Island Species Research Articles

In situ diversification and adaptive introgression in Taiwanese Scutellaria.

Island habitats provide unique opportunities to study speciation. Recent work indicates that both ex situ origination and in situ speciation contribute to island species diversity. However, clear evidence of local adaptation of endemic plant species on islands requires in-depth studies, which are scarce. This study underscores the importance of local adaptation in maintaining species boundaries by examining how adaptive introgression, hybridization, and local adaptation contribute to genetic variation in island species. Multilocus genome scanning of 51 nuclear genes was used to investigate the evolutionary relationships of the Scutellaria species complex on Taiwan Island and assess the role of in situ diversification in generating high endemism and genetic diversity. Interspecies introgressions were detected by phylogenetic networks and ABBA-BABA-based analysis, suggesting ongoing or recent speciation processes. Coalescent-based simulation identified hybrid speciation in Scutellaria taiwanensis and Scutellaria hsiehii, with evidence of hybridization between more than two parental species. Genotype-environment association studies revealed that the influence of climate, particularly precipitation- and temperature-related factors, contributed to adaptive genetic divergence between species. Additionally, adaptive introgression related to environmental pressures that may have facilitated the colonization of new island habitats were identified. This research illustrates how hybridization, introgression, and adaptation shaped the evolutionary histories and divergence of this island-endemic plant species complex and sheds light on the multifaceted mechanisms of speciation on semi-isolated islands.

The formation, maintenance, and loss of island biodiversity.

Due to the unique geographical features of large numbers, isolated by water and diverse formation histories, islands have become natural laboratories for ecological and evolutionary research. Islands have a high proportion of endemic species and disharmony in representing the species compared with that in the continent, which provides a good opportunity to explore the formation of island biodiversity. In this review, we focuse on island ecosystems and describes the progress of research in island biogeography in recent years from three aspects: formation, maintenance, and loss of island biodiversity. First, we take several representative island systems in the world as examples to introduce the formation mechanism of island biodiversity from species dispersal and immigration, population establishment and selection, species evolution and adaptive radiation. Based on the Equilibrium Theory of Island Biogeography, we then review the species-area relationship and species-isolation relationship and focus on the research progress in community assembly, including Diamond's rule, nestedness pattern, the framework of community structure of island biota, and species interactions on islands. We also discuss the high extinction risks of island species threatened from natural and human disturbances, especially from the impact of habitat loss and change, climate change, alien species invasion and the synergistic effect of these factors on the loss of island biodiversity. Finally, based on the status of island biodiversity, we summarized the current conservation practices for island biodiversity and possible research frontiers in island biogeographic studies.

Genomic data reveal that the Cuban blue-headed quail-dove (Starnoenas cyanocephala) is a biogeographic relict.

Islands are well known for their unique biodiversity and significance in evolutionary and ecological studies. Nevertheless, the extinction of island species accounts for most human-caused extinctions in recent time scales, which have accelerated in recent centuries. Pigeons and doves (Columbidae) are noteworthy for the high number of island endemics, as well as for the risks those species have faced since human arrival. On Caribbean islands, no other columbid has generated more phylogenetic interest and uncertainty than the blue-headed quail-dove, Starnoenas cyanocephala. This endangered Cuban endemic has been considered more similar, both behaviourally and phenotypically, to Australasian species than to the geographically closer 'quail-dove' (Geotrygon s.l.) species of the Western Hemisphere. Here, we use whole genome sequencing from Starnoenas and other newly sequenced columbids in combination with sequence data from previous publications to investigate its relationships. Phylogenomic analyses, which represent 35 of the 51 genera currently comprising the Columbidae, reveal that the blue-headed quail-dove is the sole representative of a lineage diverging early in the radiation of columbids. Starnoenas is sister to the species-rich subfamily Columbinae, which is found worldwide. As a highly distinctive evolutionary lineage lacking close modern relatives, we recommend elevating the conservation priority of Starnoenas.

