Struggle for Existence?

A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year; otherwise, on the principle of geometrical increase, its number would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with physical conditions of life. It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage. Although some species may be now increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them….

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 numbers of a species.  Charles Darwin

1 個解答

• 3 星期前

試解:

Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year; otherwise, on the principle of geometrical increase,

"on the principle of geometrical increase" 指出

族群大小原則上是指數增加的:  N(t) = N(0) e^(rt)

或者, 用微分方程來表示: N'(t) 隨著 N(t) 而變 (可想成: 成比例).

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 numbers of a species.

"The amount of food for each species of course gives the extreme limit to which each can increase" 指出

食物限制了族群的成長:  N(t) ≦ M.

用微分方程來表現, N'(t) 大概和 M-N(t) 成比例.

"the serving as prey to other animals, which determines the average numbers of a species." 指出

族群將被其他動物狩獵, 決定了一物種的平均大小.

用微分方程來表現, N'(t) 與其獵食者數量有關. 若不做多族群

聯立模型, 可把 N'(t) 與一外部決定的常數 k 相關聯.

合併以上三項考慮, 可用一個簡單的微分方程式來表現:

N'(t) = k N(t)(M-N(t))

解此微分方程式, 得

N(t)/(M-N(t)) = [N(0)(M-N(0))] e^(kMt)

N(t) = M N(0) e^(kMt)/(M+N(0)e^(kMt))