"Tom, go to http://www.nature.nps.gov/stats/viewreport.cfm and peruse away. It doesn't parse by individual park, but go back a ways and the trend is clear."
As I suspected, based on my own observations, gross statistics can be deceiving. I dug a little deeper and came up with the link below for Sequoia NP, my particular concern. Visits have bounced around between 686,600 in 1974 and 1,120,000+, and were 1,006,583 in 2011. That is a doubling since I first started hiking in Sequoia, which pretty much confirms what I have experienced down thru the years.
Kings Canyon is more in line with your trend, but is on the rise the past couple of year. I think one would have to address each park separately on the basis of their particular selfish interest. The source is the Public Use Statistics Office of the NPS, for those interested in pursuing the matter. In any case, I think it is a side show to the main points in contention here.
Nick - Thanks for focusing the discussion exactly where it belongs and in the detail required to make the point, where the rubber meets the rocks, so to speak.
"Control for this and Wilson and Seney (1994) found that feet and bike tires had similar impacts on erosion and soil displacement, while horses had the most. Thurston and Reader (2001) found the same. The former was done outside Bozeman, the later in Ontario, suggesting some generalizability across population densities and soil types."
I would feel a whole lot better if the more fragile desert and high alpine areas of the Sierra had been included, ditto the Cascades. I don't think you can generalize from the lowland deciduous forests of Ontario to those environments, nor from the Bozeman area to the Cascades for that matter, where recovery is a painfully slow process due to the extreme environmental conditions. As you say, the subject has not been well researched. Even so, for the reasons Nick so clearly pointed out, it is hard to see how they reached those conclusions.
Edited for content:
"Control for this and Wilson and Seney (1994) found that feet and bike tires had similar impacts on erosion and soil displacement, while horses had the most."
I just read this report, out of respect for you and because it is one of the few out there that attempts to be comprehensive. The authors themselves admitted that the results were inconclusive, and that the diversity of soil types among other things made it very difficult to draw meaningful conclusions. Interestingly, they also lumped motorcycles in with bikes and foot traffic; I found this puzzling in light of the data from a previous study that indicated motorcycles created severe erosion channels when going uphill due to the torque applied for propulsion. This same point would also seem to make sense when it comes to bicycles, as Nick pointed out in his post. As you said, the subject is not well researched. In the absence of more convincing studies, I remain to be convinced and will rely on what my eyes and common sense tell me. That said, I commend you for posting the study. It was an interesting read, and a well intentioned first step on what I suspect will be a long path to understanding the subject well enough to manage incredibly diverse trail systems.
As you said in your first post, the subject is now out in the open, and the discussion, while a bit heated at times, has been been very civil and darned interesting to boot. Thanks for getting it going.