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Sunlight represents the external energy source without which ecosystems cannot sustain themselves.
The transformation (conversion) of light energy to chemical energy, which is the only mode of energy usable by cells of all components of an ecosystem, whether they are producers, consumers or decomposers, is done through a process called photosynthesis. Therefore, photosynthesis - whether carried out by plants or microorganisms - is the only process of energy entry into an ecosystem.
We often have the impression that the earth receives a daily amount of light, greater than it really needs. In a way this is true, since as efficient as ecosystems are, they are able to harness only a small part of radiant energy.
About 34% of sunlight is estimated to be reflected by clouds and dust; 19% would be absorbed by clouds, ozone and water vapor. Of the remaining 47%, which reaches the earth's surface, much of it is still reflected or absorbed and transformed into heat, which may be responsible for the evaporation of water in the heating of the soil, thus conditioning the atmospheric processes.
Photosynthesis uses only a small portion (1 to 2%) of the total energy reaching the earth's surface. Importantly, the values quoted above are average values and not specific to any locality. Thus, the proportions may - although not much - vary according to the different regions of the country or even the planet.
An important aspect for understanding energy transfer within an ecosystem is the understanding of the first fundamental law of thermodynamics that says: “Energy cannot be created or destroyed, but transformed”. As an illustrative example of this condition, one can cite sunlight, which as a source of energy can be transformed into work, heat or food as a function of photosynthetic activity; but in no way can it be destroyed or created.
Another important aspect is that the amount of available energy decreases as it is transferred from one trophic level to another. Thus, in the examples given above of food chains, the grasshopper obtains, by eating the leaves of the tree, chemical energy; However, this energy is much smaller than the solar energy received by the plant. This transfer loss occurs successively until it reaches the decomposers.
And why does this occur? The explanation for this energy decrease from one trophic level to another is the fact of each organism; require much of the energy absorbed to maintain their vital activities such as cell division, movement, reproduction, etc.
The text about pyramidswill then show the biomass proportions from one trophic level to another. As we move from one trophic level to the next, the number of organisms decreases and the size of each one increases (biomass).