Most arches built today contain a single building block at the top that is the most important piece. This special piece can be found in the arches of soaring cathedrals, doorways in temples, and even simple buildings made out of wooden blocks. It is called a keystone, and it holds everything else together. Remove the keystone and the building or doorway is likely to collapse. The same thing is true in nature. Certain species of animals and plants are so important to their ecosystems, that if they disappear, the whole system may collapse. They are called keystone species. Some keystone species are large, like white rhinos, while others are quite small, like honey bees. But size doesn't matter in an ecosystem. All living things rely on other species to survive. A keystone species plays an especially large role that affects many different species in an ecosystem. Some keystone species are at the top of a huge ecosystem like the Greater Yellowstone Ecosystem, while others may affect a tiny ecosystem in a river or forest. Whether the ecosystem is big or small, the result of a keystone species disappearing or being greatly reduced is the same. Just like one falling domino can cause many others to fall, the loss of a keystone species can lead to the extinction of many other species. Today scientists are focusing more attention on preserving the natural balance in ecosystems. Identifying and protecting keystone species is an important part of their work.