In physical geography, a wetland is an environment at the interface between terrestrial ecosystems and aquatic systems. In essence, wetlands are ecotones. Wetlands are typically highly productive habitats, often hosting considerable biodiversity and endemism. By absorbing the force of strong winds and tides, wetlands protect terrestrial areas adjoining them from storms, floods, and tidal damage. The plants in wetlands help to filter pollutants in the water. Fresh water marshes are often on river floodplains. Intertidal wetlands provide an excellent example of invasion, modification and succession. The invasion and succession process is establishment of seagrasses. These help stabilize sediment and increase sediment capture rates. The trapped sediment gradually develops into mud flats. Mud flat organisms become established encouraging other life forms changing the organic composition of the soils. In the salt marshes there is greater species diversity, nutrient recycling, and niche specialisation making it one of the most productive ecosystems on Earth.