The sun is the main source of energy in all ecosystems. Plants harvest all their energy through photosynthesis, then other organisms eat the plants (and other producers) to gain energy. Without the sun this process would never happen. 

The majority of energy in a food chain is lost as respiratory heat. Whenever an organism takes food, breaks it down, and converts it to energy, heat is a byproduct that contains the energy lost. About 66% of the energy in a food chain is lost due to respiratory heat. No energy is lost to decomposers, rather, it is transferred to them. The decomposers respire, and create heat as well. 

Efficient foraging, also known as economical foraging, is the process by which organisms attempt to maximize their energy return for energy expended. In other words, if a lion was hunting she would want to find food that would give her more energy from eating it than she would spend hunting it.

A heterotroph consumes other organisms to obtain energy. Photoautotrophs use sunlight to make glucose in the process of photosynthesis. In photosynthesis, organisms combine carbon dioxide with water to produce sugar molecules. Chemoautotrophs use chemicals to build molecules and obtain energy.

There are various forms of symbiosis. In mutualism both species benefit while in commensalism only one species benefits and the other species is left unaffected. In parasitism, one species is harmed while the other benefits. Last, predation is defined a predator-prey relationship in which one species feeds on another.

It is true we derive energy from eating meat. And it is also true that the cow derives its energy from the grass it eats; however, ultimately the sun provides the raw energy for the grass to synthesize biomolecules that the cow uses to synthesize its biomolecules after eating the grass. When we consume a cow, we convert its energy to energy we can in the form of molecules such as glucose. The ultimate or very first energy source is the sun.

In a standard food pyramid, organisms are divided into trophic levels based on their means of gaining nutrients. As one moves upwards through trophic levels, the number of organisms that can be sustained decreases. This is because energy is lost between each level. Typically, about 90% of the energy in one trophic level is lost during transfer to the next highest level; this leaves on about 10% of the energy to be used by the consumer. Because of this disparity, it is very difficult to maintain large populations at higher trophic levels. This explains why lower level organisms can easily flourish (such as ants), while higher level organisms can easily become endangered (such as tigers).