Is it not supposed to be Capitalism?
When it fell most people lost trading routes and trading partners since Rome was a big city and everybody went there to trade
Farming can be done by a single individual but when multiple individuals work together production speeds up and over time when newer tactics and new foes of farming were discovered it attracted more people to either try it themselves or work on somone else's farms creating a civilization
The proponents of nativism wanted the US government to favor the interests of native-born people over the interests of immigrants. They did not want the immigrants to take their jobs. The natives felt that the immigrants were taking their jobs and land which would lead to overpopulation thus felt this would overburden the government if they were unable to take care of their families.