Examples of Energy Transfer
Think of how much of a plant's energy is needed by a rabbit to chew, digest, produce wastes, create the body heat it needs to live, race away to safety, or to breed. If the rabbit just ate slowly and frequently, created minimal wastes, slept often, and moved about slowly, it would be much fatter when it was eaten, perhaps by a fox. But, since the rabbit has a more active life than this, much of the energy that entered the rabbit as plant tissue is unavailable to the fox.
Similarly, when an eagle catches a fish, much of the energy the fish absorbed in its food will have been used up during its normal life processes and is not available to the eagle.
Cattle farmers do not want their cows running around because too much of the energy the cows absorb from eating hay would be lost as it changed into heat energy and muscle motion and so the cows would become too lean. In the bygone days of the huge cattle drives across the Great Plains of the United States, a major concern of the cattle owners was how thin the cattle would be when they reached the markets in distant cities. A person eating beef from one of those cows would be getting hardly any of the energy the cow got from its food. The cattle owners hoped there would be time to let the cows stand around and eat for a few weeks to fatten them up. Beef from those cows would contain much more of the energy the cows took in when they ate.
Eating mostly plants may not be for all of us but is certainly an efficient route to obtaining energy for our life processes!
Eating shark meat is one of the most useless ways of getting energy. Now of course a shark could provide many meals but, if we ate the fish lower in the food chain that were needed to support the shark, we would have food for weeks.
Think of farming and cows again: all the care, cost and time to raise a few cattle compared to the much lower cost of just the few weeks to grow the grain and corn that fed them. The cattle food would support more of us for a much longer period than the beef from the cows themselves.