# Species-area relation graph

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)

• Interesting question. Can you please add some reference and eventually two graphs that show different slope depending on the range of areas used? Note also that if $C$ designates a constant (which I think it does), then $log C$ is a constant too and could well just be replaced by $K$ (or just write $C$). You should also make clear that $S$ is species richness and $A$ is the area even though it is quite intuitive. Oct 12, 2015 at 18:59
• @Remi.b The full log notation is useful to highlight that the basic relationship is a power law: $S = CA^z$ Oct 12, 2015 at 19:10
• Perhaps because when analyzing large areas, adjacent masses of considerably different climatic conditions are successively bought under analysis, and will hence increase the richness faster than compared to small areas where increasing the analyzed area is still more likely to be confined to the same geo-climatic zone. Just speculations, though. Will get back with a better answer. Oct 13, 2015 at 3:16
• Right, @Satwik Pasana. Also consider that evolutionary history becomes increasingly important when you're looking at large areas. Oct 13, 2015 at 10:12
• @satwik pasani That makes sense. Actually when I had asked the question I had a wrong notion of what species richness is but now what you say seems logical. Add it as an answer ! Oct 13, 2015 at 10:40