Final answer:
The immigrant population significantly contributed to the closing of the American frontier by filling available lands and contributing to the full settlement of territories. This led to the Census Bureau eliminating the "frontier" category in 1890, signaling the end of free land and the shaping of the American character, as suggested by the Frontier Thesis. The closing of the frontier spurred the search for new markets beyond the US borders.
Step-by-step explanation:
The growth in the immigrant population in the United States had a profound impact on the closing of the American frontier. As the country's boundaries stretched westward driven by the Westward Expansion during the 19th century, the frontier was deemed "closed" by the Census Bureau in 1890. The Homestead Act of 1862 and events like the California Gold Rush attracted not only domestic migrants but also a large number of immigrants seeking new opportunities.
This influx of people resulted in increased settlement, meaning that nearly every acre of arable land was claimed, contributing to the Census Bureau's decision to eliminate the category of "frontier". Immigrants came from various countries and their movements into the Midwest and Western regions played a significant role in the settlement of these areas. This rapid settlement signaled the end of the available free or cheap land, leading to what was perceived as the 'closing' of the frontier.
Historian Frederick Jackson Turner's Frontier Thesis suggested that the frontier had played a crucial role in shaping the American character, defined by qualities such as egalitarianism, hard work, and independence. The end of the frontier raised concerns about the future of these values and the direction of American history. The closing of the frontier also necessitated the search for new markets outside the country's borders for America's surplus agricultural and industrial products.