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1. Effect of topographic complexity on species richness in the Galápagos Islands

   https://doi.org/10.1111/jbi.14230  

   Roell, Yannik E. ; Phillips, John G. ; Parent, Christine E. ( August 2021 , Journal of Biogeography)

   The accumulation of species through time has been proposed to have a hump‐shaped relationship on volcanic islands (highest species richness during intermediate stages of an island's lifespan). Change in topographic complexity (TC) of islands over time is assumed to follow the same relationship. However, TC can be measured in different ways and may not have the same impact across taxonomic groups. Here, we quantify TC across the Galápagos Islands and test the assumption that TC follows a predictable trajectory with island age. Subsequently, we ask whether including TC improves statistical models seeking to explain variation in species richness across islands.

   Galápagos Archipelago, Ecuador.

   Native and endemic terrestrial animals and plants.

   For each island, we generated eight TC indices from a 30‐m resolution digital elevation model. We tested for a relationship between each index and island age, and whether it significantly contributes to observed variation in species richness, using 11 different models for 12 taxonomic groups across the Galápagos Islands.

   Four TC indices were significantly negatively correlated with either island age or ontogenetic age and only one index followed the hump‐shaped relationship with age. No index consistently contributed to the variation in species richness for all taxonomic groups. However, for all 12 taxonomic groups, incorporating at least one TC index in modelling species richness improved one or more models. The most common TC index improving models was standard deviation of slope, although each index improved at least five models across all taxa. Different factors predicted taxon‐specific richness, and habitat diversity was significant for all taxa.

   Topographic complexity is an important component influencing species richness, but its impact likely differs among taxonomic groups and different scales. Therefore, future studies should incorporate broad, multi‐dimensional measures of TC to understand the biological importance of TC.

    

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2. Richness and resilience in the Pacific: DNA metabarcoding enables parallelized evaluation of biogeographic patterns

   https://doi.org/10.1111/mec.16575  

   Kennedy, Susan ; Calaor, Jerilyn ; Zurápiti, Yazmín ; Hans, Julian ; Yoshimura, Masashi ; Choo, Juanita ; Andersen, Jeremy C. ; Callaghan, Jackson ; Roderick, George K. ; Krehenwinkel, Henrik ; et al ( December 2023 , Molecular Ecology)

   Islands make up a large proportion of Earth's biodiversity, yet are also some of the most sensitive systems to environmental perturbation. Biogeographic theory predicts that geologic age, area, and isolation typically drive islands' diversity patterns, and thus potentially impact non‐native spread and community homogenization across island systems. One limitation in testing such predictions has been the difficulty of performing comprehensive inventories of island biotas and distinguishing native from introduced taxa. Here, we use DNA metabarcoding and statistical modelling as a high throughput method to survey community‐wide arthropod richness, the proportion of native and non‐native species, and the incursion of non‐natives into primary habitats on three archipelagos in the Pacific – the Ryukyus, the Marianas and Hawaii – which vary in age, isolation and area. Diversity patterns largely match expectations based on island biogeography theory, with the oldest and most geographically connected archipelago, the Ryukyus, showing the highest taxonomic richness and lowest proportion of introduced species. Moreover, we find evidence that forest habitats are more resilient to incursions of non‐natives in the Ryukyus than in the less taxonomically rich archipelagos. Surprisingly, we do not find evidence for biotic homogenization across these three archipelagos: the assemblage of non‐native species on each island is highly distinct. Our study demonstrates the potential of DNA metabarcoding to facilitate rapid estimation of biogeographic patterns, the spread of non‐native species, and the resilience of ecosystems.

    

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3. Phylogeography of the highly dispersible landscape‐dominant woody species complex, Metrosideros , in Hawaii

   https://doi.org/10.1111/jbi.13650  

   Stacy, Elizabeth_Ann ; Sakishima, Tomoko ( June 2019 , Journal of Biogeography)

   Little is known about how diversification occurs within long‐lived, highly dispersible and continuously distributed groups. We examined the distribution of genetic variation within the woody genusMetrosiderosacross the Hawaiian Islands for insights into how diversification occurs within this animal‐pollinated, wind‐dispersed group. Among Hawaiian plants,Metrosiderosis unique in its formation of continuous stands within islands that span a remarkable range of environments and comprise numerous predominantly single‐island taxa.

