Productivity in Ecosystems
Productivity is the rate of synthesis of energy containing organic matter by any trophic level per unit area in unit time. Production ecology is the branch of ecology that deals with rate of production of organic matter in different components of an ecosystem.
Productivity is of the following two types; Primary productivity and Secondary productivity.
On this earth only the photosynthetic organisms have the ability to utilize radiant energy and manufacture organic substances using water and carbon dioxide. The amount of energy accumulation in green plants through the process of photosynthesis is termed as primary productivity. It is of the following two sub types:
Gross primary productivity (GPP)
The total organic matter which the producers synthesize in the process of photosynthesis per unit time and area is termed as gross primary productivity. It includes the weight of organic matter added in the body of the producer plus the losses suffered by them due to respiration , grazing and other damages.
Net primary productivity (NPP)
It is the weight of the organic matter which the producers store in a unit area/volume per unit time. Net primary productivity is equal to the rate of organic matter created by photosynthesis minus the rate of respiration and other losses.
Net primary productivity (NPP)= Gross primary productivity (GPP)-Respiration
Net primary productivity over time appears as plant growth and it is also available to the next link in the food chain, i.e., the primary consumers (i.e., herbivores). Productivity and yield are two different parameters. The productivities are rates and yield of an agricultural crop, on the other hand, is a measurable weight or volume of harvestable material.
Net primary productivity is directly proportional to the rate of production of oxygen (i.e., during photosynthesis) and to the rate of depletion of carbon dioxide. Measurement of either of these two can be useful in the calculation of net primary productivity.
Net primary productivity depends on climate and other factors which affect photosynthesis. Low temperature, drought, absence of essential minerals, low light intensity, etc., reduce the productivity significantly.
According to a rough estimate the annual net primary productivity is approximately 170 billion tons (i.e., dry weight) of organic matter. Of this total, 115 billion tons are produced on land, and oceans which occupy about 70% of the earth surface, produce only 55 billion tons. Humans harvest about 1.2 billion tons per year as plant food.
Differences between Net primary productivity and Gross primary productivity
|Net primary productivity||Gross primary productivity|
|1. It is the amount of organic matter which the producers store per unit time and area.||1. It is the amount of organic matter which the producers synthesize per unit time and area.|
|2. Net primary productivity is equal to organic matter synthesized by photosynthesis minus rate of respiration and other loss.||2. Gross primary productivity is equal to rate of increase in body weight of producers plus loss suffered through respiration and damages.|
The rate of resynthesis of organic matters by the consumers is termed as secondary productivity. The producers (i.e., autotrophs) trap the solar energy which the heterotrophs (i.e., consumers) use for themselves . The herbivores directly consume the producers. The secondary consumers (i.e., primary carnivores) consume the herbivores. The secondary carnivores consume the primary carnivores. All these consumers store some amount of energy in their tissues. This stored energy is termed as secondary productivity.
It depends upon the loss while transferring energy containing organic matter from the previous trophic level plus the consumption due to respiration. Respiration loss is about 20% for autotrophs, 30% for herbivorous and up to 60% in case of carnivorous. Therefore net productivity decreases with each trophic level.
Yield of an ecosystem depends upon the trophic level exploited by man. It is usually T2 or second trophic level (i.e., herbivores) which is exploited on land for protein while in ocean the trophic levels harvested by man are T3, T4, T5.
Of the net primary production available in temperate forests, herbivores (e.g., caterpillars, aphids, deer and other herbivorous animals) eat only about 1 to 3%.In some other communities 10% or more of vegetation may be eaten by herbivorous. Not all of this becomes secondary productivity. On an average, approximately 10% of the food material consumed by different types of consumers is absorbed by them and the rest is egested. Furthermore, of the food material absorbed, about two third is used in respiration. Thus only about 15% of ingested food appears as secondary productivity.
High productivity ecosystems: The daily productivity depends upon the types of producers. The ecosystems occur in areas such as tropical forests, flood plains, coral reefs, areas of upwelling, sugarcane fields, etc. Maximum productivity occurs in coral reefs, followed by estuaries (areas of upwelling) and sugarcane fields.
Average productivity ecosystems: Common agricultural crops and temperate forests belong to this category.
Less productivity ecosystems: Ecosystems such as grassland ecosystems and savannah are best examples of less productivity ecosystems.
Low productivity ecosystems: Ecosystems such as deep sea and arid lands are examples of low productivity ecosystems. Low productivity of arid lands or deserts is due to non availability of water accompanied by high temperature in hot deserts and low temperature in cold deserts. Light (decreases with depth) and nitrogen limits the productivity in oceans.
Net productivity in world ecosystem
Whittaker (1970) and Leith (1974 ,1975) have compiled data about the net productivity of major ecosystems of the world. Tropical rain forests, tropical deciduous forests, temperate forests, savannah, temperate grasslands and desert scrub show the maximum primary production rate.
The yield is very high in the tropical areas as compared to cold and dry lands. The reason for higher yield of crop lands is subsidization of solar powered ecosystem with man made machines . Also extra nutrients added by man besides providing protection against pests and parasites. The productivity of all natural habitats gets augumented wherever the solar powered ecosystem is subsidized e.g., tidal waves in coastal estuaries, wind and rain in tropical rainforest, flowing water in streams, etc.