The roles of isolation and interspecific interaction in generating the functional diversity of an insular mammal radiation

Communities that occupy similar environments but vary in the richness of
closely related species can illuminate how functional variation and
species richness interact to fill ecological space in the absence of
abiotic filtering, though this has yet to be explored on an oceanic island
where the processes of community assembly may differ from continental
settings. In discrete montane communities on the island of Sulawesi, local
murine rodents (rats and mice) richness ranges from 7 to 23 species. We
measured 17 morphological, ecological, and isotopic traits, both
individually and grouped into 5 multivariate traits in 40 species, to test
for the expansion or packing of functional space among nine murine
communities. We employed a novel probabilistic approach for integrating
intraspecific and community-level trait variance into functional richness.
Trait-specific and phylogenetic diversity patterns indicate dynamic
community assembly due to variable niche expansion and packing on multiple
niche axes. Locomotion and covarying traits such as tail length emerged as
a fundamental axis of ecological variation, expanding functional space and
enabling the niche packing of other traits such as diet and body size.
Though trait divergence often explains functional diversity in island
communities, we found that phylogenetic diversity facilitates functional
space expansion in some conserved traits such as cranial shape, while more
labile traits are overdispersed both within and between island clades,
suggesting a role of niche complementarity. Our results evoke
interspecific interactions, differences in trait lability, and the
independent evolutionary trajectories of each of Sulawesi’s 6 murine
clades as central to generating the exceptional functional diversity and
species richness in this exceptional, insular radiation.