Source pool diversity and proximity shape the compositional uniqueness of insular mammal assemblages worldwide

Islands have been the test bed of several theories in community ecology,
biogeography, and evolutionary biology. Progress within these disciplines
has given a more comprehensive and mechanistic understanding of the
processes governing variation in species richness among islands. However,
it remains unclear whether these same processes also explain variation in
species and phylogenetic composition among islands. Integrating theory
from ecology and biogeography, we infer the roles of dispersal, selection,
and stochasticity on the composition of insular assemblages within
archipelagos. We further assess the influence of source pool diversity and
connectivity on the compositional uniqueness of insular assemblages.
Island systems worldwide. We compiled data on species composition of
non-volant mammals on ∼200 islands in nine archipelagos distributed
worldwide from the literature. We used variation partitioning to quantify
the relative influence of the environment (selection) and geographic
distance (dispersal) relative to a null model (stochasticity, randomness)
on taxonomic and phylogenetic compositional turnover within archipelagos.
We then used a linear mixed model to gain further insight into the
underlying mechanisms shaping variation in assemblage composition among
islands at a global scale. Specifically, we assessed the influence of
source pool diversity, isolation from the source pool, and island
characteristics on compositional uniqueness. Our results suggest that
within-archipelago variation in the composition of insular mammal
assemblages is associated with stochastic or unmeasured processes rather
than abiotic selection or dispersal limitation. The diversity and
proximity of the source pool, as well as some island characteristics,
explained variation in phylogenetic, but not taxonomic, compositional
uniqueness globally. Within archipelagos, the largely unexplained
variation in compositional turnover points to the overwhelming influence
of extinction mediated by ecological drift or other stochastic processes,
which obscures or overrides the signature of selection and/or dispersal.
Globally, isolated islands associated with highly diverse source pools
exhibit high phylogenetic uniqueness whereas well-connected islands
associated with small source pools show the opposite trend.
Phylogenetically unique assemblages also tend to occur on islands with a
small elevational span and low annual temperature variation. Taken
together, our results suggest that source pool diversity, along with the
potential for colonization from those pools, plays an important role in
shaping the composition of insular mammal assemblages worldwide.