We tested the hypothesis that aridification of Australia during the Pleistocene promoted the isolation and divergence of three lineages of a migratory fish. We found support for this using an integrative framework of environmental and genomic modelling.

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Above: Golden perch, Macquaria ambigua. Photo credit: Peter Unmack.

The Australian landscape has not always been so arid. In fact, if you travelled back to the early Cenozoic, in place of the central deserts you would find rainforests and lavish wetlands supporting very different ecosystems than we see today. But over millions of years, the plants and animals of Australia have had to adapt to a gradually drier climate, with heightened aridification during the glacial phases of the Pleistocene (~2.6 million to 11.7 thousand years ago) having caused major changes to the distribution and connectivity of populations. Despite having a good understanding of how this aridification has impacted terrestrial species, less is known about how it has affected the diversification of freshwater taxa.

Cover article (free-to-read for two years):
Booth, E. J., Sandoval-Castillo, J., Attard, C. R., Gilligan, D. M., Unmack, P. J. & Beheregaray, L. B. (2022). Aridification-driven evolution of a migratory fish revealed by niche modelling and coalescence simulations. Journal of Biogeography,   49,  1726– 1738. https://doi.org/10.1111/jbi.14337

In this study, we were curious to understand how historical aridification of Australia has influenced the evolution of an iconic freshwater fish, the golden perch (Macquaria ambigua). Specifically, we wanted to find out whether the divergence of three lineages of golden perch from three different river basins has been driven (or at least reinforced) by aridification. Previous research has found strong genetic differentiation between these lineages, and it has been proposed that they are actually three different ‘cryptic’ species. Clarifying this taxonomic ambiguity is important for the management and conservation of golden perch, especially since the species is regularly stocked from hatcheries into rivers and impoundments to support its recreational fishery.

We already had a ton of genome-wide data from previous work, so we thought we’d repurpose it to run some complex ‘coalescent’ models to better understand when these lineages diverged and experienced demographic changes. But what we needed first were some specific hypotheses to test. Rather than making hypotheses up out of thin air (or should I say water?), we used an environmental data set to develop contemporary and historical species distribution models. These models allowed us to predict how the amount of suitable habitat for golden perch has changed over time and theorize about how population sizes and connectivity of the lineages might have fluctuated between glacial and interglacial times.

Species distribution models for three golden perch lineages that are endemic to three major Australian drainage basins: Fitzroy (FIT), Lake Eyre (LEB), Murray–Darling (MDB). We found support for reduced habitat availability during the Last Glacial Maximum (~21 ka) compared to the present day.

What’s really cool is that our two independent data sets (environmental and genomic) found support for the same story. We discovered that during the Last Glacial Maximum (~21 thousand years ago), all three lineages had much smaller population sizes compared to the current day. Furthermore, the connectivity of suitable habitat between drainage basins was reduced at that time. This supports the idea that aridification caused the isolation (and facilitated the divergence) of the three golden perch lineages. We also found phylogenetic support for the delimitation of these lineages as separate species.

Our paper provides an exciting new insight into the diversification of freshwater taxa in Australia, and this integrative analytical framework could be applied to other study systems in the future. This research also has relevance for understanding how anthropogenic climate change might affect the connectivity of freshwater lineages, but that’s a story to explore elsewhere … 

Written by:
Emily Booth
PhD candidate, Molecular Ecology Laboratory, College of Science and Engineering, Flinders University, Adelaide, South Australia, Australia

Additional information:
Twitter: @molecolflinders; @EmilyJBooth
https://molecularecology.flinders.edu.au

Phylogeography is celebrating its 35th birthday; Geogenomics its 8th. We asked authors of papers in a recent special section of Journal of Biogeography to reflect on how these two approaches can increase our understanding of the distributions of genetic diversity.

Above: Cover for the Geogenomics virtual issue .