Forest and Isolation Determine Bat Species Richness on Mediterranean Islands: Conservation Implications

ABSTRACTBackgroundThe study of island species richness has gone far beyond traditional theories, as more and more studies attempt to include new predictor variables besides island area. Despite the rich bat diversity of Mediterranean islands, no studies have examined the factors influencing chiropteran species richness in such a geographic context.AimsWe used Greek islands as a model to detect drivers of bat species richness based on a broad species occurrence dataset from islands of the Aegean and Ionian Seas, which was explored using 10 potential predictor variables.Materials and MethodsInitially, we comprehensively evaluated all available data and sources for 53 Greek islands, deemed essential. This resulted in a reduced set of 28 islands. Predictors were grouped into three categories: island area, landscape diversity and island isolation. Generalised linear models were employed to identify the most significant factors explaining bat species richness in the 28 islands.ResultsTree cover and the distance between the islands and their nearest coast (mainland or island) best performed to predict bat species richness on Greek islands.DiscussionWe show the important role of landscape composition and degree of isolation in influencing the distribution of bat species on islands.ConclusionWe highlight the importance of forest habitats and the vital role of their management and protection, especially in the insular systems of the Mediterranean basin, where the risk of forest degradation or loss due to wildfires and land use change is especially high.

Patterns of Genomic Heterogeneity in a Classic Field Cricket Mosaic Hybrid Zone

ABSTRACTBarriers to gene exchange can be semi‐permeable; some genes are expected to freely flow across species boundaries whereas others, under divergent selection or responsible for reproductive isolation, might not. Genome scans in recently diverged species have identified divergent genomic regions, a pattern that has often been interpreted as islands of restricted introgression in a background of relatively free gene exchange (“genomic islands of speciation”). Areas of high differentiation, most located in the X chromosome (females XX, males X0), have been identified in the hybridizing field crickets Gryllus firmus and Gryllus pennsylvanicus. These species were assumed to follow an islands of speciation model, with highly differentiated areas interpreted as areas of reduced introgression. We sequenced the G. firmus genome to localize previously studied SNPs and sample a larger area around them in 8 allopatric populations (4 of each species). We use these data to test expectations for the islands model, in which non‐introgressing areas should have both high absolute and relative differentiation. We find that in the allopatric populations, the areas with high relative differentiation (mostly X‐linked), previously interpreted as non‐introgressing, do not have high absolute differentiation as would be expected under the “islands model.” We also show that the estimated divergence time based on nuclear DNA is about 4× older than that estimated based on mtDNA (800 K vs. 200 K years ago). We discuss the implications of our results for introgression into allopatric populations.

Sexual size dimorphism as a determinant of fighting performance dimorphism in Anolis lizards.

Rensch's Rule describes a pattern of interspecific allometry in which sexual size dimorphism (SSD) increases with size among closely related species (i.e., among a group of related species, the largest ones tend to show more male-biased SSD). Sexual selection is often invoked to explain Rensch's Rule, as larger male body size is assumed to be favoured by sexual selection for increased fighting performance in contests for mating opportunities. Often, however, the correlation between size and performance is not well described. We studied a sexually selected performance trait, bite force in Anolis lizards, to determine whether patterns of SSD are linked to size-associated patterns of performance dimorphism at the macroevolutionary level, as expected under the sexual selection hypothesis for Rensch's Rule. Additionally, we tested whether allometric patterns of performance dimorphism differ between mainland and island species, as the latter have likely evolved under a stronger sexual selection regime. We found that SSD overwhelmingly explains the relationship between performance dimorphism and size in anoles, as expected under a sexual selection model for Rensch's Rule. However, residual performance dimorphism was higher in island than in mainland species, suggesting that these groups differ in performance dimorphism for reasons unrelated to size. Head size dimorphism was associated with residual performance dimorphism, but did not fully explain the difference in performance dimorphism between island and mainland species. Together, these findings highlight the need to interpret Rensch's Rule patterns of body size evolution cautiously, as allometric patterns of performance dimorphism and size dimorphism might not be equivalent.

