The influence of spatial scale on cross-taxon congruence patterns and prediction accuracy of species richness

A comparison of species richness patterns of butterflies and birds was made using data from two grids of squares (small squares 137.5 km on a side and large squares 275 km on a side) covering western North America. Using geos tatistical procedures, we found that the spatial patterns of species richne ss of these two taxa were related. The influence of grain size on the stren gth of this relationship was investigated by analysing the two data sets. F or both data sets, the number of butterfly species in a square was a statis tically significant predictor of the corresponding number of bird species. However, cross-validation techniques showed that the marginal improvement i n prediction accuracy clue to including butterflies as a predictor was grea ter in the large-square data. We explored the effect of areal extent on cro ss-taxon congruencies by investigating species richness patterns in four su bsets of the small-square data. In regions with smaller areal extent, the c ross-taxon congruence patterns were not substantially different from the pa ttern found in the full data set. Finally, using data-splitting techniques, we explored the relationships between prediction accuracy of species richn ess, sample sizer areal extent of the sample, and grain size.