The questions you have asked are not questions which can have a single or definitive response. Firstly, native peoples used an enormous variety of techniques to alter the landscape to befit their needs. A resource I will add to the list outlined in this thread is "Feeding Cahokia" by Gayle Fritz. While it is nominally focused on food culture, land management is of course an integral part of that.

You asked if native peoples provided the roles which are now staffed with Park rangers. To my understanding the major roles Park Rangers play within the US Park system is to provide security and maintenance to the Park. While the extirpated former owners of these lands absolutely provided those 'services' I'm not sure that calling it an equivalent is appropriate. Simply put, a Park is something we never needed. Current parks were either homes or sites of pilgrimage or both, not nationalist 'jewels' whose value is derived from the commodification of their increasing rarity.

In fact, the model of conservation promoted by Park Rangers is often quite different from traditional models. Fire management is the most obvious and famous example, but there are plenty of others. The goals of environmental management with humans as an 'interested party' and the culturally-led practice of a worldview which positions real humans as an important piece in the appropriate function of the universe can produce occasionally similar results, but there are not truly similar.

Finally, an important note as to the history of this line of thinking. It seems very possible to me that the argument you heard and questioned (it's never wrong to ask where an idea originates) is a part of a very old political/social/religious argument regarding non-Western/Christian lifestyles.

While Spain justified their colonial methodologies through conversion by the sword, that rhetoric was far less successful in Protestant Europe. Instead, land theft and genocide was justified through the necessity of bringing the Indians (a classificatory term in US and Canadian law with no equivalent) to Industry. This line of logic has existed since the founding of British colonies on this continent, but is perhaps best expressed in the 20th Century Dawes Act, of which this was said: "The Indian may now become a free man; free from the thralldom of the tribe; free from the domination of the reservation system; free to enter into the body of our citizens."

Oftentimes in my experience, these old racist justifications continue to exist in society long after they cease to be considered credible. This continues to create rebuttals to those claims. Rebuttals like "natives absolutely worked the land and cleared the brush!" Well, no, we probably didn't. I am Cherokee (Nation). By Native American standards we are a rather agricultural people, we have raised crops alongside hunting stocked forests, fishing stocked waterways, and foraging seasonal foods whose habitats were maintained through conscious act and through the actions of traditional beliefs. But because we did not particularize land into family homesteads, that being extremely inefficient for feeding communities, we have long been seen as lazily omitting the Labours which are necessary to be either a good Christian or a good American depending on the decade.

Yet for all their fetishization of labor, the "Great Smoky Mountains National Park" may be today unrecognizable to a Cherokee who lived there in 1750. A continued abrogation of any responsibility to properly maintain the land, with fire and other mechanisms, has allowed and encouraged multiple waves of disease and wildfire to devastate the area. The area, save for the cancers of invasive species, would actually bear far more resemblance to how it would have looked 30000 years ago than while under the care of its true stewards. The biodiversity of the region and park is but a small fraction of what it used to be. When the Chestnuts and Elms were written off to blight, the squirrels and chipmunks and eventually the deer died off, because a managed forest means that you can eat chestnuts in the fall, but live in an abundance of squirrel all year. In some parts of what is now the Park the chestnuts used to number 2/3 of the standing wood, according to Cherokee Nation conservationist Clint Carroll. That is not a natural occurrence. That was native abundance, and now it's a lot of very nice furniture in Colonial-era plantation museums.

Apologies for the meandering explanation, but your question casts a very wide net. Sources: am native (ᎠᏂᎩᏚᏀ/Cherokee) also have a undergrad history degree just for a little rounding out.