ข้อที่ 13. The “balance of nature” is not an empty phrase. Nature provides a population to occupy a suitable environment and cuts down surplus population to fit the available food supply. One means of reducing surplus population is predators; others are parasites and diseases. Also, population density produces nervous disorders and even drives animals to mass migrations, like the lemmings of Norway which plunge into the sea. That predator populations mount to control other animals has long been known. Many years ago, the Hudson’s Bay Company records revealed that the fox population went up and down about a year after the rabbit population had gone up and down. Sometimes a situation occurs in which the predator population is reduced to a level below that which nature can readily replace. On Valcour Island in Lake Champlain (New York), a costly campaign resulted in the elimination of predatory animals only to have birds and small animals (including grouse and hares, popular game) increase for four years afterward. Then, lacking predator control, nature resorted to disease to cut down these populations. Jamaica had an example of nature’s persistence in providing animals for existing habitat. Sugar planters, about 75 years ago, imported mongooses to control rats. The mongooses killed off the rats and, with plentiful food, multiplied. Rats became scarce and the mongooses ate poultry, lambs, kids, kittens, puppies, and wildlife. Eventually, food became scarce and the mongoose population declined. The phrase “resulted in” (line 13) can be replaced by _________