Low bird diversity in the Fynbos plant diversity hotspot: Quaternary legacies in the current distributions of passerine birds

The Fynbos Biome or Cape Floristic Region is well-known for its plant diversity. This diversity does not seem to carry over at higher trophic levels, and in particular in birds. Here, we analyzed the passerine bird assemblages of the Fynbos Biome at the quarter degree resolution with occupancy models and citizen science data. Multivariate analysis of distribution summary metrics revealed a gradient of specialist to generalist species, in which range-restricted, specialist species responded positively to increases in number of competing species and included ancient lineages. Wide-ranging, generalist species exhibited stronger affinities for the arid Karoo Biomes than for the other neighboring biome (Albany Thicket) and for the other mesic biomes in South Africa. Both results are explained by the Quaternary legacy hypothesis: the combined effects of habitat filtering, isolation by distance, and limited in situ diversification, acting throughout the Quaternary, and caused by the contrasting winter-rainfall regime of the region and the low phylogenetic diversity and original adaptations of the plant assemblages.