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Endemic Plants
Endemism, the occurrence of species within narrow geographical ranges, is second in importance only to species richness as a measure of biodiversity. Endemic plants are native to a particular place and found only there. Centers of endemism are areas where many endemic species occur together. Endemism correlates generally but not exclusively with species richness. The greatest numbers of endemics occur on islands, on mountain peaks, around desert springs, and in other unusual and isolated habitats. Arid zones are particularly rich in endemic succulent plants. Serpentine soils (composed primarily of hydrated magnesium silicate) typically support many endemic species wherever they occur, but particularly in the tropics.
Of the 250,000 known vascular plant species, 170,000 are tropical or subtropical endemics. In the U.S., states with the highest numbers of endemic plants generally have the highest plant species richness, except for Hawaii, which has the second-highest number of endemics but the fourth-lowest number of native plant species. This high degree of endemism is a consequence of Hawaii's isolation, which has enabled unique and complex ecosystems that include large numbers of endemics to evolve. Because the endemics on oceanic islands have evolved in relative isolation over thousands of years, they are particularly vulnerable to change wrought by humans and introduced species.