You're right that animal-based foods are much more calorie-dense than plant-based foods. But an animal contains roughly 1/10 of the calories of all the plants that collectively went into feeding it.
The animal's only energy source is plants. Some portion of that energy goes into producing muscle and organ tissue. Some of it is converted to heat. The rest is used for respiration, digestion, thinking, walking around, dreaming, and all the other ongoing processes of life. Animals don't undergo photosynthesis, nor do they spontaneously generate energy.
Unless there's something else you're getting at here. Energy loss and trophic levels is a pretty well-understood idea.
Admittedly, there is a bit more nuance to the issue; for example, a cow can produce milk and meat from grass, which, for humans, creates available nutrients where there were none before. However, we could also plant that field of grass with corn or potatoes and get many more calories for the same amount of water and sunlight.
The animal's only energy source is plants. Some portion of that energy goes into producing muscle and organ tissue. Some of it is converted to heat. The rest is used for respiration, digestion, thinking, walking around, dreaming, and all the other ongoing processes of life. Animals don't undergo photosynthesis, nor do they spontaneously generate energy.
Unless there's something else you're getting at here. Energy loss and trophic levels is a pretty well-understood idea.
http://kids.britannica.com/students/assembly/view/90132
Admittedly, there is a bit more nuance to the issue; for example, a cow can produce milk and meat from grass, which, for humans, creates available nutrients where there were none before. However, we could also plant that field of grass with corn or potatoes and get many more calories for the same amount of water and sunlight.