The ultimate source of energy for organisms found in an ecosystem is sunlight. This is converted to chemical energy by photosynthesising organisms and passed as food between other organisms. The Sun’s light energy is captured and used by green plants and algae during photosynthesis, to make new biomass.
Energy flows through the community of an ecosystem in one direction with primary producers harvesting energy through photosynthesis or chemosynthesis.
The majority of autotrophs are photoautotrophs that harness the energy of the sun to make biomass. This energy is then passed on to consumers through feeding pathways.
The energy contained within producers and consumers is ultimately passed to the decomposers that are responsible for the constant recycling of nutrients.
A food chain is a linear series of feeding relationships in which there is a transfer of food from one organism to the next. Food chains name the organisms at each link in the chain, and give us a descriptive flowchart of the passage of energy and nutrients through the trophic levels.
A food chain shows how energy is passed through a series of animals. The arrows in a food chain show the flow of energy. As an organism is eaten, energy is transferred to the organism that ate it.
Within most ecosystems, organisms feed upon a variety of resources and many food chains co-exist.
Linkages form between different chains creating a complex network of connections. A food web is a complex network of inter-related food chains.
Food chains and webs provide only qualitative information about the feeding relationships between organisms.
A different concept for representing feeding relationships is the pyramid of numbers where trophic levels are displayed as a series of steps of differing widths.
The area of each step in the pyramid is proportional to
the numbers of organisms present at each trophic level.
Pyramids of numbers present difficulties for comparing different ecosystems and producers of different sizes; a better representation of trophic levels within ecosystems is to represent organisms at each level in terms of total dry mass per square metre of area.
The dry mass of living material determined for a given trophic level is termed the biomass.
A pyramid of biomass represents the total dry mass (in grams per square metre of area) of all the organisms in each trophic level at a particular time; such pyramids take into account the size of the organisms represented in the pyramid.
A more accurate representation of the energetics of food chains is obtained by measuring the energy content of material and displaying this information as a pyramid of energy.
Energy pyramids are constructed using energy values determined from a given area over a specified period of time for each trophic level (usually kJ m-2 yr-1).