Biogeography is an integrative discipline, as is reflected in familiar conjunctions including bio⋅geography, phylo⋅geography, and macro⋅ecology. One recently introduced term — geo⋅genomics — represents interdisciplinary approaches using “large-scale genetic data to test or to constrain geological hypotheses” (sensu Baker et al. 2014, p.38). Geogenomics is eight years young, arguably in its foundational phase, and its relationship with other disciplines in biogeography is developing. A special section in the Journal of Biogeography — which is running for the three months (v.49.9–49.11) and being compiled into a virtual issue — reviews the origins of geogenomics and compiles a suite of new studies that reflect geogenomics’ current purview (as practiced by biogeographers). A major contention of this special section is that Geogenomics has potential to substantially change the way we combine genetics and geology to increase rigor and insight when answering biogeographic questions. Here, we provide perspectives from some of the authors of papers in the special section on what they see in the future for Geogenomics. We posed three questions to them:
– What are the predominant limitations on (or opportunities for) advances in Phylogeography?
– What do you find new and exciting about Geogenomics?
– Can Geogenomics change the future of Phylogeography? (And/or what else will be needed?)
Their answers, and links to their papers, are provided below. We hope the special section and this discussion provoke thought and stimulate advances in this rich area. We look forward to publishing more on these topics in the coming years!

Editorial: (Free to read online for a year.)
Dawson, M.N., Ribas, C.C., Dolby, G.A. and Fritz, S.C. (2022) Geogenomics: Toward synthesis. J Biogeogr. 49(9):1657–1661. https://doi.org/10.1111/jbi.14467 

The special section on Geogenomics was conceived and edited by Paul Baker (Duke University, USA), Greer Dolby (University of Alabama at Birmingham, USA), Sherilyn Fritz (University of Nebraska – Lincoln, USA), Anna Papadopoulou (University of Cyprus, Cyprus), and Camila Ribas (Instituto Nacional de Pesquisas da Amazônia, Brazil) in association with the chief editors.  

Key attributes in geogenomic thinking include the bidirectionality of process and inference, and the integrative iteration of ecological, evolutionary, and earth processes, scaling from generational to geological times. See Dawson et al. (2022) for more detail.

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Barbosa, W. E. S.,  Ferreira, M.,  Schultz, E. D.,  Luna, L. W.,  Laranjeiras, T. O.,  Aleixo, A., &  Ribas, C. C. (2022). Habitat association constrains population history in two sympatric ovenbirds along Amazonian floodplains. Journal of Biogeography,  49,  1683– 1695. https://doi.org/10.1111/jbi.14266

Molecular biology has advanced a lot in the last decade, and the development of new sequencing techniques and a decrease in sequencing costs made a lot of phylogenomic studies possible. Given the availability of different sequencing platforms and sequencing strategies, strategic planning in phylogenomic studies has become more difficult, and analysis of all these data remains at a high cost. In addition, such complex data analysis requires scientists to be trained in bioinformatics, which makes them capable of solving the problems that arise throughout the stages of data processing and analysis.

What I find most exciting about geogenomics is the possibility of comparing dates of geological or climatic events with biological data at various scales with robust data, which allows even more accurate reconstructions of Earth‘s history.

Geogeonomics can make phylogeographic studies more interdisciplinary. In phylogeographic studies, biologists use dating and geological interpretations to understand the processes governing the distribution of closely related lineages. Meanwhile, in geogenomics, geologists can use genomic data as a proxy for their earth history studies. Thus, through bioinformatics, biologists and geologists can work together to test hypotheses as well as formulate new ones and obtain increasingly robust reconstructions.

– Waleska Barbosa

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Northern chamois (Rupicapra rupicapra) inhabit steep terrain in Central Europe and are well-adapted to cold climate. They recolonized the European Alps after the glaciation. Photo credit: Flurin Leugger. 

Leugger, F.,  Broquet, T.,  Karger, D. N.,  Rioux, D.,  Buzan, E.,  Corlatti, L.,  Crestanello, B.,  Curt-Grand-Gaudin, N.,  Hauffe, H. C.,  Rolečková, B.,  Šprem, N.,  Tissot, N.,  Tissot, S.,  Valterová, R.,  Yannic, G. &  Pellissier, L.(2022).  Dispersal and habitat dynamics shape the genetic structure of the Northern chamois in the Alps. Journal of Biogeography,  49,  1848– 1861. https://doi.org/10.1111/jbi.14363

Phylogeographic studies at the very beginning were purely descriptive, linking molecular data to geography, e.g., investigating the spatial distribution of genotypes (Avise et al., 1987[1]). Early phylogeographic studies were based on mtDNA markers which included only few 100s base pairs and only describe female-related patterns (Avise 1998[2], 2009[3]). Compared to the multi-locus or more recently used whole genome analyses (see also response below), they provide limited insights into the history of the study organism. Advances in statistical approaches and modelling helped to overcome some limitations, i.e., to receive more realistic estimations or investigate additional species-environment relations (Knowles, 2009[4]) and test hypothesis of genetic diversity over time. The spatio-temporal resolution of genetic and/or geographic data is until now often a limiting factor in phylogeographic studies.

