Describe the source of food for the first 3 levels in the food chain and explain why it takes less land, water, air to produce a gram of plant tissue to eat than it does to produce a gram of animal tissue to eat.

This question is incomplete as the specific food chain is not provided. In general, a food chain will have a primary producer at the base of the chain. This is an organism that is able to utilise a form of energy to convert carbon dioxide in the atmosphere to a carbohydrate. For terrestrial food chains, these are typically plants, who by utilising the energy from the sun, are able to convert carbon dioxide to carbohydrates. The subsequent level in the food chain may be a primary consumer, typically herbivores, that consume plant matter. Subsequent links in the chain would typically be secondary consumers, who would be carnivores, omnivores or decomposers. Carnivores would typically be predators and would be at the apex of the food chain. Energy is lost from one link to the subsequent link in the food chain, through basic respiration and inefficient energy transfer from one link to another. This explains why more resources (land, water and air) are required to grow meat rather than plant matter. More of the sun's energy is available within the lower trophic levels in a food chain, before much of it is lost as energy moves up the food chain. An easy example to illustrate this is that much of the livestock raised in the USA is fed grain. If people rather ate the grain than the livestock, they could obtain all their required energy from a smaller amount of grain then would be needed to raise the meat they require. 

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