Islam has the concept of "jihad by birth rate" as seen in this tradition:
Narrated Ma'qil ibn Yasar:
A man came to the Prophet (peace_be_upon_him) and said: I have found a woman of rank and beauty, but she does not give birth to children. Should I marry her? He said: No. He came again to him, but he prohibited him. He came to him third time, and he (the Prophet) said: Marry women who are loving and very prolific, for I shall outnumber the peoples by you. (Sunan Abu Dawud, Book 11, Number 2045)
Although numbers do not say anything about truth, cf. More Is Better?, they are an essential aspect in regard to the power of Islam.
The Muslim sections of the world's population are not the only ones that are growing fast, but the following article has some serious observations about Western Civilization being defeated by this very strategy of taking over a society by a substantially larger birth rate:
It's the Demography, ... (The real reason the West is in danger of extinction) by Mark Steyn
Though serious, this issue of birth rate is only one aspect of many. We recommend a study of Biblical Eschatology in regard to Islam, e.g. the book
Will Islam Be Our Future?
See also: MUSLIM POPULATION FIGURES
Other arguments where birth rates feature prominently: