[Serious] Why defecation?

[John 6IX Breezy]

This applies to any system, not just biological ones, that consume fuel and produce waste. A locomotive that uses 100 pounds of coal per mile and belches ten pounds of carbon into the atmosphere is clearly less efficient than an equivalent one that uses 10 pounds per mile and emits one pound of carbon (or whatever the actual figures are - it doesn’t really matter). I can state this is true without knowing how locomotives work. 

Sorry for the double response. This comparison between cars and animals didn't sit right with me but I wasn't familiar enough with cars to respond. After doing a bit of research I'm convinced that the concept of "waste" is not comparable across the two categories.

To understand why, we need to take an ecological approach. Coal exists in deposits and are therefore isolated from the ecosystem. When coal is mined and subsequently burned it is introducing emission into the ecosystem that negatively affects the balance of the environment. For example, Sulfur Dioxide contributes to acid rain and Carbon Dioxide contributes to the green house effect. It is this introduction of new chemicals into an ecosystem that makes the waste of car emission incomparable to the waste found in feces.

Biological ecosystems in comparison have two key properties: Energy enters into it through sunlight and passes through. And chemicals are constantly being recycled. Carbon in a biological ecosystem is never "wasted" it is simply recycled constantly between living and nonliving things. Plants, take up ingredients from the ground, and pass it herbivores, which pass it to carnivores. And when an animal defecates or dies, decomposers transform them back into inorganic matter to be taken up by plants again. 

In other words. Cars produce waste because their emissions introduce chemicals into the environment that measurably pollute the environment. Animals do not have such waste; their feces continues a cycle of chemicals within an ecosystem, and nothing is lost or introduced into the environment. The word waste when it comes to animals is perhaps a misnomer when compared to anything like car emissions.