CHAPTER III. STRUGGLE FOR EXISTENCE.
4. NATURE OF THE CHECKS TO INCREASE.
The causes which check the natural tendency of each species to increase are
most obscure. Look at the most vigorous species; by as much as it swarms
in numbers, by so much will it tend to increase still further. We know not
exactly what the checks are even in a single instance. Nor will this
surprise any one who reflects how ignorant we are on this head, even in
regard to mankind, although so incomparably better known than any other
animal. This subject of the checks to increase has been ably treated by
several authors, and I hope in a future work to discuss it at considerable
length, more especially in regard to the feral animals of South America.
Here I will make only a few remarks, just to recall to the reader's mind
some of the chief points. Eggs or very young animals seem generally to
suffer most, but this is not invariably the case. With plants there is a
vast destruction of seeds, but from some observations which I have made it
appears that the seedlings suffer most from germinating in ground already
thickly stocked with other plants. Seedlings, also, are destroyed in vast
numbers by various enemies; for instance, on a piece of ground three feet
long and two wide, dug and cleared, and where there could be no choking
from other plants, I marked all the seedlings of our native weeds as they
came up, and out of 357 no less than 295 were destroyed, chiefly by slugs
and insects. If turf which has long been mown, and the case would be the
same with turf closely browsed by quadrupeds, be let to grow, the more
vigorous plants gradually kill the less vigorous, though fully grown
plants; thus out of twenty species grown on a little plot of mown turf
(three feet by four) nine species perished, from the other species being
allowed to grow up freely.
The amount of food for each species, of course, gives the extreme limit to
which each can increase; but very frequently it is not the obtaining food,
but the serving as prey to other animals, which determines the average
number of a species. Thus, there seems to be little doubt that the stock
of partridges, grouse, and hares on any large estate depends chiefly on the
destruction of vermin. If not one head of game were shot during the next
twenty years in England, and, at the same time, if no vermin were
destroyed, there would, in all probability, be less game than at present,
although hundreds of thousands of game animals are now annually shot. On
the other hand, in some cases, as with the elephant, none are destroyed by
beasts of prey; for even the tiger in India most rarely dares to attack a
young elephant protected by its dam.