Agriculture uses resources. This is about as simple as it gets. Agriculture is a method by which resources are transformed into food energy with which humans can sustain their lives. Ultimately there are two kinds of resources that agriculture uses in order to achieve this: solar energy and chemicals found in the earth. Both of these resources are ultimately quite limited. We have found new ways to increase the number of resources in the Earth, specifically by using oil, which has been discussed at length. However, solar energy is also limited. Indeed, on a yearly basis the amount of solar energy that arrives on Earth does not vary exceptionally, as such we can deal with solar energy as a constant energy input per year.

Since agriculture is humanity’s main food supply and population is a function of food supply (see thesis #4). We can readily assert that humans are made of solar energy, one to three times removed. As the human population is currently continuing to increase, this would indicate that an ever increasing proportion of Earth’s yearly supply of solar energy is being apportioned to humans and the few species that the majority of humans use as food (see Unlocking the Food and Why People Starve). As such an ever decreasing proportion of this energy is available to other species. When this decrease is coupled with the amount of land desertified by agriculture and other practices and the amount of land paved for roads and cities, a disturbing pattern develops.

The unfortunate fact of the matter is that humans are not exceptional to the laws of physics. The first law of thermodynamics is quite clear: nothing can be created or destroyed. As such, as humans continue to expand we reapportion solar energy once used by other species for our own use. This causes a noticeably large increase in the extinction rate of other species, and as those species die, diversity is lost. This makes agriculture one of the leading destroyers of diversity in the world. While many would not see a problem with this, that is solely due to their own short-sightedness. The position of humanity as an omnivore makes us very adaptive to living off of many different food supplies, but that is irrelevant as we do not live off many different food supplies. The current population of Earth is maintained only through the distribution of wheat, rice, rye, and barley. Grains. Without these very closely related plants our current population would have been unachievable. And it would be impossible to maintain it without them. No other food can be grown as densely, gives us as much energy per pound, or as been so thoroughly domesticated. As with any living thing these grains are reliant on many different insects, bacteria, and other species. If the wrong combination of these species were to become extinct, grain would become extinct as well. While this may seem unlikely right now, it should be remarked upon that as the number of extinct species increases the likelihood of a “keystone� species dying off increases to the point of certainty. This would have the obvious cascade reaction on agriculture and on the majority of the human species.

To summarize:
Agriculture causes a large population of people reliant on a narrow category of plants for survival.
This extra population exists at the expense of other species.
Many of those other species become extinct.
As the number of extinct species increases, the likelihood of a cascade reaction causing the extinction of the afore mentioned plants increases.
Therefore: After X amount time agriculture tends to kill itself off.

While this would certainly mean a rather large percentage of humanity would die off, it would not necessarily mean the extinction of humanity. Omnivores have the advantage of feeding off diverse foods; however, grains are not omnivores.