where SJ is the similarity index, c is the number of shared species between the two sites and a and b are the number of species unique to each site. As you can see, this index differs from Jaccard’s i...where SJ is the similarity index, c is the number of shared species between the two sites and a and b are the number of species unique to each site. As you can see, this index differs from Jaccard’s in that the number of species shared between the two sites is divided by the average number of species instead of the total number of species for both sites.