   Hawaiian Islands.

   Metrosideros.

   We performed population genetic analyses of variation at nine nuclear microsatellite loci from 1,486 adults of 23Metrosiderosmorphotypes sampled from five main Hawaiian Islands plus additional Pacific Island populations.

   American Samoa and Tahiti populations clustered most closely with the older islands. Results also revealed isolation by distance across the archipelago, clustering of populations predominantly by island, and evidence of multiple colonizations or back‐colonizations of three islands. The number of genetic clusters peaked on islands of intermediate age, coincident with peak morphotype richness. All islands comprised a broad range of genetic distances among taxa with the greatest overall genetic distance observed on Oahu. The two taxa that are distributed broadly across the archipelago were weakly but significantly differentiated only on volcanically active Hawaii Island, where they partition early‐ and late‐successional environments. One of these taxa was positioned centrally both within individual‐island splitstree networks and across the archipelago‐wide network.

   Distance‐dependent gene flow contributes to isolation ofMetrosiderosacross islands, especially on terminal islands. Morphological diversity likely accumulates rapidly within this group, likely associated with differential adaptation across heterogeneous environments, but isolation of gene pools through speciation within continuousMetrosiderosstands likely requires persistent disruptive selection where environments are stable for long periods. The generalist, wet‐forestM.polymorphavar.glaberrimamay play a central role in the generation of the group's many, largely island‐endemic, taxa.

    

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4. The importance of historical ecology for interpreting evolutionary processes in plants of oceanic islands

   https://doi.org/10.1111/jse.12673  

   Stuessy, Tod F. ( September 2020 , Journal of Systematics and Evolution)

   Oceanic islands and archipelagos are natural laboratories for investigating patterns and processes of evolution. Islands change with the course of time, resulting in a dynamic ontogeny over millions of years. The combined forces of tectonic plate subsidence and erosion from waves, wind, and rainwater bring about substantial geomorphological change over millions of years, until islands eventually disappear under the sea. Added to these long‐term natural changes to the environment of the islands are the changes caused by human activities in recent centuries. After humans reach a previously unpopulated island, they utilize the natural resources for their own survival, cutting forests for making houses, boats, and firewood. The size of the human population and the length of time on the island determine the degree of environmental impact. Evolutionary processes in plants of oceanic islands take place during ontogeny of the islands, resulting in population divergence, speciation, and hybridization. Due to the dramatic alterations suffered by many islands over millions of years, the present patterns of distribution and ecology of species within endemic groups may have little to do with the patterns when the species originated. Understanding these environmental changes is fundamental to infer a founder effect, reasons for levels of genetic variation within and among populations, and modes of speciation. Special caution must be exercised while making comparisons between groups located on islands of different geological ages and that have endured differing environmental modifications from humans. Examples are provided from the Juan Fernández Archipelago and Lord Howe Island.

    

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5. The evolution of trait variance creates a tension between species diversity and functional diversity

   https://doi.org/10.1038/s41467-022-30090-4  

   Barabás, György ; Parent, Christine ; Kraemer, Andrew ; Van de Perre, Frederik ; De Laender, Frederik ( December 2022 , Nature Communications)

   Abstract It seems intuitively obvious that species diversity promotes functional diversity: communities with more plant species imply more varied plant leaf chemistry, more species of crops provide more kinds of food, etc. Recent literature has nuanced this view, showing how the relationship between the two can be modulated along latitudinal or environmental gradients. Here we show that even without such effects, the evolution of functional trait variance can erase or even reverse the expected positive relationship between species- and functional diversity. We present theory showing that trait-based eco-evolutionary processes force species to evolve narrower trait breadths in more tightly packed, species-rich communities, in their effort to avoid competition with neighboring species. This effect is so strong that it leads to an overall reduction in trait space coverage whenever a new species establishes. Empirical data from land snail communities on the Galápagos Islands are consistent with this claim. The finding that the relationship between species- and functional diversity can be negative implies that trait data from species-poor communities may misjudge functional diversity in species-rich ones, and vice versa. 

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