Forget-me-not phylogenomics: Improving the resolution and taxonomy of a rapid island and mountain radiation in Aotearoa New Zealand (Myosotis; Boraginaceae)

Island and mountain systems represent natural laboratories for studies of species radiations, but they often present several challenges for phylogenetic inference and species delimitation. The southern hemisphere forget-me-nots (Myosotis, Boraginaceae) comprise a geologically recent radiation centred in Aotearoa New Zealand, a mountainous archipelago, with about 50 species that are morphologically and ecologically divergent but lack genetic variation sufficient to resolve phylogenetic relationships and species boundaries using standard DNA Sanger sequencing markers, AFLPs, or microsatellites. Many of these Myosotis species are geographically restricted in alpine areas, uncommon or threatened, have polyploid and dysploid genomes, and are of high taxonomic and conservation priority. Here we present phylogenomic analyses using target-capture of Angiosperms353 baits, and genome skimming of whole plastomes and nrDNA, to improve resolution of the radiation, explore biogeographic and morphological patterns within it, and address specific taxonomic questions for each species. Our comprehensive sampling includes over 300 individuals representing nearly all species from New Zealand and Australia, which is ∼ 2–3 × more taxon sampling and ∼ 80–120 × more molecular data than previously published for Myosotis. Exploration of different data filtering, curation and analyses (coalescent vs. concatenation) improved the resolution of the Angiosperms353 tree, which despite short backbone branches with low support values, showed taxonomic and geographic patterns, including multiple switches between ebracteate and bracteate inflorescences and multiple expansions within New Zealand from Te Waipounamu South Island to Te Ika-a-Māui North Island, Rakiura Stewart Island, subantarctic islands, and Australia. Some of these patterns were also seen in the genome skimming datasets, and comparison of the three datasets was useful for improving our understanding of the taxonomy and resolution of this radiation. Although this phylogenomic study does not fully overcome all of the challenges regarding species delimitation of this rapid island and mountain species radiation, it nevertheless makes an important contribution to an integrative taxonomic revision of the southern hemisphere species of Myosotis.

How to Count Parrots: Comparing the Performance of Point and Transect Counts for Surveying Tasman Parakeets (Cyanoramphus cookii)

Obtaining precise estimates of population size and trends through time is important for the effective management and conservation of threatened species. For parrots (Psittaciformes: Psittacidae), obtaining such estimates can be challenging, particularly for cryptic species that occur in low densities in complex and/or fragmented habitats. We used a statistical resampling approach with the aim to compare the reliability and precision of counts for the critically endangered Tasman parakeet (Cyanoramphus cookii) that were taken using two methods on Norfolk Island (Pacific Ocean), namely, fixed-point counts and line transect counts. The detections obtained during fixed-point counts had better estimated precision (0.274) than line transect counts (0.476). The fixed-point method was also more efficient, yielding 1.338 parakeet detections per count compared to the 0.642 parakeet detections per count obtained by the line transect method. Although Tasman parakeets can be detected by either of these methods, our research demonstrates that the fixed-point method is more precise and reliable. These findings can help prioritise resources for the long-term monitoring of recovering populations of this species and similar island species.

How Do Species Traits and Biogeographical Factors Determine the Fate of Amphibians After Long‐Term Fragmentation?

ABSTRACTAimIdentifying key factors that render certain species more vulnerable to fragmentation is vital for elucidating processes underlying extinction and targeting conservation priorities. However, few studies have explored the delayed ecological responses of species following isolation. To bridge the gap, we conducted comprehensive analyses of correlates of extinction vulnerability and biogeographical variation in amphibians over long‐term fragmentation.LocationZhoushan Archipelago, China.MethodsWe sampled the occupancy of amphibians on 37 land‐bridge islands. We calculated three metrics of extinction vulnerability (population extinction rate, island occupancy frequency and species nestedness ranking) for each species and correlated these variables with eight species' traits. We further explored biogeographical variations in amphibians by relating five biogeographical variables to species' probability of occurrence and calculated the threshold for the persistence of each species on islands.ResultsSpecies with low natural abundance, larger egg sizes, smaller clutch sizes or restricted geographical distributions were more likely to experience higher population extinction rates and species nestedness rankings, while lower island occupancy frequencies across islands. Although most species were found on larger islands, we observed significant increases in the occurrence probabilities with island area for five species. The estimated areas with a 50% chance of occurrence ranged from 0.39 to 199.5 km2. Interestingly, the likelihood of occurrence of Hyla chinensis (treefrog) was negatively related to distance to the mainland after controlling for the effect of area.Main ConclusionsOur study highlights the variation in the fragmentation sensitivity of amphibians. Species distribution was primarily regulated by area‐related extinction, particularly for those with ‘slow’ life history strategies or restricted ranges. Overall, management efforts should focus on species with extinction‐prone traits and landscape features that threaten the persistence of populations. Future studies should consider the sequential separation of island populations and the interaction of traits to reveal the fate of species to fragmentation.

