There are less large herbivores in North America than pre-colonization

I'm not sure where you derived the 80 million estimate from, but a quick glance shows CAN - US - MEX currently over 105 million in beef alone.

Also, you should keep in mind domestic stocks do not necessarily reflect domestic consumption. Brazil currently boasts a beef population of over 210 million (!), much of which arrives on plates right here in the USA.
"In 2014, Brazil exported about 37,000 tonnes of processed beef to the United States valued at $217 million, USDA data showed.

Brazil's total beef exports were 1.56 million tonnes worth $7.2 billion. Its top buyers were Hong Kong, Russia and the European Union, according to beef export association Abiec.

In five years, Brazil could export 100,000 tonnes of fresh beef to the United States annually, the country's agriculture ministry estimated in a statement."

http://www.reuters.com/article/2015/06/29/us-usa-usda-meat-imports-idUSKCN0P92EL20150629


Other Sources:
http://agebb.missouri.edu/mkt/bull12c.htm

http://www.indexmundi.com/agriculture/?country=mx&commodity=cattle&graph=dairy-cows-beginning-stocks

http://www.statcan.gc.ca/tables-tableaux/sum-som/l01/cst01/prim50a-eng.htm

http://beef2live.com/story-world-cattle-inventory-ranking-countries-0-106905

Read 1491 by Charles C. Mann.

Indians managed game populations pretty well, it wasn't until smallpox killed off the majority of them that buffalo populations exploded.

Also, those weights and percentages don't hold up.

A modern beef cow is roughly 1350 lbs at slaughter:
http://beefmagazine.com/genetics/0201-increased-beef-cows

Average wild Bison cow are only about 1000 lbs:
http://www.nps.gov/yell/learn/nature/bisonfaq.htm


The numbers everyone is quoting is including bulls for the bison, but not for the beef cattle.

A quick Google search reveals that the average American Buffalo weighs roughly 1500lbs while the average modern cattle cow weighs in at 1300lbs.

That puts them at a roughly similar size, but the difference is that the cow population keeps on growing and polluting sources of water because these cattle are concentrated in very large numbers when in captivity. They're not free to just roam and spread out over the world, they are bundled up and their excrement ends up in very large chunks in specific areas rather than spread out as nature would have it, which ends up causing some serious damage to those areas (water sources mainly). We then feed them so much that they produce much more CO2 than normal cows did just a few decades ago, so that's affecting our atmosphere.

Also, we weren't taking down whole forests when bisons were roaming the earth.

You overlooked some very important details my friend.

I do not agree. A fully grown Holstein Fresian milking cow weighs around 400 - 500 kg, a female bison the same. Even if your numbers were right, they'd make no sense, because double weight does not equal double food and water intake, nor double methane production.
Also you do not include the amounts of pigs, turkeys and chicken in domestic holdings. Furthermore, the US is not self sufficient when it comes to meat production but imports meat from other countries. Climate change is a global phenomenon and not just happening within US borders.
Something most people do not know, though, is that organic beef and milk is even worse for the climate than conventional produce, because animals are less efficient (produce less meat and milk per day/animal) and use even more land, because land productivity in organic farming is lower. Also a more grass based diet, like common in organic farming, leads to an increase in methane production compared to a more grain based diet, as common in conventional farming.

http://www.fao.org/news/story/en/item/197623/icode/

One obvious difference I haven't noticed elsewhere is that any given population of cattle, say X million, is not equivalent to a wild population of X million bison in one very important respect. Over the life-time of one bison, many new cattle would have been grown from scratch because they are slaughtered so young. Growing big animals from birth takes more feed than to keep adult animals happy.
Also bison roamed in fields that did not displace humans at the time. Now all cattle need grazing and feed-land specially set aside. Certainly, if you do the numbers, North America (the whole surface area) is just not big enough for grass-fed cattle roaming around like the bison once did to feed the demand of North America. Not even close. The X million cattle are slaughtered intensively and new ones grown constantly to keep up. A static population of bison would not last long before it ran out considering the number of people today. But everything has changed.
The bison of yesteryear existed in an almost empty land. Grass fed beef is unfortunately not a solution to anything or a good analogy to the huge bison populations that used to exist.
I thought the movie did a good job of explaining all that.
If we all became vegan then much land could go back to being natural forest or functioning grassland with the proper balance of wild-life on it. Like a nature reserve. We don't eat animals in a nature reserve so just think about it as the nature reserves becoming bigger. I don't see the problem with that. It would be awesome, I think.