How do scientists estimate species numbers?

Question

2 different methods. Oh, and why does the total number of species range from 5 million to 30 million? thanks =]

Answer 1

Usually species counts are done by assumptive reasoning. They take a pre-determined  area, for example, 10 miles.By doing a count in that area.  it can be assumed that in similar  habitats within the count  area would be( the count total multiplied by the count area