In the first of two fascinating programmes from Israel by Peter Day, Professor Omer Moav of Hebrew University said secular Jews typically have two or three children, Arab Israelis four or five, and ultra Orthodox Jews eight. Orthodox children study little apart from the bible, and are a huge drain on the economy because they don't work when adult, and even if they did would have few useful skills.
By no means the country's only challenge, but clearly significant.
[P.S. an Israeli comments on the threat to "that one"]