Buffalo extermination - environmental catastrophe or savior?

Given my Wall Street Journal article earlier this week, I've received a large number of questions and comment about beef cattle production and the environment.  One comment on the piece in the WSJ made an observation that had never occurred to me.

One of the big concerns with beef production is methane emissions.  Methane is a more potent greenhouse gas than carbon.  Cattle are ruminants, and their digestion produces methane (which is released not from the back-end of the cow as is typically asserted but rather the front-end).  

In any event, it seems a common presumption of many who are worried about this issue is that if we got rid of all the beef cattle in the US (or at least drastically reduced their numbers), that would be a great thing because we could significantly cut greenhouse gas emissions and help curb climate change.

In fact, we did something very much like that in the US in the mid to late 1800s, and it is almost universally considered a tragedy.

According to some environmental groups, there was once more than 20 million bison roaming the Great Plains.  This number may not be far fetched.  According to one academic paper, the bison carrying capacity of the Great Plains in 1860 was estimated between 13.78 to 20.67 million bison.   According to EPA calculations, American bison generate as much or more methane as do beef cattle on a per-head basis (compare table A-184 to A-187).  

In 1990, there were only about 50,000 head of bison in the US.  Today there are less than 200,000.  Thus, there has been a 100 fold reduction in bison numbers since the mid 1800s.

Were these bison causing climate change back in the 1800s?  Is it a great victory for the environment that they were almost eradicated?    

Logically consistency would seem to dictate that we think about the methane emissions of the ~20 million American bison in the 1800s the same way we think about the methane emissions of the ~29 million beef cattle in the US today.   I suspect the total amount of methane emissions from 1860s bison population and 2014 US beef cattle population are roughly similar (according to the EPA, feedlot beef cattle have much lower per-head methane emissions than bison - about half as much).  [addendum: a reader subsequently emailed me and correctly pointed out that, including dairy cattle, there are actually more than 90 million cattle in the US today - a figure roughly 3 to 4 times more than the number of bison existing in the 1800s]

So, bison depopulation - environmental boon or ecological travesty?  Neither?  Both?