Preservationist is generally understood to mean historic preservationist: one who advocates to preserve architecturally or historically significant buildings, structures, objects or sites from demolition or degradation. Historic preservation usually refers to the preservation of the built environment, not to preservation of, for instance, primeval forests or wilderness.
A person who preserves figs, pears, peaches, blackberries, blueberries, plums and apples!
Persons who work to preserve ancient or endangered languages are called language preservationists.
Preservationist is also sometimes used in the natural environmentalist field, but while the natural environment conservationist movements preserve ecosystems and the natural environment, this movement is widely known as conservation or environmentalism.
Preservationism has been defined by Richard Heinberg in his book Powerdown: Options and Actions for a Post-Carbon World as distinguishing survivalist groups who wish merely to survive a collapse of civilization from preservationist communities who wish to preserve as much of human culture as is possible in the event of collapse.
Some of the notable historic preservationists who are or have been advocates for the protection of the built environment include: