If you’re an American citizen, did you know that you can expand the territory of the United States all by yourself? All you have to do is find an island that isn’t claimed or inhabited by citizens of another country, and take “peaceable possession” of it.
Oh, and the island has to be covered in bird shit.
If you expand your personal and national dominion in this manner, you can thank the Guano Islands Act of 1856. The law was passed in the middle of a period of what might be called guano mania.
As the global population grew in the 1800s, farming became more intensive. The increased demands of farming had depleted soils in many parts of the world of phosphate, potassium, and nitrogen, which are necessary for most plants to grow. Traditional methods of fertilization — which mostly involved putting manure on fields — weren’t cutting it. So people began looking for alternative sources of these nutrients to augment their soils.
They found them on tiny islands out in the ocean. In some waters, like those west of South America, rocks jutted above the surface of the Pacific. These islets were no good for traditional human habitation; there was little fresh water, and very little soil to grow crops. Instead, there were immense colonies of seabirds, hundreds of thousands per island, which rested, nested, and, crucially, defecated there. In the sunny, dry weather off the coasts of Chile and Peru, this excrement never washed away. It dried and piled up into what looked like mountains.
Though these mountains of excrement were, by all accounts, repellent, they were economically valuable because they contained concentrated deposits of — you guessed it — phosphates, potassium, and nitrogen. Crush that stuff up and spread it on your field, and you’d grow a lot more food.