Food Chains
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.
Food Webs
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.
Pyramids of Number
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 Biomass
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.
Pyramids of Energy
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).
Energy Inefficiency
Pyramids of energy show how little energy passes from one trophic level to the next. Despite this, all energy in an ecosystem ultimately comes from the Sun, which powers the growth of plants and the energy available to all other organisms.
Energy from the sun reaches the Earth’s atmosphere at an average rate of 8.4 J cm-2 min-1, but the amount of this energy utilised during photosynthesis is (on average) only 1 to 5%, However, it is enough to drive nearly all life on Earth.