More than half of the radiant energy that reaches the earth’s atmosphere is reflected. Depending on the latitude and cloud cover, the solar energy that reaches the earth in a year is estimated to be 5.25 million kjlm2/yr. This quantity is known as the solar constant. This large flow of energy reaching the earth is reduced (exponentially) as it passes through clouds, water vapours and other atmospheric gasses. Hence, the amount that actually reaches the earth’s ecosystems is about 1.0-2. million KJ/m2/yr. When this solar energy falls on the ecosystem, about 20% is reflected by the vegetation; 30% is used in evaporating water from the leaves (transpiration), 40% warms up the plants, soil and the air leaving only about 1% to be used in photosynthesis for making new organic matter in the leaves of plants. When this 1% is absorbed by plants, some of it is lost as heat. Assuming 200 units of this energy was absorbed by the leaf, 150 units, representing 75% is lost as heat while 50 units (25%) is changed to chemical energy in food. But all this is not passed on to the consumer; about 7.5% of the chemical energy in food is used by autotrophs for its activities. 17.5% stored in autotrophs can be passed on to the primary consumer (herbivore).
However, only a small proportion of this energy gets into the bodies of the final consumers. In autotrophs, the loss in energy affects primary production because photosynthesis is dependent on solar energy. Energy, which may be defined as the capacity of doing work, exists in many different forms. Thus, a moving train possesses mechanical energy, and does work in overcoming the frictional resistance to its movements. A fire possesses heat energy, and does work in heating a room or perhaps in converting water to steam. An accumulator possesses chemical energy, and does work in sending an electric current round a circuit. This electrical energy may then do work in ringing a bell or starting up a motor car engine. Similarly, sound energy does work in setting the air in vibration and eventually, causing the ear to hear; while light energy causes the eye to see.
As a result of the careful work of physicists over the centuries, it has gradually become clear that energy can be transformed from one form to another but cannot be destroyed. This is the law of conservation of energy. In nature, such transformations are governed by laws of thermodynamics.