The Geometric Increase in Population

The rule is that every organic being naturally increases at such a high rate. If not destroyed, the earth would be covered by the progeny of a single pair.

There is no exception to this.

Even slow-breeding man has doubled in 25 years. At this rate, in a few thousand years, there would be no room for his progeny.

Linnaeus has calculated that if an annual plant produced only 2 seeds and their seedlings next year produced 2, and so on, then in 20 years there would be a million plants.

The elephant is the slowest breeder of all known animals.

It breeds when 30 years old until 90.

It brings forth 3 pairs of young in this interval.

So that at the end of 500 years, there would be 15,000,000 elephants descended from the first pair.

This rule is proven by the numerous cases of the rapid increase of wild animals when circumstances have been favourable to them for 3 consecutive seasons.

This is also proven by cattle and horses which have run wild.

Plants have been introduced and have spread throughout whole islands in less than 10 years.

The conditions of life have been very favourable. Consequently, nearly all the young have been enabled to breed.

All plants and animals tend to increase at a geometrical ratio. This increase must be checked by destruction at some period of life.

Our familiarity with the larger domestic animals tends to mislead us because we do not see them destroyed. We forget that:

• thousands are annually slaughtered for food
• in the wild, an equal number would be destroyed

The condor lays 2 eggs. The ostrich lays 20.

Yet in the same country, the condor might outnumber the ostrich.

The Fulmar petrel lays but one egg. Yet it is believed to be the most numerous bird in the world.

One fly deposits hundreds of eggs. The hippobosca lays just one.

But this difference does not determine how many individuals of the two species can be supported in a district.

Those species depend on a rapidly fluctuating amount of food.

• Having many eggs is of some importance to them as it allows them rapidly to increase in number.

But the real importance of a large number of eggs or seeds is to make up for much destruction at some period of life.

• This period in the great majority of cases is an early one.

If an animal can in any way protect its own eggs or young, a small number may be produced.

Yet the average stock is fully kept up. But if many eggs or young are destroyed, many must be produced, or the species will become extinct.

If a tree lived for 1,000 years, it would be enough for it to produce 1 seed per 1,000 years to keep its numbers up suppose that this seed were never destroyed.

In looking at Nature, it is most necessary to keep the foregoing considerations always in mind–

Never forget that:

• every organic being:
  • is striving to the utmost to increase in numbers
  • lives by a struggle at some period of its life
• heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals.
• the population will almost instantaneously increase to any amount if
  • any checks are lightened
  • the destruction is mitigated ever so little

The face of Nature is like a yielding surface with 10,000 sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.

It is unclear what checks the natural tendency of each species to increase.

The most vigorous species swarms in numbers.

• We do know not exactly what checks its population.

The Destruction of Individuals

Eggs or very young animals suffer most.

With plants there is a vast destruction of seeds.

It is the seedlings which suffer most from germinating in ground already thickly stocked with other plants.

Seedlings are destroyed in vast numbers by various enemies.

On a ground 3 x 2 feet, dug and cleared, free from other plants, I marked all the seedlings of our native weeds as they came up.

Out of the 357, 295 were destroyed, chiefly by slugs and insects.

The more vigorous plants gradually kill the less vigorous fully grown plants.

Thus, out of 20 species growing on a little plot of turf (3 x 4 feet) 9 species perished from the other species being allowed to grow up freely.

The amount of food limits how each species can increase.

The average numbers of a species is determined by the serving as prey to other animals, not by the obtaining of food.

Thus, the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin.

If such game animals were not hunted in the next 20 years in England and no vermin were destroyed, there would be less of such game animals than at present, even if hundreds of thousands of game animals are now annually hunted.

Elephants and rhinoceros are not destroyed by beasts of prey.

Climate plays an important part in determining the average numbers of a species.

Periodic seasons of extreme cold or drought are the most effective of all checks.

I estimated that the winter of 1854-55 destroyed 4/5 of the birds in my own grounds.

10% is an extraordinarily severe mortality from epidemics with man.

The action of climate seems to be independent of the struggle for existence by reducing food.

When we travel from south to north, or from a damp region to a dry, some species get rarer and finally disappear.

We wrongly attribute this to the change of climate.

But this is a very false view because each species is getting killed at some point of its life from enemies or competitors for the same place and food.

If these enemies be are favoured by the change of climate, they will increase in numbers.

• Since each area is already full with inhabitants, the other species will decrease.

When we travel southward and see a species decreasing in numbers because they are hurt by the favored species.

So it is when we travel northward, but in a lesser degree. This is because the number of species of all kinds, and therefore of competitors, decreases northwards.

Hence in going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southwards or in descending a mountain.

In the following regions, the struggle for life is almost exclusively with the elements:

• the Arctic regions, or snow-capped summits, or
• absolute deserts

Our gardens have imported plants which can well endure our climate. But they cannot:

• compete with our native plants
• resist destruction by our native animals.

This proves that climate acts in main part indirectly by favouring other species.