GLM versus CCA spatial modeling of plant species distribution

Despite the variety of statistical methods available for static modeling of plant distribution, few studies directly compare methods on a common data set. In this paper, the predictive power of Generalized Linear Models (GLM) versus Canonical Correspondence Analysis (CCA) models of plant distributio n in the Spring Mountains of Nevada, USA, are compared. Results show that G LM models give better predictions than CCA models because a species-specifi c subset of explanatory variables can be selected in GLM, while in CCA, all species are modeled using the same set of composite environmental variable s (axes). Although both techniques can be readily ported to a Geographical Information System (GIS), CCA models are more readily implemented for many species at once. Predictions from both techniques rank the species models i n the same order of quality; i.e. a species whose distribution is well mode led by GLM is also well modeled by CCA and vice-versa. In both cases, speci es for which model predictions have the poorest accuracy are either disturb ance or fire related, or species for which too few observations were availa ble to calibrate and evaluate the model. Each technique has its advantages and drawbacks. In general GLM will provide better species specific-models, but CCA will provide a broader overview of multiple species, diversity, and plant communities.