Most tribes require a specific percentage of Native “blood,” called blood quantum, in addition to being able to document which tribal member you descend from. Some tribes require as much as 25% Native heritage, and most require at least 1/16th Native heritage, which is one great-great grandparent.
Subsequently, one may also ask, how much of the Native American population is left?
About 22% of our country's 5.2 million Native Americans live on tribal lands (2010 U.S. Census). Living conditions on the reservations have been cited as "comparable to Third World," (May 5 2004, Gallup Independent).
Why did the population of the Native Americans decrease?
They now believe that widespread epidemic disease, to which the natives had no prior exposure or resistance, was the primary cause of the massive population decline of the Native Americans. One reason this death toll was overlooked is that once introduced, the diseases raced ahead of European immigration in many areas.