|Biomass Related to Energy|
Biomass is the amount of organic matter, such as animal and plant tissue, found at a particular time and place. The rate of accumulation of biomass is termed productivity. Primary production is the rate at which plants produce new organic matter through photosynthesis.
Secondary production is the rate at which animals produce their organic matter by feeding on other organisms. Biomass is an instantaneous measure of the amount of organic matter, while primary and secondary production give measures of the rates at which biomass increases.
Plant and animal biomass consists mostly of carbon-rich molecules, such as sugars, starches, proteins, and lipids, and other substances, such as minerals, bone, and shell. The carbon-rich organic molecules are not only the building blocks of life but also the energy-rich molecules used by organisms to fuel their activities.
Solar Energy and Photosynthesis
Ultimately, all energy used by organisms to produce the building blocks of life and to drive life processes originated as solar energy captured by plants. Only a small fraction, less than 2 percent, of the total solar light energy received by a plant is absorbed and transformed by photosynthesis into energy-containing organic molecules.
The rest of the sun’s energy passes out of the plant as heat. The rate at which plants capture light energy and transform it into chemical energy is called primary production.
Because plants do not rely on other organisms to provide their energy needs, they are referred to as primary producers, or autotrophs (meaning “self-feeding”). In addition to light energy, plants must absorb water, carbon dioxide gas, and simple nutrients, such as nitrate and phosphate, to produce various organic molecules during photosynthesis. Oxygen gas is also produced.
Sugars are the first energy-containing organic molecules produced in photosynthesis, and they can be changed to other, more complex, molecules, such as starches, proteins, and fats.
The energy in the sugar molecules can be used immediately by the plants to maintain their own respiration needs, stored as starches and fats, or can be converted to new plant tissue. It is the stored organic matter plus new tissue that contributes to the growth of plants and to biomass.
Because the energy-containing products of photosynthesis can be used either immediately in respiration or in the formation of new plant biomass, two types of primary production can be distinguished. Gross production refers to the total amount of energy produced by photosynthesis.
It includes both the energy used by the plant for respiration and the energy that goes into new biomass. Net production refers only to the amount of energy that accumulates as new biomass. It is only the energy in net production that is potentially available to animal consumers as food.
The rate of primary production varies directly with the rate of photosynthesis; therefore, factors in the environment that affect the rate of photosynthesis affect the rate of primary production. These factors most often include light intensity, temperature, nutrient concentrations, and moisture conditions.
Each species of plant has a specific combination of these factors that promotes maximum rates of primary production. If one or more of these factors is in excess or is in short supply, then the rate of primary production is slowed.
In contrast, Arctic tundra vegetation is exposed to reduced light intensity, very cold winters, and cool summers. Primary production there is very low. In deserts, the lack of water severely limits primary production even though light and temperature are otherwise favorable.
In aquatic habitats, rates of primary production by algae, such as phytoplankton, are determined by nutrient concentration and light intensity. As sunlight penetrates water, it is quickly absorbed by the water molecules and by small suspended particles.
Thus, all primary production occurs near the surface, as long as nutrients are available. Although thewaters of the open ocean are very clear, and sunlight can penetrate to great depths, the scarcity of nutrients reduces the rate of primary production to less than one-tenth that of coastal bays.
The energy and material needs of some organisms are met by consuming the organic materials produced by others. These consumer organisms are called heterotrophs; there are two types. Those that obtain their food from other living organisms are called consumers and include all animals. Those that obtain their energy from dead organisms are called decomposers and include mostly the fungi and bacteria.
The energy available to each type of consumer becomes progressively less at each level of the food chain. Each consumer level uses most of its food energy, about 90 percent, to fuel its respiratory activities. In this energy-releasing process, most of the food energy is actually converted to heat and is lost to the environment.
Only 10 percent or less of the original food energy is used to form new biomass. It is only this small amount of energy that is available for the next consumer level. The result is that food chains are limited in their number of links or levels by the reduced amount of energy available at each higher level.
Generally, the greater the amount of primary production, the larger the number of consumer organisms and the longer the food chain. Most food chains consist of three levels; rarely are there examples of up to five levels. It should be noted that the food chain concept is a simplified view of a more complex network of energy pathways, known as food webs, that occur in nature.
Another outcome of the reduction in energy flow up the food chain is a progressive decrease in production and biomass. The most productive level, and the one with the greatest biomass, is therefore the primary producers, or plants.
Human Threats to Primary Production
|Human Threats to Primary Production|
On land, the Arctic, subarctic, and Antarctic regions are very unproductive most of the year. Human attempts to increase primary production in the form of food or fuel crops usually involve changing the characteristics of the land, converting forests into croplands, for example, and adding large quantities of nutrients and water.
It has been estimated that humans are currently utilizing most of the easily workable croplands and that the development of additional lands for agriculture would require major changes to currently unworkable habitats, changes that would be expensive and demand much fuel energy.
The study of production processes is vitally important in understanding the ecology of natural ecosystems. Such information is necessary to manage and conserve habitats and their organisms in the face of human pressures.
These processes provide insight into the general health of ecosystems. Pollutants, such as acid rain or industrial toxic wastes, are known to reduce the primary and secondary productivity of forests and lakes.
Throughout the world, humans are reducing the biomass of the world’s primary producers through deforestation. This is particularly true in the tropics, where high population pressures have necessitated that land be cleared for agriculture and development. There is a world wide demand for lumber. One obvious consequence is the dramatic reduction in the primary and secondary production of these areas.
The clear-cutting (removal of all the trees) of tropical forests allows unprotected soils to wash away quickly during the heavy tropical rains. It will take hundreds of years for new soils to develop and for the forest to return—if it can return at all.
Deforestation is also harmful in that tropical forests form a major part of the world’s life-support system. For millions of years these forests have buffered the earth’s atmosphere by producing the oxygen gas needed by animals and by removing carbon dioxide and other toxic gases.
The low level of carbon dioxide in the atmosphere is believed to have moderated the earth’s temperature, counteracting the so-called greenhouse effect. It is therefore of great importance to understand and preserve these forests and other primary producers of the world.