资料题：According to this passage, the phrase"balance of nature" means that（　）。

The"balance of nature"is not an empty phrase.Nature provides a population to occupy a suitable environment and cuts down surplus population to ft the available food supply.One means of reducing surplus population is predators(食肉动物); others are parasites and diseases.Also, population density produces nervous disorders and even dives animals to mass migrations, like the lemming(旅鼠)of Norway who plunge into the sea.That predator populations mount to control other animals has long been known.Many yearsago, 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 t 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(松鸡) and hare(野兔), 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 mongoose(猫鼬) to control rats.The mongooses killed of 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 mongoose population declined.