The mice need more structural vegetation to live in because of water which surrounds the marshland, especially in tidal marshes because the water levels can rise. The difference between a tidal marsh and a diked marsh is that tidal marshes are located on coastal areas and water levels can rise with the tides, while diked marshes can be compared to ponds. Evaporation and rain fall are the main things that can cause a change in the water levels in diked marshes. Grasslands are often present near marshes which the mice live in, and in winter, when water levels tend to rise, they choose to migrate to them. A plant called fat hen is useful to Salt Marsh Harvest mice because it is comfortable for the mice to live in and it provides adequate coverage from water.
Coverage is very important to the mice.
Salt Marsh Harvest mice are found in tidal marshes in the San Francisco Bay area. Before the 1800's, there were 734 square kilometers of tidal marshes. Today however, animals that occupy these marshes only have about 152 square kilometers of habitable marshland. Some of the marshes are polluted and disturbed by humans. The marshes are located near Suisan Bay, San Pablo Bay, and San Francisco Bay.