Epidemics as a Check

When a species increases inordinately in numbers in a small tract due to highly favourable circumstances, epidemics often ensue.

• This generally happens to our game animals

Some of these epidemics are due to parasitic worms have been disproportionably favoured by the crowding of animals.

This is a struggle between the parasite and its prey.

On the other hand, in many cases, a large stock of individuals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation.

Thus, we can easily raise plenty of corn and rape-seed, &c., in our fields because the seeds are in great excess compared with the number of birds which feed on them/

The birds cannot increase proportionally to the supply of seed as their numbers are checked during winter.

It is troublesome to get seed from a few wheat or other such plants in a garden.

• I have in this case lost every single seed.

This necessity of a large stock of the same species for its preservation explains why:

• very rare plants are sometimes extremely abundant in the few spots where they do occur
• some social plants being social, that is, abounding in individuals, even on the extreme confines of their range.

In such cases, a plant could exist only where the conditions of its life were so favourable that many could exist together.

• Thus, they would save each other from destruction.

The following probably come into play in some of these cases:

• The good effects of frequent intercrossing
• The bad effects of close interbreeding

The checks and relations between organic beings, which have to struggle together in the same country, are complex and unexpected.

An estate in Staffordshire had a large and extremely barren heath never been touched by man.

but several hundred acres of exactly the same nature had been enclosed twenty-five years previously and planted with Scotch fir.

The change in the native vegetation of the planted part of the heath was most remarkable, more than is generally seen in passing from one quite different soil to another:

The proportional numbers of the heath-plants were wholly changed.

12 species of plants (not counting grasses and carices) flourished in the plantations, which could not be found on the heath.

The effect on the insects was still greater. 6 insectivorous birds were very common in the plantations, which were not to be seen on the heath. The heath was frequented by 2-3 distinct insectivorous birds.

The introduction of a single tree created such a potent effect.

I saw the importance of enclosure near Farnham, in Surrey.

Here there are extensive heaths, with a few clumps of old Scotch firs on the distant hill-tops: within the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so close together that all cannot live.

When I ascertained that these young trees had not been sown or planted, I was so much surprised at their numbers that I went to several points of view, whence I could examine hundreds of acres of the unenclosed heath, and literally I could not see a single Scotch fir, except the old planted clumps.

But on looking closely between the stems of the heath, I found a multitude of seedlings and little trees, which had been perpetually browsed down by the cattle.

In 1 square yard, at a point some hundreds yards distant from one of the old clumps, I counted 32 little trees.

One of them, judging from the rings of growth, had during 26 years tried to raise its head above the stems of the heath, and had failed.

As soon as the land was enclosed, it became thickly clothed with vigorously growing young firs.

Yet the heath was so extremely barren and so extensive that no one would ever have imagined that cattle would have so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the Scotch fir; but in several parts of the world insects determine the existence of cattle.

Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of these animals when first born.

The increase of these flies must be habitually checked by some means, probably by birds.

Hence, if certain insectivorous birds (whose numbers are probably regulated by hawks or beasts of prey) were to increase in Paraguay, the flies would decrease–then cattle and horses would become feral, and this would certainly greatly alter (as indeed I have observed in parts of South America) the vegetation: this again would largely affect the insects; and this, as we just have seen in Staffordshire, the insectivorous birds, and so onwards in ever- increasing circles of complexity.

We began this series by insectivorous birds, and we have ended with them. Not that in nature the relations can ever be as simple as this.

Battle within battle must ever be recurring with varying success.

Yet in the long-run the forces are so nicely balanced, that the face of nature remains uniform for long periods of time, though assuredly the merest trifle would often give the victory to one organic being over another.

Nevertheless so profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being; and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!