Raisa Nikula

Long-distance dispersal: a framework for hypothesis testing

Tests of hypotheses about the biogeographical consequences of long-distance dispersal have long eluded biologists, largely because of the rarity and presumed unpredictability of such events. Here, we examine data for terrestrial (including littoral) organisms in the Pacific to show that knowledge of dispersal by wind, birds and oceanic drift or rafting, coupled with information about the natural environment and biology of the organisms, can be used to generate broad biogeographic predictions. We then examine the predictions in the context of the origin, frequency of arrival and location of establishment of dispersed organisms, as well as subsequent patterns of endemism and diversification on remote islands. The predicted patterns are being increasingly supported by phylogenetic data for both terrestrial and littoral organisms.

Kelp genes reveal effects of subantarctic sea ice during the Last Glacial Maximum

The end of the Last Glacial Maximum (LGM) dramatically reshaped temperate ecosystems, with many species moving poleward as temperatures rose and ice receded. Whereas reinvading terrestrial taxa tracked melting glaciers, marine biota recolonized ocean habitats freed by retreating sea ice. The extent of sea ice in the Southern Hemisphere during the LGM has, however, yet to be fully resolved, with most palaeogeographic studies suggesting only minimal or patchy ice cover in subantarctic waters. Here, through population genetic analyses of the widespread Southern Bull Kelp (Durvillaea antarctica), we present evidence for persistent ice scour affecting subantarctic islands during the LGM. Using mitochondrial and chloroplast genetic markers (COI; rbcL) to genetically characterize some 300 kelp samples from 45 Southern Ocean localities, we reveal a remarkable pattern of recent recolonization in the subantarctic. Specifically, in contrast to the marked phylogeographic structure observed across coastal New Zealand and Chile (10- to 100-km scales), subantarctic samples show striking genetic homogeneity over vast distances (10,000-km scales), with a single widespread haplotype observed for each marker. From these results, we suggest that sea ice expanded further and ice scour during the LGM impacted shallow-water subantarctic marine ecosystems more extensively than previously suggested.

Oceanic rafting by a coastal community

Oceanic rafting is thought to play a fundamental role in assembling the biological communities of isolated coastal ecosystems. Direct observations of this key ecological and evolutionary process are, however, critically lacking. The importance of macroalgal rafting as a dispersal mechanism has remained uncertain, largely owing to lack of knowledge about the capacity of fauna to survive long voyages at sea and successfully make landfall and establish. Here, we directly document the rafting of a diverse assemblage of intertidal organisms across several hundred kilometres of open ocean, from the subantarctic to mainland New Zealand. Multispecies analyses using phylogeographic and ecological data indicate that 10 epifaunal invertebrate species rafted on six large bull kelp specimens for several weeks from the subantarctic Auckland and/or Snares Islands to the Otago coast of New Zealand, a minimum distance of some 400–600 km. These genetic data are the first to demonstrate that passive rafting can enable simultaneous trans-oceanic transport and landfall of numerous coastal taxa.

Poleward bound: biological impacts of Southern Hemisphere glaciation

Postglacial recolonisation patterns are well documented for the Northern Hemisphere biota, but comparable processes in the Southern Hemisphere have only recently been examined. In the largely terrestrial Northern Hemisphere, recession of ice after the Last Glacial Maximum (LGM) allowed various taxa, including slow-moving terrestrial species, to migrate poleward. By contrast, the Southern Hemisphere polar region is completely ringed by ocean, and recolonisation of Antarctica and the sub-Antarctic islands has thus presented considerable challenges. Although a few highly dispersive marine species have been able to recolonise postglacially, most surviving high-latitude taxa appear to have persisted throughout glacial maxima in local refugia. These contrasting patterns highlight the importance of habitat continuity in facilitating biological range shifts in response to climate change.

Passive rafting is a powerful driver of transoceanic gene flow

Dispersal by passive oceanic rafting is considered important for the assembly of biotic communities on islands. However, not much is known about levels of population genetic connectivity maintained by rafting over transoceanic distances. We assess the evolutionary impact of kelp-rafting by estimating population genetic differentiation in three kelp-associated invertebrate species across a system of islands isolated by oceanic gaps for over 5 million years, using mtDNA and AFLP markers. The species occur throughout New Zealands subantarctic islands, but lack pelagic stages and any opportunity for anthropogenic transportation, and hence must rely on passive rafting for long-distance dispersal. They all have been directly observed to survive transoceanic kelp-rafting journeys in this region. Our analyses indicate that regular gene flow occurs among populations of all three species between all of the islands, especially those on either side of the subtropical front oceanographic boundary. Notwithstanding its perceived sporadic nature, long-distance kelp-rafting appears to enable significant gene flow among island populations separated by hundreds of kilometres of open ocean.

Evolutionary consequences of microhabitat: population-genetic structuring in kelp- vs. rock-associated chitons.

Rafting has long been invoked as a key marine dispersal mechanism, but biologists have thus far produced little genetic evidence to support this hypothesis. We hypothesize that coastal species associated with buoyant seaweeds should experience enhanced population connectivity owing to rafting. In particular, invertebrates strongly associated with the buoyant bull‐kelp Durvillaea antarctica might be expected to have lower levels of population‐genetic differentiation than taxa mainly exploiting nonbuoyant substrates. We undertook a comparative genetic study of two codistributed, congeneric chiton species, assessing population connectivity at scales of 61–516 km, using ≥186 polymorphic AFLP loci per species. Consistent with predictions, population‐genetic differentiation was weaker in the kelp‐associated Sypharochiton sinclairi than in the rock‐associated S. pelliserpentis. Additionally, while we found a significant positive correlation between genetic and oceanographic distances in both chiton species, the correlation was stronger in S. pelliserpentis (R2 = 0.28) than in S. sinclairi (R2 = 0.18). These data support the hypothesis that epifaunal taxa can experience enhanced population‐genetic connectivity as a result of their rafting ability.

Comparison of population-genetic structuring in congeneric kelp- versus rock-associated snails: a test of a dispersal-by-rafting hypothesis

Phylogeographic studies indicate that many marine invertebrates lacking autonomous dispersal ability are able to achieve trans-oceanic colonization by rafting on buoyant macroalgae. However, less is known about the impact of rafting on on-going population-genetic connectivity of intertidal species associated with buoyant macroalgae. We hypothesize that such species will have higher levels of population-genetic connectivity than those exploiting nonbuoyant substrates such as rock. We tested this hypothesis by comparing nuclear multilocus population-genetic structuring in two sister topshell species, which both have a planktonic larval phase but are fairly well segregated by their habitat preference of low-tidal bull-kelp holdfasts versus mid-to-low tidal bare rock. We analyzed population samples from four sympatric sites spanning 372 km of the east coast of southern New Zealand. The sampled region encompasses a 180 km wide habitat discontinuity and is influenced by a stable, northward coastal current. The level of connectivity was high in both species, and neither of them showed significant correlation between genetic and geographic distances. However, a significant negative partial correlation between genetic distance and habitat discontinuity was found in the rock-associated species, and estimates of migrant movement between sites were somewhat different between the two species, with the kelp-associated species more often yielding higher estimates across the habitat discontinuity, whereas the rock-associated species more often exhibited higher estimates between sites interspersed by rock habitats. We conclude that for species with substantial means of autonomous dispersal, the most conspicuous consequence of kelp dwelling may be enhanced long-distance dispersal across habitat discontinuities rather than a general increase of gene flow.