First, I will talk about South american colonization by spanish conquistadors because i don't really know north america settlementToday, ethnologist prefer using the using the term of "ethnocide" instead of "genocide"  to designate what happened to native americans. It is admitted that first settlers had not the wish to destroy american people like Hitler did. It wasn't a premeditated undertaking.However, it's true spanish and english people wanted to take their lands, and for this they had to cut contacts between possible rebel fractions and with that all rests of what could unite natives like their culture or religion. Ethnocide means destroying culture, without respect of human differences. http://en.wikipedia.org/wiki/EthnocideThey forbad religious practices, just like traditional funerals... That's why I'm surprised to read that

Regarding to the quantitative evolutions of native populations, we can neither say that it was a genocide because spanish people needed indians to reduce them in slavery. They were very violent during conquest wars and even after. Diseases also explain how natives population decreased very quickly. But spanish monarchy needed indians for manual activities and taxes (That's why monarchy sent religious orders to maintain indians populations and control settlers activities). Yet i admit that some demographic estimations are pure speculation.But, even if spanish could see how native populations could't stand diseases and technologies differences, they continued to impose them hard works like gold and silver mining (see Potosi mining and terrible conditions for natives). So, if it was't a planned project like what we call a genocide, it's was an ethnocide which had quantitative repercussions on natives mental hearth (and suicide propensity for instance) and which were accompanied of serious physical violences.