A Renaissance of Atoll Ecology

The approximately 320 atolls of the world, scattered across the tropical oceanic basins, constitute a unique type of ecosystem in that they are each an integrated unit consisting of island, coral reef, and lagoon components. Atolls have a complex geology, ecology, and biogeography, which can be fully appreciated only by transcending the classic boundary thinking of marine and terrestrial realms. The atolls we observe today were shaped by Quaternary sea-level fluctuations, which imposed strong environmental filters on their communities. As entirely biogenic, reef-borne structures, the islands of atolls depend upon marine productivity, which catalyzes island community assembly. Island species communities exist in complex dynamic equilibria with the surrounding oceanographic conditions. Energy fluxes and element cycles of the atoll system readily cross habitat boundaries and create a productive, diverse, and biomass-rich ecosystem on land and underwater. Past human disturbances and future global change put atolls at the forefront of conservation and ecological restoration.

Parrots live in smaller groups on islands.

Species living on islands are predictably different from their mainland counterparts in morphology and behaviour, but the source of these differences is still debated. Islands, in particular, are characterized by depauperate predator communities. Relaxed predation pressure might explain why living in groups, a common anti-predator adaptation in animals, is considered less likely on islands. However, the empirical evidence for this effect is scant and alternative explanations have been overlooked. For instance, smaller groups might be more common because island species are less studied, because a more stable food supply associated with benign climate on islands favours territoriality, or because the population density is too low to allow the formation of larger groups. I examined the determinants of foraging group size in parrots, a large worldwide avian order with many island populations. Using a multivariable phylogenetic framework, I found that foraging group size was smaller on islands than on the continents controlling for ecological variables known to influence group size such as diet and body size. In addition, the island effect persisted after controlling for research effort, climate variables and population density, suggesting that impoverished predator communities are an important driver of group size on islands.

Stable detection frequency of the threatened Christmas Island Boobook Ninox natalis, 2012–2022

ABSTRACT Both island species and raptors are at particularly high risk of extinction but few island raptor populations have been the subject of long-term monitoring. To determine trends in abundance in the Christmas Island Boobook, surveys were conducted annually from 2012–2017, in 2019 and in 2022. Across the survey period the population appears to have been either stable or to have increased slightly. Almost no part of the island lacked owls during the most recent survey. The detection rate averaged 1.56 (out of four surveys at a site); it was lowest in 2013 (1.22) and highest in 2022 (2.38). Detection was more likely on nights with low wind, at wetter sites and in closed vegetation, rather than in open or very low vegetation. In a separate analysis on the same data, the top-ranked dynamic occupancy-detection model found that occupancy increased with increasing elevation and vegetation height. No effect of a suspected threat, the presence of invasive yellow crazy ants Anoplolepis gracilipes at a site, could be detected. We recommend ongoing monitoring and research, potentially using automated recording devices and the tracking of individual owls to understand and refine the assumptions underpinning the interpretation of survey results.

First sighting of a pair of Ryukyu robin Larvivora komadori Temminck, 1835 (Aves, Passeriformes) during breeding season on Jeju Island, Republic of Korea: a nesting attempt?

The Ryukyu robin (Larvivora komadori Temminck, 1835) is an endemic inhabitant of the Ryukyu Archipelago, Japan. In 2018, we made several birding excursions to Jeju Island, Republic of Korea. Within the 1100 Altitude Wetland protected area on May 28 and May 30 we encountered a pair of birds, which, according to reference literature and bird guides we identifi ed as Larvivora komadori. That was the fi rst visual sighting of the Ryukyu robin during the breeding season on Jeju Island, which is separated from the nearest breeding site by more than 200 kilometers of open sea. This fact can be regarded as a potential ability for this endemic island species to spread to new islands. Possible pathways for natural introduction of the Ryukyu

A rapid increase in tropical species of grouper (Perciformes: Serranidae) in the temperate waters, the Goto Islands, Japan.

