Bird clades with less complex appendicular skeletons tend to have higher species richness

Area of Science:

• Evolutionary Biology
• Comparative Anatomy
• Biodiversity Science

Background:

• Species richness varies significantly across taxonomic groups, but the drivers of this heterogeneity are not fully understood.
• Morphological diversity (disparity) is known to be decoupled from taxonomic diversity, yet the role of morphological complexity remains less explored.

Purpose of the Study:

• To investigate the relationship between morphological complexity and species richness in extant birds.
• To determine if increased morphological complexity correlates with reduced clade diversity.

Main Methods:

• A novel index of morphological complexity was developed based on the differentiation of fore and hind limb pairs.
• This index was applied to a dataset of 983 extant bird species.
• Statistical analysis was performed to test the correlation between morphological complexity and species richness.

Main Results:

• A significant negative correlation was found between the species richness of bird clades and their mean morphological complexity.
• Clades with higher morphological complexity exhibited a tendency to occupy fewer dietary and habitat niches, indicating greater ecological specialization.
• Ecological specialization, as a proxy for complexity, was also negatively correlated with species richness.

Conclusions:

• Greater morphological complexity in the appendicular skeleton appears to impede the generation and maintenance of species diversity in birds.
• This may be due to evolutionary "entrenchment" in specific morphologies and ecologies that are less conducive to further diversification.