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.
Climate plays an important part in determining the average numbers of a
species, and periodical seasons of extreme cold or drought seem to be the
most effective of all checks. I estimated (chiefly from the greatly
reduced numbers of nests in the spring) that the winter of 1854-5 destroyed
four-fifths of the birds in my own grounds; and this is a tremendous
destruction, when we remember that ten per cent. is an extraordinarily
severe mortality from epidemics with man. The action of climate seems at
first sight to be quite independent of the struggle for existence; but in
so far as climate chiefly acts in reducing food, it brings on the most
severe struggle between the individuals, whether of the same or of distinct
species, which subsist on the same kind of food. Even when climate, for
instance, extreme cold, acts directly, it will be the least vigorous
individuals, or those which have got least food through the advancing
winter, which will suffer the most.
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