Muskrats, mink, voles, and mice are common. Marshes serve as nurseries for young fish. They also stabilize soil and improve water quality by filtering sediments and absorbing nutrients. The organic matter, called peat, accumulates faster than it decomposes, and open wetlands fill with peat over time. Bogs are specialized, acidic peatlands derived mostly from species of sphagnum moss.
Bogs are nutrient poor and generally form in basins with no inlet or outlet for surface water.
The lack of fresh water causes the acidity to increase to a level that is toxic to all but specialized plants. Bogs start as floating mats of vegetation over the open water of a basin. As the mats grow and thicken, the basin begins to fill in with accumulated peat. The older the bog, the drier it becomes, until trees are able to grow. Older bogs are dominated by spruce and larch trees in the dry areas where the mat is thickest, by shrubs in the intermediate zones, and by herbs in the saturated soils.