A pair of song-thrushes may raise two broods a year, with
five or six nestlings in each brood; every
autumn an oak tree may produce many thousands of acorns. Yet the numbers of oak trees and
thrushes in the countryside are not increasing. This is because animals and plants have to
compete for food, water, light and space, and a great many do not survive to become adults.
Numbers are therefore held in check.
A woodland of, say, five hectares (12.3 acres) can support
a limited number of thrushes, voles,
tawny owls and stoats. If, for a time, the number of voles increases beyond a certain point, there
will not be enough food to go round, and some will starve or become diseased. Also the numbers of
birds and beasts of prey (stoats, weasels, owls and hawks) will increase, and before long the
number of voles will be reduced. This is called the balance of nature.
Sometimes man disrupts this balance. Farmers and gamekeepers
from time to time shoot or
poison stoats and owls, so that the numbers of mice and voles increase to a level where they eat
so many nuts and acorns that few are left to grow up into trees to replace those that die or are
felled.
