Biodiversity is also called biological diversity. It is the variety of life found in a place on Earth or often the total variety of life on Earth. A common measure of this variety called species richness is the count of species in an area. Biodiversity encompasses the genetic variety within each species and the variety of ecosystems that species create. Although examining counts of species is perhaps the most common method used to compare the biodiversity of various places in practice biodiversity is weighted differently for different species. The count of animal species that live on land is much higher than the count of those that live in the oceans because there are huge numbers of terrestrial insect species. Insects comprise many orders and families and they constitute the largest class of arthropods which themselves constitute the largest animal phylum. In contrast there are fewer animal phyla in terrestrial environments than in the oceans. No animal phylum is restricted to the land except a few and other animal phyla occur exclusively or predominantly in marine habitats. Some species have no close relatives and exist alone in their genus whereas others occur in genera made up of hundreds of species. Given this one can ask whether it is a species belonging to the former or latter category which is very important. On one hand a taxonomically distinct species which is the only one in its genus or family for instance may be more likely to be distinct biochemically and so be a valuable source for medicines simply because there is nothing else quite like it. On the other hand although the only species in a genus carries more genetic novelty a species belonging to a large genus might possess something of the evolutionary vitality that has led its genus to be so diverse. A second way to weight species biodiversity is to recognize the unique biodiversity of those environments that contain few species but unusual ones. Dramatic examples come from extreme environments such as the summits of active Antarctic volcanoes and hot springs or deep sea hydrothermal vents. The numbers of species found in these places may be smaller than almost anywhere else yet the species are quite distinctive. More generally areas differ in the biodiversity of species found only there. Species having relatively small ranges are called endemic species. On remote oceanic islands almost all the native species are endemic. Endemic species are much more vulnerable to human activity than are more widely distributed species because it is easier to destroy the entire habitat in a small geographic range than in a large one. The idea of biodiversity also encompasses the range of ecological communities that species form. A common approach to quantifying this type of diversity is to record the variety of ecological communities and area may contain. It is generally accepted that an area having say both forests and prairies is more diverse than one with forests alone because each of these assemblages is expected to house different species. This conclusion however indirectly based on differences in vegetation structure or appearance rather than directly on lists of species. Loss of various biodiversities due to human intervention is an alarming issue in the present day world. Various species are dying due to changes in their habitat which are human.