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).