Wetlands are ecosystems, which are natural regions whose plants and animals act as a community. Wetlands have such poor drainage that they are essentially bodies of standing water. Swamps and marshes receive some groundwater, which is water within the Earth that rises or can be brought up to the surface. For that reason, swamps and marshes contain more minerals than do some other types of wetlands. Bogs, for example, have a thick bottom layer that the groundwater cannot pass through, so most of their water comes from rain and has fewer minerals than groundwater. Because of their greater mineral content, which creates a richer environment for living things, swamps and marshes tend to have more varied types of plants, such as trees. Bogs, on the other hand, generally can support only smaller plants, such as grasses.