The downtrend in population-weighted visits was first discovered
in a 2004 paper by Oliver and other colleagues. This earlier paper
may be downloaded here.

Their results of the 2006 paper were very surprising. There were
basically only 4 variables that explained 97.5% (almost all) of the
decline in visits to National Parks. These were: 

time spent on the internet
time spent playing video games
time spent watching movies (both in theaters & at home)
oil prices

As oil (& therefore gas) prices moved up, park visits went down.This
made sense because mostly people drive to National Parks, and if
gas prices go up people would drive less.

Most surprisingly, as the time people spent on electronic media
increased, park visits decreased. Though correlation is of course
not causation, it still seems likely that people are increasingly
choosing these electronic media over nature.

Oliver and Patty coined the word videophilia to describe this
phenomenon. It is in contrast to the word biophilia, coined by E.
O. Wilson in 1984. Biophilia describes people's supposed innate
affinity or need for natural areas.