what does stepper slopes in species area relationship graph mean??

In graphical forms, steeper slopes means slopes which are closer to vertical axis, or in other words they make larger angle with horizontal axis.

In case of species area relationship given by Alexander von Humboldt, steeper slopes (Z values in the range of 0.6 to 1.2) are obtained when species relationships are plotted among very large areas like the entire continents. Steeper slopes signifies that number of species found increases faster than the area explored in very large areas like the entire continents.