So, the second law of thermodynamics says that in a closed system entropy always increases. A different way of saying this is that no process is ever 100% efficient- some energy is always going to be lost as heat. Now, when herbivores eat, they convert the energy stored in the plant material they eat, the air they breathe, and the water they drink (minus the excrement they get rid of) into 1) the energy they use to stay alive, and 2) the chemical bonds of the tissues of their own body. But as they do this, some energy from their food inputs is lost as heat. The second law of thermodynamics prohibits this process (or, in general, any other process) from being completely, totally, 100% efficient. At most it can be 99.9999...(and so on)% efficient. -- So in the real world, how efficient are actual animals? In fact they're no where even close to 99.999...% efficient. On the ecosystem level, on average only about 10% of the energy available to one level of the food chain is transferred to the next level. So when a herbivore (or herbivore population) eats a plant (or a plant population), very very roughly, it's only getting about 10% efficiency. And when a primary carnivore eats an herbivore, that process is only 10% efficient, so the primary carnivore is only getting (0.1*0.1) = 1% of the total energy of the original plants. And now, we can talk about carnivorophagism. When, in turn, a secondary carnivore eats a primary carnivore, its actions capture again only about 10% of the energy available in the primary carnivore. So in total, secondary carnivores capture (0.1*0.1*0.1) = only 0.1% of the original energy available in the original plant. This is why in ecosystems plants always generally exist in huge amounts in comparison to herbivores, and herbivores exist in huge amounts in comparison to the primary carnivore that preys upon them, and so on. Think of an African savanna. How much grass exits there? A humongous amount. How many primary herbivores that eat the grass, like gazelles? Much fewer, but still quite a high number, herds of them. How many primary carnivores of those gazelles exist, say, lions? Much fewer, now only a couple. What about predators of lions? Why is the lion an apex predator of the savanna? It might not be necessarily because lions are large, strong, fearfully fast creatures. It's possible they aren't eaten by something else because it's too inefficient to (exclusively) eat them. -- This is one very general, very simplified reason why carnivorophasim is never going to be completely as efficient as the consumption of primary carnivores or herbivores or plants, and thus not as environmentally friendly.I have heard that eating meat has far greater environmental effects than following a vegetarian diet. How does entropy come into it?
That's an interesting point. It is simplifying, but quite interesting, to look at the steps in the consumption chain -- the crops, chickens, and pigs -- as stores of energy, basically as batteries. I would think that the author's point that "the organisms must expend energy to stay alive" would be more significant than the inefficiency of conversion from consumed to consumer flesh. Adult organisms continue to consume but do not significantly change their body chemistry. I suppose this is mitigated by the goal in commercial farming to fatten up the products as quickly as possible and then slaughter them without wasting any more resources on them. I always thought the corresponding process in cars interesting.