A species that has a significant physical impact on its environment and thereby creates habitats for other species and underpins the community of organisms. Foundation species facilitate species diversity in a wide range of aquatic and terrestrial environments; they can occupy any trophic level, from primary producers to predators. Many are plants that are dominant in a community and provide shelter, shade, or substrate for other species. For example, the seagrass Thalassia testudinum colonizes coastal sediments, modifying water flow and stabilizing the substrate sufficiently to allow settlement by bottom-dwelling invertebrates, while also protecting the invertebrates from predators. However, foundation species need not be numerically abundant in a community. An example is the beaver (Castor spp.), whose dams on rivers have a wide-ranging impact by creating pools upstream. Hence it is also a keystone species.