Searching for a predictive model for species richness of Iberian dung beetle based on spatial and environmental variables

In Mediterranean countries, inventories of many animal groups, particularly insects, are incomplete or nonexistent. Hence, a feasible spatial picture of unequally surveyed areas is required to ascertain which faunistic surveys are good enough to produce reliable estimates of species richness. We used generalized linear models to build a multiple-regression function through which we predicted the distribution of Iberian dung beetle species richness. Given the scarcity and unevenness of the species-richness spatial distribution, the number of records of a dung beetle database (BANDASCA), falling within each of the 50 x 50 km grid squares, was chosen as a measure of the sampling effort for that square. Examining the asymptotic relationship between the number of dung-beetle species and database records for each physioclimatic Iberian subregion, we found that 82 grid squares (32% of the total) were adequately sampled. Dung-beetle species richness was related in each of these 82 cells to 24 explanatory variables. Curvilinear functions, interaction terms, and the significant third-degree polynomial terms of latitude and longitude were included to model species-richness distribution. The final model accounted for 62.4% of the total deviance after we eliminated seven outlier squares, with maximum elevation, grassland area, land-use diversity, forest area, geological diversity, interaction of terrestrial area and maximum elevation, and interaction between calcareous rock and geological diversity and latitude being the most significant independent variables. The residuals of the function were not spatially autocorrelated, and we validated the final model by a jackknife procedure. Large and environmentally complex hotspots in the Iberian Central, Baetic, and Subbaetic mountain ranges stand out from the emerging map of species richness. Further detailed research is required to determine the complementarity of the faunas of these two main hotspots, the key question in conservation planning for a dung-feeding beetle.