On May 26, 1924, the U.S. government enacted the eugenics-inspired Immigration Act of 1924, which completely prohibited immigration from Asia. The law reflected the desire of Americans to isolate themselves from the world after fighting World War I in Europe, which exacerbated growing fears of the spread of communist ideas. It also reflected the pervasiveness of racial discrimination in American society at the time.
A biggie: the transcontinental railroad. Because it would economically improve wherever it was, both the North and the South competed for the railroad to be in their respective territory, evidenced by the Kansas-Nebraska Act and the Gadsen Purchase and induction of California as a free state in 1860.
In addition: people could get land cheap because it was abundant and might leave to start afresh economically.
There's other stuff I can't remember as clearly, but there's a little and I hope it helps :)
The answer would be Wind erosion
For humans, population density is the number of people per unit of area, usually quoted per square kilometer or square mile (which may include or exclude, for example, areas of water or glaciers). Commonly this may be calculated for a county, city, country, another territory or the entire world.
