Barriers prevent species from spreading into new locations and help determine their natural range.
A clam, for example, could not move from the Pacific to the Atlantic coast by itself, even though the habitat there may be suitable. The land would represent a physical barrier. Mountain ranges, oceans and deserts are examples of physical barriers that can limit the dispersal or range of particular species. In the case of small organisms, an outcropping of rocks on a sandy beach or even a tide pool may form the edge of their dispersal range. When a species' salinity, temperature and chemical requirements are not met in a new area, these too, become physical barriers.