The species-area relationship (SPAR) is one of the cornerstones of ecological science. We use information about the geographic distribution of species to deduce the SPAR from a new model; our results complement existing ones that explain SPARs on the basis of equilibrium theory or species-abundance relationships. We assume that each species is characterized by a geographic range and level of abundance (occupancy). We use a mixture of analytical (in one dimension) and numerical (in two dimensions) methods to create SPARs that are concave and can often be parametrized by a power law with exponent less than 1. We show that the main features of the SPAR depend upon the way that one censuses patches and on the characteristics (range and occupancy) of the species. Our approach identifies the key field variables that need to be measured and have implication for conservation, particularly when one estimates the number of species lost after habitat destruction.
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