@Adverb,
Ive heard this term and always thought that it meant something like an overloading of delicate environments with tons of tourists which negatively alter the sites environment.
An example of "mass tourism" in my mind is the bazzillion s of tourists that visit the Galapogos Islands. The tourists have knowingly or unwittingly spread diseases , foreign seeds, and even invasive animals on several of the islands in the archipelago.
Its kind of trashy in some of the areas most frequented by tour boats and fly-ins and the infrastructure is in dire need of upgrades so that they can handle the existing load. The result to the aquatic environment around several islands is one of spreading "dead zones" made up of water contaminated with coliforms and nitrogen compounds.
Peoples are pigs,and we aint much in a hurry to change.
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