Body size and trophic levels explain global asymmetric response of tetrapod diversity to climate effects

Although climate-based hypotheses are widely used to explain large-scale
diversity patterns, they fall short of explaining the spatial variation
among taxonomic groups. Integrating food web and metabolic theories into
macroecology is a promising step forward, as they allow including explicit
taxon-specific traits that can potentially mediate the relationship
between climate and diversity. Our investigation focuses on the role of
body size and trophic structure in mediating the influence of contemporary
climate and historical climate change on global tetrapods species
richness. We used piecewise structural equation modeling to assess the
direct effects of contemporary climate and climate instability of species
richness and  the indirect effects of climate on tetrapod
richness mediated by community-wide species traits. We found that birds
and mammals are less sensitive to the direct effect of contemporary
climate than amphibians and squamates. Contemporary climate and climate
instability favored the species richness in mammals and amphibians.
However, for birds and squamates, this link is only associated with
contemporary climate. Moreover, we demonstrated that community-wide traits
favored the species richness gradients of tetrapod groups, except
amphibians, but this link depends on traits and taxonomic groups.
Specifically, bird communities with smaller bodies and bottom-heavy
structures support higher species richness. Squamates also tend to be more
diverse in communities with prevalence of smaller bodies, while mammals
are correlated with top-heavy structures. Moreover, we show that higher
contemporary climate and climate instability reduce the species richness
of birds and mammals through community-wide traits and tend indirectly
improve squamate species richness. Thus, we showed the significance of
body size and trophic structure effect in driving a global asymmetric
response of tetrapod diversity to climate effects highlights the
limitations of applying "typical" climate-based hypotheses.
Furthermore, by combining multiple theories, our research contributes to a
more realistic and mechanistic understanding of diversity patterns across
taxonomic groups.