Is species richness mediated by functional and genetic divergence? A global analysis in birds

Unravelling why species richness varies shows such dramatic spatial
variation is an ongoing challenge. Common to many theories is that
increasing species richness requires a compensatory trade-off on an axis
of species’ ecology. Spatial variation in species richness may also affect
genetic diversity if large numbers of coexisting, related species result
in smaller population sizes. Here, we test whether increasing species
richness results in differential occupation of morphospace by the
constituent species, or decreases species’ genetic diversity. We test for
two potential mechanisms of morphological accommodation: denser packing in
ecomorphological space, and expansion of the space. We then test whether
species differ in their nucleotide diversity depending on allopatry or
sympatry with relatives, indicative of potential genetic consequences of
coexistence that would reduce genetic diversity in sympatry. We ask these
questions in a spatially explicit framework, using a global database of
avian functional trait measurements in combination with >120,000
sequences downloaded from GenBank. We find that higher species richness
within families is not systematically correlated with either packing in
morphological space or overdispersion but, at the Class level, we find a
general positive relationship between packing and species richness, but
that packing is comparatively greater in tropical points relative to their
species richness. We find limited evidence that geographical co-occurrence
with closely related species or tropical distributions decreases
nucleotide diversity of nuclear genes; however, this requires further
analysis. Our results suggest that avian families can accumulate species
regionally with minimal tradeoffs or cost, implying that external biotic
factors do not limit species richness.