This map shows the huge amount of land the federal government owns in the American West

Since Saturday, armed anti-government protesters comprising a self-styled militia have occupied a federal wildlife refuge in Oregon.

At the heart of the protest is the militia members' desire to transfer ownership of the land from the federal government to private hands. Ammon Bundy, the apparent leader of the militia, told reporters earlier this week that the protesters would not leave the land until such a plan was in place.

Federal land ownership has been the source of a long-standing dispute among government agencies and many in more rural farming and ranching regions. It's especially prevalent in the western portion of the United States.

The reason? The federal government owns a huge amount of land there. The US Geological Survey maintains maps of federally owned land. According to the USGS, here is the land owned by the US government, in purple:

Business Insider/Andy Kiersz, data from USGS

According to The New York Times, in fact, the federal government lays claim to about 47% of all land in the American West. That compares with about 4% east of the Mississippi River. Oregon is among the states where the federal government owns a majority of the land.

The reason is simple: As the US expanded across the West, it took territory by purchasing, or taking, the land. It proceeded to sell the land to private owners or state or local governments, doing so with ease in the Midwest but running into more roadblocks in the West.

"The many mountainous, arid and difficult-to-reach tracts of land in the West simply weren't attractive to farmers. Settlers claimed the few valleys where farming was feasible and built towns," The Times explained.

"The only thing most of the remaining land was good for was grazing, but cattle ranchers and sheep herders needed large tracts of land to feed their livestock, not the smaller parcels they could claim through homestead policies.