When species richness is plotted vs area, the graph follows the equation : log S = log C + Z log A where Z is the slope of the line. Z values are usually in the range of 0.1 to 0.2 but if very large areas like entire continents are analyzed, the slope is much steeper. (0.6-1.2).
Why is it so ?
(Here S = species richness and A= area under consideration)
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).