The rate of biomass production or the amount of food energy produced or obtained or stored by a particular trophic level per unit area in a unit time is called productivity. It can also be defined as the energy accumulated in plants by photosynthesis. The unit of productivity is gm/m2/year or kcal/m2/year.
According to Odum there are three types of productivities – primary productivity, secondary productivity and community productivity.
Type # 1. Primary Productivity:
Primary productivity is the productivity at the producer level. It can be termed as the amount of organic matter produced by the plants from solar energy in a given area during a given period of time. Approximately 1-5% of solar energy that falls on the plant is converted to organic matter.
Primary productivity is of two types:
i. Gross Primary Productivity (GPP):
This refers to the total amount of organic matter produced. This also includes what is used by the producer in respiration. This can also be defined as total energy captured by the photosynthetic organism. This will depend on the photosynthetic capacity of the producer and environmental factors. Mean net primary production is high in tropical rain forest while it is the lowest in deserts.
ii. Net Primary Productivity (NPP):
This productivity refers to the gross production minus the loss by respiration and decomposition. This is also called as apparent photosynthesis or net assimilation.
NPP = GPP – Respiration
It can also be defined as the balance energy or biomass remaining after meeting the cost of respiration of producers. It is the net stored energy in the green plants. This is the net accumulation of biomass which serves as food for herbivores and decomposers. NPP is said to be a measure of amount of organic matter produced in a community in a given time available to the heterotrophs.
Type # 2. Secondary Productivity:
This refers to the productivity at the consumer level. The secondary productivity reflects only the utilisation of food for production of consumer biomass. It can be referred to as the net rate of increase in biomass of heterotrophs. The secondary productivity acts as food to the next trophic level (Fig. 3).
Type # 3. Community Productivity:
This is the rate of net synthesis of organic matter by a community per unit time and area.
Energy Flow in Ecosystem:
The main function of an ecosystem is to transfer energy from one level to the other level. The flow of energy is based on the two laws of thermodynamics. First law of thermodynamics states that the energy can neither be created nor destroyed. It can however be transformed from one form to the other.
For example, solar energy changes into energy of food and heat. Second law of thermodynamics states that during this transformation some amount of energy is always lost in the form of heat. For example, when energy in the form of food is transferred from one organism to the next, some energy is lost at every step.
Sun is the only source of energy for all living organisms. Sunlight can be divided into three categories – UV light which is reflected back by ozone, infrared light which helps in heating and visible light which is also called as photo synthetically active radiation (PAR). Out of the incident solar radiation, 30% of it is reflected back, 20% gets absorbed in the atmosphere and 50% is visible light.
Out of this 50% of visible light only 2-10% is utilised for photosynthesis for gross primary productivity. Remaining 40-48% gets dissipated on the Earth as heat. Some energy is utilised by the plants and what is left behind is net primary productivity which is only 0.8-4% of incident solar radiation.
From the above factors it is clear that energy flows in a single direction, and keeps on decreasing at every level.
Trophic levels represent the feeding levels. Trophic levels are made on the basis of steps found in the food chain. There are usually around 4-5 trophic levels in a food chain and energy flows in only one direction in these food chains.
First trophic level is occupied by the producers which are autotrophs. They have got the capacity to utilise solar energy. Second level is occupied by the primary consumers which are the herbivores. Secondary consumers are carnivores and omnivores and occupy the third trophic level. Fourth trophic level is for the tertiary consumers which are large carnivores. (Fig. 5)