Six types of species-area curves

Macroecological studies infer ecological processes based on observed patterns. An often used measure of pattern is the species-area curve. Insufficient attention has been paid to the variety of methods used to construct those curves. There are six different methods based on different combinations of: (1) the pattern of quadrats or areas sampled (nested, contiguous, noncontiguous, or island); (2) whether successively larger areas are constructed in a spatially explicit fashion or not; and (3) whether the curve is constructed from single values or mean values. The resulting six types of curves differ in their shapes, how diversity is encapsulated, and the scales encompassed. Inventory diversity (alpha) can either represent a single value or a mean value, creating a difference in the focus of the measure. Differentiation diversity (beta) can vary in the extent encompassed, and thus the spatial scale, depending on the pattern of quadrat placement. Species-area curves are used for a variety of purposes: extrapolation, setting a common grain, and hypothesis testing. The six types of curves differ in how they are used or interpreted in these contexts. A failure to recognize these differences can result in improper conclusions. Further work is needed to understand the sampling and measurement properties of the different types of species-area curves.