The ___ in native populations created the need for workers.

Indigenous peoples

Explanation:Indigenous peoples, also referred to as first peoples, first nations, aboriginal peoples, native peoples (with these terms often capitalized when referred to relating to specific countries), or autochthonous peoples, are culturally distinct ethnic groups who are related to the earliest known inhabitants of a particular geographic region.

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain

to act quickly in times of danger to the united states

because you have to act quickly if you want to live/survive.