Kīpuka as crucibles of evolution
Our data provided strong evidence for our first hypothesis, that we
would document a species-area effect relating to community-wide
isolation. Lava flows pose a strong dispersal barrier to core forest
arthropods (Roderick et al. 2012, Carson et al. 1990), so we did recover
a strong species-area effect consistent with previous results from
spiders (Vandergast et al 2004), Drosophila flies (Mueller 2015),
birds (Flaspohler et al. 2010), and root fungi associated with M.
polymorpha (Vannette et al. 2016). The isolation of communities by lava
also caused considerably higher community turnover between kīpuka than
between equivalently spaced sites in continuous forest. Even kīpuka
isolated by only a few 100 m of lava sometimes showed nearly complete
community turnover. As further predicted by our first hypothesis, the
community-wide isolation between kīpuka also translated into genetic
differences between populations of individual species, as has already
been shown in a few spiders (Vandergast et al. 2004, Roderick et al.
2012) and flies (Mueller 2015). The regrowth of forest after volcanic
eruptions on Hawaiʻi can take from several hundred to several thousand
years, providing ample time for genetic differentiation between
populations in individual kīpuka by genetic drift (Lourenço et al.
2017). Allopatric isolation, a major driver of speciation for organisms
across the tree of life, can happen over long geological time periods
and large geographic distances–through continental drift or the
emergence of mountain ranges– but we also know that even very short
geographic distances can suffice to isolate populations (Johnson &
Munshi-South 2017). Our analysis suggests that fragmentation by lava
flows could pose a possible means for such microallopatric
differentiation for Hawaiian arthropods.
The extreme community turnover between neighboring kīpuka means that
individual arthropod species are exposed to very different communities
of interacting species in different kīpuka, and hence different
selective pressures. For example, a single species present in different
kīpuka might interact with different competitors, predators or
parasitoids. Interspecific interactions are highly important drivers of
speciation (Thompson 2005), so these distinct biotic communities may
accelerate the differentiation of taxa between kīpuka. Besides the
possibility kīpuka provide for adaptation to different biotic
environments, kīpuka may provide opportunities for adaptation along
climatic gradients. For instance, adjacent kīpuka are frequently located
along pronounced temperature and precipitation gradients in Hawaiʻi,
such that the isolation of species in these kīpuka could foster
phenotypic differentiation through local climate adaptation. In a
continuous forest this adaptation would likely be swamped by gene flow.
While here we focused on kīpuka distributed within a relatively similar
climatic zone, an old kīpuka system distributed across a climatic
gradient would provide an excellent setup for studying the impacts of
microallopatry on rates of phenotypic differentiation. A final means by
which kīpuka may foster differentiation is through the merging of
formerly isolated kīpuka: admixture of the isolated populations they
contain could provide the raw material for rapid evolutionary responses,
as has been suggested as the underlying cause for many adaptive
radiations (Marques et al. 2019).