In the Great Lakes region, sand dunes are one of the most striking landforms, and in Michigan they became part of the coastal scenery only 3-4,000 years ago when a receding lake level exposed abundant sand to prevailing westerly winds. With constant onshore wind and the binding ability of grasses, the world's largest collection of freshwater coastal dunes formed here. Michigan's position in the Great Lakes basin and the importance of strong onshore winds help explain this phenomenon, and some of the finest examples of these formations are found in Michigan State Parks. Sand dunes also vividly show the interrelationships of living things and their environment, and it was within the dunes that the science of ecology was recognized. Scientists observed dunes changing from barren sand to ridges of grasses that over time shared space with shrubs and trees, a process in which one plant community replaces another known as succession. Coastal sand dunes can be divided into four distinct zones or habitats: beach, foredune, trough or interdunal pond, and backdune. This division is based on the presence of particular plant and animal species associated with certain land features and microclimates, and what sets these coastal sand dunes apart from those in deserts and along oceans is their life.