$A$ species-area curve is drawn by plotting the number of species against the area. Why is the slope of the regression line steeper when a very large area (like an entire continent) is considered compared to smaller areas?

(N/A) Ecologists have discovered that the value of $Z$ (slope of the regression line) generally lies in the range of $0.1$ to $0.2$,regardless of the taxonomic group or the region (whether it is the plants in Britain,birds in California,or molluscs in New York state,the slopes of the regression line are remarkably similar).
However,if you analyze the species-area relationships among very large areas,such as entire continents,you will find that the slope of the line is much steeper,with $Z$ values typically in the range of $0.6$ to $1.2$.
For example,for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents,the slope is found to be $1.15$.