The species-area relationship (SAR) is one of a few general patterns in ecology, yet the universality of its shape and slope has been questioned. Using geometrical considerations and analysis of various data on nested SARs from all available spatial scales, I will explore constraints which are imposed upon the shape and slope of the SAR at multiple spatial scales.
Although the exact shape of the SAR cannot be universal across various taxa, its slope is strongly constrained at very local scales in which the number of individuals becomes limited, and also at very large scales where limits of species geographical ranges play a role. These two opposite constraints lead to triphasic SARs revealing predictably high local slopes (the derivatives) of the relationship in logarithmic space for very small and very large scales, and lower slopes in between. The intermediate phase is most variable and may reveal biologically relevant information concerning details of the structure of species distribution. Universality of the SAR shape at large spatial scales enables estimation of extinction rates with area loss, which appears to be predictably dependent on mean range size of given taxon within a continent.