The vegetation of a given district can only support a certain number of
animals, and the different kinds of plant-eaters will compete together
for it. They will also have insects for their competitors, and these
insects will be kept down by birds, which will thus assist the mammalia.
But there will also be carnivora destroying the herbivora; while small
rodents, like the lemming and some of the field-mice, often destroy so
much vegetation as materially to affect the food of all the other groups
of animals. Droughts, floods, severe winters, storms and hurricanes will
injure these in various degrees, but no one species can be diminished in
numbers without the effect being felt in various complex ways by all the
rest. A few illustrations of this reciprocal action must be given.
_Illustrative Cases of the Struggle for Life_.
Sir Charles Lyell observes that if, by the attacks of seals or other
marine foes, salmon are reduced in numbers, the consequence will be that
otters, living far inland, will be deprived of food and will then
destroy many young birds or quadrupeds, so that the increase of a marine
animal may cause the destruction of many land animals hundreds of miles
away.
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