The rabbit limit

Consider you are living only off rabbits.

You need to invest energy in hunting.

But you can never invest more energy in catching rabbits than you get from eating them. You will starve to death in that way, faster the more rabbits you catch.

This is what I call the rabbit limit, or, more scientifically: EROI (Energy Return on Investment)
It can never be lower than 1 if you want the activity to be sustainable

However, human beings have found out a way to cheat. We use fossil fuels to do the job instead. (Consider that you use a car to hunt the rabbit.Then, it will be very easy, since you don't have to invest your precious energy in running! But the car is propelled by fossil fuels, not rabbits. Also, the construction of the car need fossil fuels, not rabbits.)

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