is the amount of chemical energy in a consumer’s food, that gets converted into new biomass in a given time. This is usually only about 10 percent of the total energy that was available on the lower trophic level. Not all the food gets into the consumer. Not all the food that gets in gets digested. Not all the energy created by the food that is digested gets turned into new biomass. Most is used for cellular processes during the organism’s life, and lost as heat.

Efficiency An organism’s production efficiency is the percentage of energy converted into new biomass compared to the total assimilated. The rest is used in cellular respiration. A community’s trophic efficiency is the percentage of energy that is transferred from one trophic level to the next. The rest is lost as heat or was never eaten in the first place.

Energy pyramid with 10% trophic efficiency Energy units are joules per unit area per unit time. (J/m2/y)

A biomass pyramid looks the same

And so does a population pyramid

Most communities have about 5-20 % trophic efficiency This means that only 5 to 20 percent of the total energy is actually passed up to the next trophic level. This explains why there are greater numbers of organisms at the lower trophic levels than the higher ones. There is less energy available to tertiary consumers than primary consumers. This also explains why few communities can support more than four trophic levels.

What happens to the biomass? Biomass that is converted into energy is ultimately lost from the ecosystem in the form of heat. Most of the matter (carbon, hydrogen and oxygen) is recycled in the form of carbon dioxide and water through cellular respiration. Other matter such as nutrients and minerals must be recycled into the soil or water through decomposition. Saprotrophic bacteria and fungi (decomposers) perform this function.