Concerted recolonization and co-dispersal of host and epibiota
Despite the potential challenges of inferring fine scale temporal
differences in expansion timing among species (Gehara et al.2017), the different τ pulse buffer priors we used in theMultiDice analysis in addition to the post hoc comparison
of independent fastsimcoal simulations consistently supported
synchronous, post-uplift demographic expansions across theDurvillaea hosts and their specialized consumer species. These
finding highlight rapid biological responses to disruption, and indicate
that community reassembly can be concerted when taxa have tight
ecological links. These findings also support the contemporary
host-tracking hypothesis, showing that temporal patterns of lineage
diversification are strongly congruent across different trophic levels
(Becerra 2003; Nicholls et al. 2010). Although our studied
species are co-distributed and can be subject to similar environmental
changes and vicariance events, the inferred synchronous demographic
expansion indicates a strong ecological interdependency between the
macroalgal hosts and their holdfast epifauna.
Macroalgal rafting is considered an important dispersal mechanism in the
assembly of biological communities of isolated coastal ecosystems (Thiel
& Gutow 2005; Fraser et al. 2011; Gillespie et al. 2012;
Nikula et al. 2013). Buoyant Durvillaea macroalgae, in
particular, have high rafting propensity (Fraser et al. 2011)
and, when detached, can retain diverse epifaunal macroinvertebrate
assemblages that have the potential to survive and even brood throughout
their rafting journey (Miranda & Thiel 2008; Waters et al.2018). In particular, holdfast-burrowing epibiota such asLimnoria and Onithochiton , which have no planktonic larvae
and limited autonomous dispersal capacity, are expected to rely heavily
on their rafting hosts for dispersal. In keeping with this prediction,
our multi-population demographic modelling reveals a tight co-dispersal
pattern whereby uplifted coasts were broadly colonized from northern
source populations. While this dispersal route runs counter to
prevailing offshore winds and currents, previous studies have shown that
strong storms and surface waves can underpin counter-current dispersal
in rafting assemblages (Fraser et al. 2018b).
The spatial genomic structure of the intertidal Durvillaea hosts,L. segnis and O. neglectus indicates a higher admixture
between the uplift and non-uplift clusters in the invertebrates compared
to the kelps. Such patterns of local admixture may reflect the more
extended opportunities for local, active dispersal in invertebrates
among neighboring locations, and/or their patchier local distributions
which may provide more opportunities for the establishment of dispersing
individuals. In contrast, the densely populated kelps prevent the
establishment of dispersing recruits and restrict effective gene flow
through high-density blocking (Waters et al. 2013).