In 1944, the US Coast Guard released 29 reindeer onto remote St. Matthew Island. The reindeer flourished, subsisting on the excellent vegetation, and their population soared to more than 6,000 . When the researchers returned in 1966, all but 42 of the reindeer were dead.
The killer was not disease, hunters, environmental change, or ninjas. The killer was overconsumption. St. Matthew Island is the native home of very few animals (just voles and foxes) and a great number of delicious and nutritious lichens which make the perfect diet for reindeer. With ample food supply, the population exploded, and when the island was surveyed in 1957, the average body mass of reindeer on the island exceeded the global average by 25% in females and 47% in males. The birth rate was high and the mortality rate was low. In short, the reindeer were seeing unbelievable success.
But this growth process could not continue indefinitely. By 1963, the population density had reached 46.9 reindeer per square mile, a density which the natural resources simply could not sustain. The average body mass dropped to levels more in keeping with those of domestic herds. The unsustainably large population had completely eliminated the lichen on the island, leaving only sedge grass on which the reindeer could sustain themselves. Over the course of the next year, the population crashed due to intense competition over resources that could only sustain a small fraction of the population, dropping to a mere 42 (41 females, 1 male). When the scientists returned in 1966, the male was not expected to survive another year, and the fawn survival rate was precisely zero. What had begun as an ideal ecosystem turned out to be the demise of the reindeer.
The occurrences on St. Matthew Island stand as a foreboding testament to the potential future of humanity, and the threat is not limited merely to food, but to natural resources in general. We consume more wood (and other resources) every year than the planet is capable of replenishing. The world population continues to rise at an alarming rate, which it can only do because of the ever-increasing amount of land that is devoted to agriculture. When we run out of land to farm, there is little that we will be able to do besides watch as countless people starve our population drops to a fraction of its current level.