Well at each step up the food chain energy is transferred to the consumer. But with each transfer most of the energy is lost and "wasted" and so becomes unsustainable. For example a herbivorous animal like the cow and other vegetarian animals take in the energy from the green plants ( made by photosynthesis) But then this step of the food chain becomes unsustainable and wasteful ,as far as feeding everyone on earth, because the return of energy from the cow ,say, to a meat eater is only about 10 per cent of what the cow received from the plants. This is because 90 per cent is "wasted" on keeping the cow alive and growing beefier for the waiting carnivores.So it follows that the only way to feed the millions of children and adults that starve to death (needlessly) every year is for the world to become sustainable . This can be reached on a vegetarian diet as it is a diet that has no waste and also almost nil pollution. So each step up the food chains and food webs is a disaster for the worlds peoples as the whole system is unsustainable. Much energy is lost the higher the rung.