During Augustus's reign slaves accounted for more than one-third of the population of Rome; in Alexandria, possibly two-thirds. The quantity of slaves available resulted in bargain prices being paid for them; an ordinary unskilled slave was worth about 500 gold coins, a slave specializing in a particular trade, fifteen hundred to two thousand. Consequently anyone who needed a manual worker, whether he was a landowner, a business man or a craftsman, preferred to use slaces rather than free men. This proved a further cause of society's disintegration.
As a reult of it a large body of people sprang up in the great cities, particularly in Rome, who were largely unemployed all the time. They consisted of uprooted peasants, free workmen who could no longer find work, freed slaves, andforeigners from every corner of the Empire. This was an excellent breeding-ground for all political cankers, and for all forces of demoralization. The hard-working Roman of olden days became the 'client,' the parasite, paid for his doubtful loyalty by the sportula.
Jimmy Akin has commented pretty sensibly on the issue of immigration reform and the recent protests. He also mentioned the economic realities behind the current mess (although I can't find that post right now).
Mind you, I'm all for legal immigration, particularly from our neighbors down south. They're coreligionists, and it can only bode well for the Church to have an infusion of these people. Several of our parishes have large Hispanic communities, and we're doing what we can to help assimilate the newcomers into our broader commuunity.
That said, the problem is with those who come here illegally. They might have legitimate reasons for doing so (persecution, poverty, crime). However, the problem isn't why they come but how they get here and what happens next.
For those who are smuggled in, if they make the trip safely, they can spend their years being exploited by business owners who don't pay overtime, don't pay minimum wage, and don't provide safe working conditions. Because the cheap labor pool is available, the wages for some jobs (the so-called undesirable ones*) drop, so some people would rather collect unemployment thank work. So we get pool of unemployed along with a pool of exploited people.
For those who come in and have forged papers, they pay taxes but cannot claim tax refunds or derive social security benefits. They might not be as exploitable, but they certainly don't gain the true benefits of a free society either.
So we have an unemployed class and an exploited class. Add in gladitorial sports, and we're ripe for just the sort of collapse that felled Rome.
It took another two hundred years for the Romans. Will it take that long for us?
UPDATE: In addition to the aforementioned, Daniel-Rops mentions the decline of the birthrate and the increase of adultery, as well as the increase of abortion and exposure. He also throws out this memorable line: "The substitution of the State edict for the individual conscience is always a sure sign of decadence, in every country and in every age."