Of islands on islands: A natural experiment reveals the pronounced
effect of habitat fragmentation on arthropod community assembly in
Hawaiʻi
Abstract
The term ‘habitat fragmentation’ is frequently associated with the
biologically-destructive activities of human development, but an
important evolutionary hypothesis posits that much of the biodiversity
we see today was generated by episodic, natural habitat fragmentation.
This hypothesis suggests that fragmentation can serve as a ‘crucible of
evolution’ through the amplifying feedbacks of colonization, extinction,
adaptation, and speciation. Interrogating the generality of this
hypothesis requires measuring the repercussions of fragmentation at
intra- and interspecific levels across entire communities. We use DNA
metabarcoding to capture these repercussions from the scales of
intraspecific differentiation to community composition in a megadiverse,
ecologically foundational group, arthropods, using a natural habitat
fragmentation experiment on patches of wet forest isolated by
contemporary Hawaiian lava flows (kīpuka). We find a pronounced effect
of area in kīpuka cores, where the taxonomic richness supported by a
kīpuka scales with its size. Kīpuka cores exhibit higher intra- and
interspecific turnover over space than continuous forest. Additionally,
open lava, kīpuka edges, and the cores of small kīpuka (which are
essentially entirely “edge”) host lower richness, are more
biologically homogeneous, and have higher proportions of non-native taxa
than kīpuka cores. Our work shows how habitat fragmentation isolates
entire communities of habitat specialists, paving the way for genetic
differentiation. Parsing the extent to which differentiation in kīpuka
is driven by local adaptation versus drift provides a promising future
avenue for understanding how fragmentation, and the different isolated
communities created through this process, may lead to speciation in this
system.