The north used the rising number of immigrants to .The south felt no need to employ immigrants on their plantations because

The north used the rising number of immigrants to staff the industrial economy (factory jobs, warehouse jobs, etc...) the south felt no need to employ immigrants on their plantations because the invention of the cotton mill had made slavery a more economical option.

Explanation:

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disease was the number one killer of native peoples. they had had little exposure to the diseases of europe. this led to some villages being almost completely wiped out and some tribes destroyed.