Of islands on islands: Natural habitat fragmentation drives microallopatric differentiation in the context of distinct biological assemblages

An important evolutionary hypothesis posits that much of the biodiversity
we see today was generated by the interplay of colonization, extinction,
adaptation, and speciation during episodes of natural habitat
fragmentation. To interrogate the generality of this hypothesis, we
leverage the natural experiment provided by arthropod communities in
kīpuka—patches of Hawaiian wet forest isolated by lava flows. With DNA
metabarcoding, we provide the first simultaneous exploration of ecological
and evolutionary characteristics in the kīpuka system. At both species-
and haplotype-equivalent scales, we find that diversity scales with kīpuka
area, and that kīpuka accelerate the distance decay of similarity relative
to continuous forest. Open lava, kīpuka edges, and the centers of the
smallest kīpuka host lower inter- and intra-specific richness, are more
biologically homogeneous, and have higher proportions of non-native OTUs
than kīpuka centers for the order we could test (Araneae). Our work shows
how natural habitat fragmentation stimulates, in tandem, ecological shifts
in community composition and genetic differentiation. The strong
signatures of both ecological and evolutionary processes highlight the
importance of studying both processes simultaneously if we are to
understand how biological communities respond to change.