…

The distinction between richness and endemism deserves closer attention, because the two measures respond to fundamentally different evolutionary dynamics. Species richness simply counts how many species co-occur in a given area, whereas endemism captures how geographically restricted those species are, and weighted versions of the metric can additionally account for the evolutionary distinctness of the lineages involved. A region can therefore be packed with species yet contain few that are confined to it, while a remote island group with a modest species list may harbour an extraordinary proportion of forms found nowhere else on Earth. The Southwest Pacific illustrates this decoupling with unusual clarity. Along the coast of eastern Australia and around the northern reaches of Aotearoa New Zealand, fish faunas accumulate large numbers of species drawn from both tropical and temperate sources, producing pronounced richness peaks. Yet the true endemism centres lie elsewhere, on small subtropical islands scattered across the Tasman Sea and the southern Pacific, where isolation has allowed lineages to diverge undisturbed by continuous gene flow from continental faunas.

The geological setting of the Southwest Pacific helps explain why these patterns take the shape they do. The region sits atop one of the most tectonically active corners of the planet, where the Pacific plate subducts beneath the Australian plate along deep ocean trenches, and where volcanic arcs, seamount chains and submerged plateaus have risen and sunk over tens of millions of years. Lord Howe Island, Norfolk Island and the Rangitāhua or Kermadec Islands all owe their existence to hotspot volcanism or arc volcanism, and each emerged from the sea as a blank slate for colonisation. Because these islands are young in geological terms and far from continental shores, their fish faunas had to be assembled almost entirely through over-water dispersal. Once founders arrived, the combination of geographic isolation, distinct current regimes and subtropical water temperatures created conditions in which small populations could diverge from their source lineages, ultimately producing species restricted to a single island or a small archipelago.

Oceanography reinforces this isolation. The East Australian Current carries warm, oligotrophic water southward along the continental slope, spinning off eddies that can transport larvae toward Lord Howe and Norfolk, while the Tasman Front separates tropical from subtropical water masses and acts as a partial filter on faunal exchange. Further east, the South Pacific subtropical gyre connects remote islands through slow, diffuse currents, and the deep basins between island groups present formidable barriers to coastal species whose larvae survive only days to weeks in the plankton. For species with extended pelagic larval durations, these currents are highways; for those with short-lived or benthic larvae, they are walls. This variability in dispersal ability helps explain why endemism is concentrated in particular taxonomic groups and why some island faunas resemble stepping stones along current pathways while others appear surprisingly idiosyncratic.

The review’s proposed scenario, in which subtropical islands act as biodiversity cradles while mainland Australia serves as the principal source of endemic lineages, aligns with a growing body of phylogenetic evidence from other marine taxa. A cradle, in biogeographic parlance, is a place where new species are generated, in contrast to a museum, which preserves old lineages over long spans of time. Islands frequently serve as cradles because their small areas, abrupt environmental gradients and isolation promote rapid divergence following colonisation. Australia, by contrast, with its vast continental shelf and long coastline spanning tropical to temperate latitudes, functions as both a museum of ancient lineages and a pump of dispersing propagules. Molecular phylogenies calibrated with fossil evidence have repeatedly shown that lineages now endemic to Southwest Pacific islands trace their ancestry back to Australian or broader Indo-Pacific ancestors, with divergence dates that often coincide with periods of sea-level fluctuation, current reorganisation or island emergence during the late Miocene and Pliocene.

Vicariance, the second pillar of the proposed scenario, deserves particular emphasis because it is often underappreciated in island systems. The traditional narrative assumes that island endemics arise almost exclusively through long-distance colonisation followed by in situ divergence, yet the review highlights that some endemic lineages were once more widely distributed and became fragmented as barriers strengthened or as intermediate populations disappeared. Sea-level lowstands during glacial cycles exposed land bridges and altered current pathways, while highstands flooded shelves and submerged stepping-stone habitats. Each of these oscillations could sever gene flow between populations that had previously exchanged larvae, converting a continuous distribution into disjunct segments destined for independent evolutionary trajectories. Distinguishing between a young endemic produced by recent colonisation and an old endemic produced by vicariance requires dated phylogenies, and it is precisely the combination of fossil-calibrated molecular trees with probabilistic biogeographic models that has made such distinctions tractable in recent years.

