Each year the density of 7 species of Odonata (dragonflies and damselflies) is monitored in a wetland preserve. If the density of each species is to be compared with the density of every other species, how many comparisons must be made?

21 comparisons

Step-by-step explanation:

Assuming that comparisons are only made with two species at a time and that the order at which species are picked does not matter, the number of comparisons that must be made is given by the combination of picking two out of 7 species:

In order to compare the density of all species, 21 comparisons are needed.