Prevalence of polygon-shaped patterns in birds and trees
The filling index procedure showed that abundance-environment
plots were polygon-shaped for 73% of North American tree and bird
species. From the 186 tree species, 65% of abundance–GDD plots and
64% of abundance–water balance plots were polygonal (i.e., the
observed filling index was significantly higher than the
line-shaped null model; Appendix S4). Moreover, 88% of bird
abundance–GDD plots and 87% of abundance– water balance plots showed
polygonal shapes (Appendix S5). Plot filling index es were
positively correlated with sample size (trees: Spearman r = 0.82 ± 0.01,
p<0.001; birds: Spearman r = 0.58 ± 0.02, p<0.001).