Global warming has resulted in rapid poleward shifts in the geographical distributions of many tropical fish species. This study conducted daily market surveys from 2008 to 2013 to investigate catch trends of seven commercially important grouper species in the temperate Goto Islands, Japan. Our results revealed that the catch numbers of tropical grouper species increased rapidly by an average of 5.9-fold (12.3-fold at maximum) within six years, whereas the temperate and subtropical species did not exhibit substantial changes. Based on the findings of several previous studies, the rapid increase in the number of tropical groupers in temperate waters was most likely caused by the successful settlement of larvae transported from tropical waters. Large-scale ocean currents may facilitate larval transport from tropical waters because the Goto Islands face the Tsushima Warm Current, which branches from the Kuroshio Current. Meanwhile, the transition processes of size distribution in tropical groupers suggest a possible hypothesis that adults migrating from tropical waters first settle in temperate waters and then enhance their populations by reproduction. Further studies are required to determine how tropical grouper species settle and how their populations increase in temperate waters.

Exploratory analysis of the genetic diversity for two Phyllodactylus species in contrasting environments

Abstract Historically, the genetic diversity of insular species and populations has generally been considered to be lower with respect to their continental counterparts. In this study, I compare for first time the genetic diversity of two gecko species, Phyllodactylus angelensis and P. magnus, that inhabit contrasting environments in Mexico (island and mainland, respectively). Utilizing genomic data for both species generated through genotyping by sequencing (GBS), I performed paired comparisons of various genetic diversity indices analyzing thousands of single nucleotide polymorphism (SNP) variants. The majority of genetic diversity indices demonstrated very similar values between both species, while the observed heterozygosity and genetic diversity within populations was higher for the insular species. The results do not support the generalized assumption of lower genetic diversity in island species. Key aspects such as the effective population size of founders, gene flow, and shared life histories may be involved in explaining the observed pattern.

Island biogeography of the megadiverse plant family Asteraceae

The megadiverse plant family Asteraceae forms an iconic component of island floras including many spectacular radiations, but a global picture of its insular diversity is lacking. Here, we uncover the global biogeographical and evolutionary patterns of Asteraceae on islands to reveal the magnitude and potential causes of their evolutionary success. We compile a global checklist of Asteraceae species native and endemic to islands and combine it with macroecological analyses and a phylogenetic review of island radiations. Asteraceae have a global distribution on islands, comprising approximately 6,000 native island species, with 58% endemics. While diversity of the family on islands is lower than expected given its overall diversity, Asteraceae are the most diverse family on oceanic islands, suggesting an exceptional ability to thrive in isolation. In agreement with island biogeography predictions, native Asteraceae diversity increases with area and decreases with isolation, while endemism increases with both. We identify 39 confirmed island radiations and 69 putative radiations, exceeding numbers for other iconic insular groups. Our results reveal Asteraceae offer immense potential for research in ecology and evolution, given their close tracking of island biogeography expectations, large number of both species and radiations, cosmopolitan distribution, and numerous undiscovered radiations.

Within-flower anther polymorphism and the floral biology of Jaltomata antillana (solanaceae) of the Greater Antilles

The small, unspecialized flowers of Jaltomata antillana (Solanaceae), typical of island species, are likely allochthonous in origin given similar congeneric close relatives in the Andes. Floral nectar, produced continuously during the life of the flower, is yellowish in color and is UV absorptive, in contrast with the white UV reflecting corolla. The day the flower opens flowers are pistillate (anthers not yet presenting pollen) for only a few hours and then become functionally hermaphroditic (anthers dehisce). Within each flower there is length variation among the stamens, size variation among anthers, and staggered anther dehiscence, collectively an apparent pollen-dispensing mechanism. Flowers close each night both when the flower will be functional the next day and at floral senescence. In a pollinator-free greenhouse, 100% of the flowers that were manually self-pollinated set fruits; however, 29% of the tagged unmanipulated flowers set fruit. Autonomous self-pollination takes place at the end of the corolla’s life and allows fruit and seed production in the absence of pollinators but results in fruits that are lighter and contain fewer seeds than fruits resulting from manual self-pollinations.

