President Grant along with Congress passed a bill on March 1st, 1872 that would make Yellowstone National Park the first in the world. The purpose of the park is to protect the unique geothermal structures and wildlife living in the park. The original park was more than 2 million acres of land, almost 3 times the size of the State of Rhode Island! However, despite the huge area, Congress did not give the park any money for maintenance and supervision, so there was not much protection until 1916, when the National Park Service was created. The year after its creation, Yellowstone became under its care and has been carefully protected since.
The creation of National Parks was the start of wildlife protection in the US and 31 years later, the first marine protected area (MPA) was created. Pelican Island National Wildlife Refuge was the first MPA to be established. Its original purpose was to protect egrets and other shorebirds inhabiting the area from extinction through plume hunting. Many birds were hunted in that period for their feathers which were used for clothing and ornaments. Today, thanks to the effort of many, we have significantly more of our ocean being protected.
Source: http://www.pbs.org/nationalparks/parks/yellowstone/