Answer:
A population would grow faster than its ability to feed itself.
Explanation:
Thomas Malthus' theory, in my personal beliefs, is remarkably accurate and quite rational. He argued that if one were to have a country/population left unchecked, as in without any form of administration, government, or central authority to balance it, that a population would thus outgrow its resources and thus result in overpopulation and a lack of necessities... something that may, perhaps, lead to eventual extinction.
This is fairly factual when you think of the contemporary age. The earth was previously believed to have a carrying capacity of about 2-40 billion people, an argument that eventually centered on around 7 billion. Today, the earth's maximum carrying capacity is generally percieved to be about 9 billion people. In this age, we currently are nearing 8 billion.
This. Is. An. Issue.
A plethora of earth's resources that life itself depends on is LIMITED. Our freshwater reserves are limited. The amount of animals on this planet, a source of food, is <em>also </em>limited. The amount of plants on this planet, significant sources of energy, food, oxygen, and all sorts of natural processes that keep everything alive, are, unfortunately, limited.
This demands that humans figure a way to require less of these precious resources, fast. By the year of 2150, we'll likely have surpassed our carrying capacity.
For the issue of food, there are options. The primary issue is that humans are omnivores, as in, we love both plants AND animals... in our stomach's, of course. A prime example is myself! Personally, I couldn't live without beef, but I <em>definitely </em>couldn't or wouldn't want to survive without spinach and broccoli, because they are absolutely delicious.
However, despite humans being omnivores, we stubbornly refuse to eat our veggies. . . meaning a mass majority of us prefer to eat meat. We breed our animals to have offspring, giving us more meat. We generically enhance or even create our meat. We love meat.
The issue being that meat is a terrible source of energy. Remember, energy comes from sources of life itself, like the sun! PLANTS take the mass majority of this energy in, not animals. Animals EAT the plants, to where as much as 80% of that initial energy source is lost, disappearing into nothing, and meaning only roughly 20% is absorbed into the animal upon eating the said plant. Then, and only then, HUMANS come to eat the animal, in which 80% of that initial 20% is also lost between these stages.
As you can see, humans end up with barely any amount of this vital energy, simply because we love meat. We feed the plants to the animals to keep them healthy so WE can then eat the said animals, thus resulting in a HUGE loss of energy. We use our land for pastures. We give other resources (like water) to the animals, again, so we can eventually consume them.
The earth is going to run out of resources at one point or another, but our current consumption habits will likely hasten this process as far as freshwater and food.
Ofc, it shouldn't need to be said that if we were ALL to switch to primarily plant-only consumption, we'd probably be set. Getting rid of all our pastures and replacing them with massive farms would give is a surplus of plants, which are remarkably better sources of energy and will thus be able to sustain humans much, much longer. We won't have to worry as much about starving.
Then again, you must ALSO worry about the fragility of plants. They can easily be detroyed by natural disastors and are dependant upon environmental conditions such as weather temperature, climate, and soil. These factors are very limiting, but then you must additionally remember the amount of care they require, as well as they are extremely vunerable to mass destruction (like droughts, burning, flooding, etc., which can wipe out a LOT at once).
Obviously it's a give-or-take thing.
Malthus said it right, three hundred years ago.
I get the length of this post was probably uneccesary but you asked a very good question that gave me an excuse to cover something in-depth.
I am inevitable.
~Troy