Species-area relation graph
Z value is a fitted constant and it ranges between 0.1 to 0.3 regardless  of the region or taxonomic groups.i.e. the slope is almost similar .Among larger areas like continent,the slope tends to be much steeper.becoz, Larger the area larger will be number of species(species richness),for larger areas the Z values tends to be 0.6 to 1.2.(NOTE:if Z is less,lesser area is enough to capture more species).