Co-occurrence based assessment of species habitat specialization is affected by the size of species pool: reply to Fridley et al. (2007)

1. Fridley et al. (2007) introduced a technique of species habitat specialization assessment based on co-occurrence analysis of large species-plot matrixes, with a continuous metric (theta value) intended to reflect relative species niche width. 2. They used simulated data in order to demonstrate the functionality of the new method. I repeated their simulation and introduced three alternative scenarios with various patterns of species pool size along a simulated gradient. Results indicated that the co-occurrence based estimation of species niche width is dependent on the size of species pool at the position of species optima. This relationship was also revealed in an analysis of a real data set with Ellenberg indicator values as surrogates for environmental gradients. 3. I introduced a modification of the original algorithm, which corrects the effect of the species pool on the estimation of species niche width: the beta diversity measure based on additive partitioning was replaced with the multiplicative Whittaker's beta. Even after this, the method can satisfactorily recover the real pattern of species specialization only for unsaturated communities with a linear relationship between local and regional species richness. 4. Synthesis. This paper corrects the algorithm for co-occurrence based estimation of species specialization, introduced by Fridley et al. (2007), which was sensitive to the changes in species pool size along environmental gradients.