Natural disasters cause species to disappear over time. Depletion of food supplies and black marketing of some species varieties are other causes of species extinction. According to an estimate, the total number of species on earth range from 20 to 50 million, but Robert May, however has made an estimate that places the global species diversity at about 7 million. The number (of species) we arrive at is merely an approximate estimate because it is difficult to determine how many species still exist.

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