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a desert is a place that has few, or sometimes even no, life forms. sometimes life forms adapt to living in deserts, but conditions tend to be extreme, and survival is challenging. some deserts can be visited but not lived in. some deserts are so inhospitable that life as we know it cannot survive in them at all. in terms of rainfall, areas that receive less than ten inches of rain a year are considered to be deserts. some deserts receive only three or four inches of rain a year. a few places do not receive any rain at all. when we think about deserts, we think about limiting factors. on earth, liquid water is necessary for life. some life forms survive periods when water is not available by becoming spores or seeds, or by becoming dormant (hibernation or estivation). some plants can survive for many years as seeds. insects and unicellular life forms can also wait out drought. sooner or later, however, liquid water is necessary. survival is essential, but it is not all of life. without growth and reproduction, life is on hold, not progressing. salinity can also interfere with an organism's use of water. fresh water fish cannot live in the ocean, and land plants watered with sea water will die. the excess salt in briny water pulls water out of the organism and dehydrates it. if you put a salt water fish in fresh water it will die, too, because the organism will retain too much water in its cells. sunlight: is the ultimate source of most of the energy used by living things on earth. plants use sunlight for photosynthesis. lack of light in caves and under deep water make these environments unsuitable for photosynthesizing plants. hydrogen sulfide: a small set of life forms live around deep sea volcanic vents, using a process called chemosynthesis to extract energy from the mineral rich hot water. without these chemicals, these cold, dark areas are almost lifeless. essential ele
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