Methodologically, the field has moved decisively beyond descriptive biogeography. Probabilistic models of range evolution now allow researchers to estimate the likelihood of dispersal, extinction and local speciation events across a phylogeny, incorporating information about geological connectivity between areas and even allowing rates of these processes to vary through time. Simultaneously, randomisation tests and spatial autocorrelation methods permit rigorous statistical evaluation of whether observed patterns of richness and endemism exceed what would be expected by chance given the underlying species pools. These tools, applied to the Southwest Pacific, have transformed anecdotal observations about unusual island faunas into quantitative statements about where, when and how biodiversity was generated. They have also revealed that the processes governing richness and those governing endemism operate at different spatial scales, so that a single conservation strategy calibrated to one metric may fail to protect the other.

The bimodal latitudinal distribution of marine species richness, with peaks at the margins of the tropics and a dip near the equator, adds an intriguing dimension to the Southwest Pacific picture. Although counterintuitive, this pattern has been attributed to the extreme and seasonally stable conditions at the equatorial centre, which may exclude species adapted to the more variable conditions found toward the edges of the tropical belt. In the Southwest Pacific, the northern waters around the Kermadec archipelago and the northern North Island of Aotearoa New Zealand sit near the subtropical convergence of faunas, where warm-water species reaching their southern limits mingle with cold-water species reaching their northern limits. This overlap of distantly related tropical taxa and closely related temperate taxa inflates local richness without necessarily producing endemism, because the constituent species each belong to broader regional faunas. The result is a richness peak that is taxonomically heterogeneous and evolutionarily young, in stark contrast to the phylogenetically distinctive assemblages of the offshore islands.

Climate change looms over these patterns. Poleward shifts in species distributions have been documented across the globe as waters warm, and regions that span multiple climatic zones, such as the Southwest Pacific, are expected to experience particularly pronounced reorganisations of their biotas. Tropical species are already extending southward along the Australian coast and into Tasmanian waters, while some temperate endemics face compression of their suitable habitat against the southern coastline. For the subtropical island endemics, whose entire global distribution may encompass a few square kilometres of reef and rocky coast, even modest shifts in water temperature, current strength or larval survival could spell the difference between persistence and extinction. The cradle function of these islands may also be compromised if the environmental conditions that promote divergence change faster than new adaptations can arise.

From a conservation standpoint, the decoupling of richness and endemism carries a practical lesson. Protected areas designed to capture the greatest number of species will naturally gravitate toward the richness hotspots of the continental coast and northern New Zealand, yet these same areas may contribute little to the preservation of range-restricted endemics, which are concentrated on isolated islands with small populations and correspondingly elevated extinction risk. Effective conservation of the region’s evolutionary heritage therefore requires a dual strategy, safeguarding both the bustling species-rich assemblages of the continental margins and the fragile, phylogenetically unique communities of Lord Howe, Norfolk and Rangitāhua. The review’s synthesis makes clear that these two goals are not redundant but complementary, each protecting a different facet of the region’s biodiversity.

Finally, the Southwest Pacific remains underexplored relative to the better-studied coral triangle to its north. Deep-reef habitats, mesophotic ecosystems and the fish faunas of remote seamounts are only beginning to be surveyed with modern genetic and imaging tools, and cryptic species almost certainly await description within even well-known genera. As genomic datasets accumulate and fossil records improve, the biogeographic scenario proposed for the region will be tested and refined, and the interplay between dispersal and vicariance, between cradles and museums, will be quantified with increasing precision. What is already evident is that the Southwest Pacific offers a natural laboratory in which the full spectrum of marine biogeographic processes, from rare transoceanic colonisation to ancient vicariant fragmentation, can be studied within a compact and geologically dynamic setting, and that the lessons learned there will inform our understanding of how marine biodiversity is generated and maintained across the world ocean.

Subject of Research: …

Article Title: The historical biogeography of marine ray-finned fishes ( Actinopterygii ) in the Southwest Pacific: species richness and endemism patterns

Article References: Samayoa, A. P., & Liggins, L. (2026). The historical biogeography of marine ray-finned fishes (Actinopterygii) in the Southwest Pacific: species richness and endemism patterns. Frontiers of Biogeography, 19, Article e204479. https://doi.org/10.21425/fob.19.204479

Image Credits: AI Generated

DOI: 10.21425/fob.19.204479

Keywords: historical, biogeography, marine, ray-finned, fishes, Actinopterygii, Southwest, Pacific, species, richness, endemism, patterns