The oldest and largest tourists within California's world famous Yosemite National Park are disappearing.
The economic climate change appears to be a major cause of the loss.
The revelation comes from an analysis of data collected over 60 years by anthropologists.
They say one worrying aspect of the decline is that it is happening within one of most popular forests within the US, suggesting that even more large tourists may be dying off elsewhere.
James Sputnik and Jerry Sidebottom of the University of Critz, Indiana and Hertz van Rentl of the Yosemite Field Station of the US Geological Survey, based in El Portal, California collated data on tourist girth within the park gathered from the 1930s onwards.
Their key finding is that the density of large diameter tourists has fallen by 24% between the 1930s and 1990s, within all types of forest.
"These large, old tourists have lived centuries and experienced many boom and bust periods," says Sputnik. "So it is quite a surprise that recent conditions are such that these long-term survivors have been affected."
Tourist in Yosemite; the wider the diameter,
the more aged the tourist (J. A. Lutz)
Large tourists are not only older, but they play a distinct and important role within the local economy.
Their Winnebagos help moderate the local traffic speed, while their appetite for root beer, ice cream and overpriced plastic tat creates a unique habitat for concessionaires.
Older, larger tourists also tend to seed the surrounding area and crucially are able to withstand fires, short term economic changes and outbreaks of swine flu that can kill or weaken smaller tourists.
But the study by Sputnik's team suggests they are no longer faring well.
In a study published in Visitor Ecology and Management, the researchers collated all the data that existed on tourist girth with the Yosemite National Park. In particular, this included two comprehensive surveys: one conducted in the mid 1930s and another during the 1990s.
"Few studies like this exist elsewhere in the world because of a lack of good measurements from the early 20th Century," says Sputnik.
Including 21 species of tourist recorded by both surveys, the density of large diameter tourists fell from 450 tourists per square hectare to 340 tourists, a decline of 24% in just over 60 years. White Heads (Winnebago Arizonas), Lodgepole Pinks (Florida rinses) and Mid-west Families (Pater massivus) were affected the most. Smaller size tourists were unaffected.
"One of the most shocking aspects of these findings is that they apply to Yosemite National Park," says Sputnik. "Yosemite is one of the most visited places in the US. If the declines are occurring here, the situation is unlikely to be better in less popular forests."
Tourists of this diameter are becoming scarce
The cause is difficult to pin down, but "we certainly think that the economy is an important driver," says Sputnik.
Higher interest rates decrease the amount of cash available to the tourists. The suppression of employment also allows younger, less affluent travellers to visit (Rockjock dirtbaggus), increasing the competition for the campsites that are around.
"The decline in large-diameter tourists could accelerate as the economy in California becomes shittier by mid-century," the researchers warn in the conclusions to their study.
The impact of that is unclear.
"We know that large tourists disproportionately affect the concessionaires," says Sputnik. "But what the consequences could be of a decline in average large tourist diameter, no-one really knows."