Species-area relationship: separating the effects of species abundance and spatial distribution

1. The species-area relationship (SAR) has long been used to describe biological diversity across spatial scales. Theoretically, we can describe SARs exhaustively from patterns of species abundance and spatial distribution; where all species are equally common and randomly dispersed, SARs typically rise steeply and then decelerate when plotted on logarithmic axes, but uneven abundances and spatial aggregation lower the SAR curve. 2. In this article, we have developed a generalized model of random placement without replacement for species-occupancy data sets to separate the effects of species abundance and spatial distribution. Observed departures from the model are used to assess the relative contributions of aggregation and unequal occupancy to curve shape. We have applied it both to modelled one- and two-species cases and to plant survey data at both small (1 x 1 m) and large (10 x 10 km) spatial resolutions. 3. In modelled distributions, common species raise the SAR most strongly at fine scales, whereas increasingly rare species add to the curve at progressively coarser scales. The effect of aggregation on SAR curvature is greatest with rare species, which lower the curve at most scales, whereas commoner species do so only at fine scales. Uneven occupancies and aggregation may result in convex, concave, S-shaped or inverted S-shaped curves in log-log space. 4. In analyses of field survey data, aggregation contributed less to lowering SAR curvature than did uneven occupancies, both in small and large scale data sets. 5. Synthesis. We have thus demonstrated that SAR shape reflects the distribution of species-occupancy and spatial patterning and provide practical methods for parsing the contributions of each to biodiversity across spatial scales. Given the prevalence of gridded occupancy biodiversity data, these methods should have broad application.