Photosynthesis is fundamental to energy flow through an ecosystem. It is achieved by the autotrophic organisms that convert the energy of sunlight into chemical energy. Energy fixation is achieved by photosynthesis .
All organisms require energy to carry out cellular activity, growth and reproduction. They obtain that energy from the food they eat.
PRIMARY PRODUCTIVITY : The amount of light energy converted into chemical energy by autotrophs in an ecosystem during a given period of time. Measured by the rate of accumulation of BIOMASS (dry weight of vegetation) in the ecosystem [usually expressed as per unit area in a given time e.g. g/m 2 /yr]
GROSS PRIMARY PRODUCTIVITY (GPP): the total primary productivity
HOWEVER: Remember that plants not only photosynthesise, they also respire . Therefore not all the material produced is stored (and available as food for the primary consumers). Some of it is used for cellular respiration and other metabolic activities by the plant itself.
The energy available to the next level in a food chain or food web is the gross primary productivity (GPP) minus the energy used by the plant during respiration (R) and is called the NET PRIMARY PRODUCTIVITY (NPP)
This can be written as an equation: NPP = GPP - R
Net primary productivity (NPP) is of interest because it represents the chemical energy available to the primary consumers (herbivores) and is the beginning of the flow of energy through an ecosystem.
Energy fixation and productivity are the basis of ecosystem productivity. In this section the aim is to discuss how that energy flows through an ecosystem, and to consider the efficiency of that energy flow.
Numbers of organisms in a given area in a given time are counted and then grouped into trophic levels.
Pyramids of numbers typically show a broad base of producers and a successive decrease in number of animals at each level
There are several situations that show inverted pyramids of numbers. For example, a tree, as the primary producer, supports large numbers of insects, which in turn are the food source of large numbers of birds or other predators
Each block = total dry mass of organisms at that trophic level.
At each level the biomass decreases . Normally a pyramid of biomass would have a broad base, getting narrower at each succeeding level.
Occasionally, however, inverted pyramids of biomass can be found where the primary consumers outweigh the primary producers. E.g. In aquatic ecosystems the primary producers are algae. They are very productive and have a high turnover rate . This means that they grow in numbers rapidly but are also eaten in large numbers by zooplankton and small fish. Thus at any given time the biomass of the producers will be less than that of the primary consumers.