On the form of species–area relationships in habitat islands and true islands

Aim We undertook the largest comparative study to date of the for m of the island species–area relationship (ISAR) using 207 habitat island datasets and 601 true island datasets. We also undertook analyses of (a) the factors in?uencing z- and c-values of the power (log–log) model and (b) how z and c vary between different island types. Location Global. Methods We used an information theoretic approach to compare the ?t of 20 ISAR models to 207 habitat island datasets. Model performance was ranked accord- ing to pre-set criteria, including metrics of generality and ef?ciency. We also ?tted the power (log–log) model to each dataset and analysed variation in parameter estimates and model ?ts as a function of key dataset characteristics using linear models and constrained analysis of principal coordinates. Results The power (nonlinear) model provided the best ?t to the most datasets, and was the highest ranked model overall. In general, the more complex models performed badly. Average z-values were signi?cantly lower for habitat island datasets than for true islands, and were higher for mountaintop and urban habitat islands than for other habitat island t ypes. Average c-values were signi?cantly lower for oceanic islands, and signi?cantly higher for inland water-body islands, than for habitat islands. Values of z and c were related to dataset characteristics including the ratio of the largest to smallest island and the maximum and minimum richness values in a dataset. Main conclusions Our multimodel comparisons demonstrated the nonlinear implementation of the power model to be the best overall model and thus to be a sensible choice for general use. As the z-value of the log–log power model varied in relation to ecological and geographical properties of the study systems, caution should be employed when using canonical values for applied purposes.