Persistent Gene Flow Suggests an Absence of Reproductive Isolation in an African Antelope Speciation Model.

African antelope diversity is a globally unique vestige of a much richer world-wide Pleistocene megafauna. Despite this, the evolutionary processes leading to the prolific radiation of African antelopes are not well understood. Here, we sequenced 145 whole genomes from both subspecies of the waterbuck (Kobus ellipsiprymnus), an African antelope believed to be in the process of speciation. We investigated genetic structure and population divergence and found evidence of a mid-Pleistocene separation on either side of the eastern Great Rift Valley, consistent with vicariance caused by a rain shadow along the so-called "Kingdon's Line." However, we also found pervasive evidence of both recent and widespread historical gene flow across the Rift Valley barrier. By inferring the genome-wide landscape of variation among subspecies, we found 14 genomic regions of elevated differentiation, including a locus that may be related to each subspecies' distinctive coat pigmentation pattern. We investigated these regions as candidate speciation islands. However, we observed no significant reduction in gene flow in these regions, nor any indications of selection against hybrids. Altogether, these results suggest a pattern whereby climatically driven vicariance is the most important process driving the African antelope radiation and suggest that reproductive isolation may not set in until very late in the divergence process. This has a significant impact on taxonomic inference, as many taxa will be in a gray area of ambiguous systematic status, possibly explaining why it has been hard to achieve consensus regarding the species status of many African antelopes. Our analyses demonstrate how population genetics based on low-depth whole genome sequencing can provide new insights that can help resolve how far lineages have gone along the path to speciation.

Genomic diversity and evolution of the Hawaiian Islands endemic Kokia (Malvaceae).

Island species are highly vulnerable due to habitat destruction and their often small population sizes with reduced genetic diversity. The Hawaiian Islands constitute the most isolated archipelago on the planet, harboring many endemic species. Kokia is an endangered flowering plant genus endemic to these islands, encompassing 3 extant and 1 extinct species. Recent studies provided evidence of unexpected genetic diversity within Kokia. Here, we provide high-quality genome assemblies for all 3 extant Kokia species, including an improved genome for Kokia drynarioides. All 3 Kokia genomes contain 12 chromosomes exhibiting high synteny within and between Kokia and the sister taxon Gossypioides kirkii. Gene content analysis revealed a net loss of genes in K. cookei compared to other species, whereas the gene complement in K. drynarioides remains stable and that of Kokia kauaiensis displays a net gain. A dated phylogeny estimates the divergence time from the last common ancestor for the 3 Kokia species at ∼1.2 million years ago (mya), with the sister taxa (K. cookei + K. drynarioides) diverging ∼0.8 mya. Kokia appears to have followed a stepping-stone pattern of colonization and diversification of the Hawaiian archipelago, likely starting on low or now submerged older islands. The genetic resources provided may benefit conservation efforts of this endangered endemic genus.

The herpetofauna of the Sporades Islands (Aegean Sea, Greece): New discoveries and a review of a century of research

The Sporades are one of the most biologically important archipelagos in the Aegean Sea (Greece) and have received priority conservation over the last 50 years. However, despite numerous early efforts, its herpetofauna is only partially described, resulting in many distributional gaps that have prevented adequate understanding and management of the resident species communities. We review one century of bibliography from the Northern Sporades and combine this information with a review of museum specimens and insights from numerous extensive field surveys over the last near-decade to provide for the first time a comprehensive picture of the reptiles and amphibians of the archipelago. We report here on 26 new island records and find that the herptile communities of the region are largely derived from the herpetofauna of the nearby Thessaly mainland, with only a few introduced taxa. There is also a small but significant set of endemic taxa in the archipelago. Island species richness declines with decreasing island size and increasing duration of island isolation. Herptile communities on smaller islands are progressively nested subsets of the communities on larger islands. The presence of reptile species depends sensitively on the condition and management of native ecosystems. While non-aquatic species maintain largely healthy populations, most populations are under pressure from the combined effects of rampant tourist development, the destruction and degradation of rare wetland habitats, and the abandonment of traditional agricultural landscapes. We provide recommendations regarding sustainable management of the local reptile and amphibian populations.