Absent immigration, America's population growth would stabilize at near-replacement levels.
Now you are taking a page from another good book on the subject of the environmental impact of growth. The Skeptical Environmentalist. (Bjorn Lomborg) His work shows that cultures reach a stablization level of growth when they reach a certain level of wealth, a level he predicts the world will reach within the next century.
I do not subscribe to the gloom and doom of "The Population Bomb" Although in my youth I joined Erlich's movement as well as the Sierra Club. The reason I do not subscribe is more or less Ayn Randian, I believe we have enough genius to solve our problems and enough land to spare for those who need more elbow room. (Yes, I mean it there is a huge amount of land in the US that is still very open and rural.)
As far as immigration is concerned, I would have at least a 5 year moritorium and I would close the border to foot traffic too. (The illegal kind) I would do this not so much to protect the environment, but to force the assimilation of the immigrants we have and keep our culture. Without that I believe losing the environment will soon follow. Anybody see the environment in Mexico? There is no doubt that immigration puts a strain on our environment and the Sierra Club has their head in a dark place if they don't see it.
As to the indians, (I am part) I believe our culture to be superior to theirs too, although I wish cooler heads could have prevailed and the land divided without bloodshed. I do believe that the indians turned towards terrorism and gave their enemies the justification to do what they wanted to do anyway, Move them off the land. (A different discussion I am sure.)
Which is why absent immigration, U.S. population growth would stabilize.
I believe we have enough genius
We have the potential for as much genius as we would ever need among the current population.
The trick to having geniuses around is to provide an experience of life that nurtures and inspires the genius latent in many men.
History shows that out of relatively small populations great concentrations of genius can arise.
America's founding fathers furnish one example: The very small population of Colonial America furnished those men (likely in good part precisely because the population was small and the continent so large).
Small Hungary contributed an unusually large percentage of early atomic scientists.
Renaissance Florence gave the world many geniuses, also born from a tiny population within the span of a generation.
Time and again we see this flowering of genius seemingly all out of proportion to the size of the population that spawns it.
If you need more genius, you set the stage for it, and it will arise.
America's founding fathers were inspired to their genius and love of liberty by their experience of living so near to wilderness.
The environment brings out the genius in men.
I believe we have enough . . . land to spare for those who need more elbow room.
No we do not.
The very best places are all filled and then some.
I for one, have no use for any land that has not the seacoast on one side and forests, meadows, and mountains on the other.
And apparently Americans feel that way too, because the coastal areas of America are packed to the gills with people, and still they crowd in more.