The connection of genomic (i.e., data from entire genomes and not only from a very restricted marker) data and landscape evolution at large spatial scales is exciting in the field of Geogenomics. Although initially defined as way to investigate geological patterns based on biological data (Baker et al., 2014[1]), Geogenomics is often perceived in a reciprocal illumination: geological or geographic data is used to improve our understanding of biodiversity and biological properties (Dawson et al., 2022[2]). New insights can be obtained by combining genomic with geographic data (and various models) compared to classical genetic analysis. For example, we[3] used genomic data of chamois across the European Alps in combination with several hundred landscape connectivity models over 20,000 years to test hypothesis on chamois’ dispersal which would not have been possible with distribution data or genetic data alone. Given that many researches use the reciprocal definition of Geogenomics (e.g., Barbosa et al., 2021[4]; Luna et al., 2021[5]; Ortego et al., 2022[6]), I argue that Geogenomics should be considered as part – or rather advancement – and not opposite of Phylogeography, where whole-genome data is used compared to the single or few genetic markers used during the emergence of Phylogeography. The core of Geogenomics is to test various hypothesis using genomic data in combination with both models and landscape data to gain new insights into biodiversity (e.g., migration) and landscape evolution (e.g., formation of migration barriers). Understanding how biodiversity evolved facing past environmental changes is paramount to predict and/or mitigate adverse effects of the current global changes where Geogenomics can contribute valuable information.

The core of Geogenomics is the hypothesis-driven approach using large-scale genomic data to improve our understanding of geological or biological properties. Advancements in genetic analysis and computational modelling are likely to foster Geogenomics in the next years and result in ever more hypothesis which can be addressed. Additionally, the tools of Geogenomics offer new possibilities to study biodiversity and landscape evolution at a finer scale than ever before. Genomic (whole-genome) data, i.e., SNP-based approaches, allows for more detailed analysis compared to classical phylogeographic studies, that is for example, on population level instead and over shorter time to estimate anthropogenic impacts. Using ancient DNA (aDNA) from fossils will offer new insights into Phylogeography. For example, aDNA could improve models about historic population changes and connectivity between populations.

– Flurin Leugger

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(Left) The Striped Woodcreeper (Xiphorhynchus obsoletus, Dendrocolaptidae), a bird species with wide distribution in the Amazon floodplains. Photo: João S. Barros. (Right) Amazonian floodplains of the Rio Branco, at the foot of the Serra Grande, Roraima, Brazil. Photo: Thiago O. Laranjeiras.

Luna, L. W.,  Ribas, C. C., &  Aleixo, A. (2022).  Genomic differentiation with gene flow in a widespread Amazonian floodplain-specialist bird species. Journal of Biogeography,  49,  1670– 1682. https://doi.org/10.1111/jbi.14257

Modern phylogeography still lacks conceptual and methodological definitions in incorporating multivariate data (e.g., environmental, biotic, behavioral, climatic, and geological variables) into biogeographic models. In this context, the predominant limitation is how we can incorporate information from multiple species traits and historical changes in the landscape (constrained by dated geological events) into a statistical framework that can infer the current distribution of genetic diversity of the species and communities. Another issue is that the incorporation of whole genomes into the discipline has just begun, and several new tools and approaches will have to be developed or adjusted to fully incorporate the massive amount of data provided by this type of DNA sequencing. 

Geogenomics can be seen as a step forward in phylogeography in terms of concept and methodological approach. This advance comes from defining the study design more rigorously, using geological constraints and high-throughput genomic technology to tackle biogeographic hypotheses. That is, it adds the context of geological history into the investigation of biogeographic patterns, or vice versa, using known biogeographic patterns to illuminate geological processes at both regional and intercontinental scales.

Explicitly using dated geological events within phylogeographic approaches can help refine the hypotheses being tested. For the past two decades, phylogeography relied on the description of spatial genetic patterns as a posteriori explanation of possible events that shaped the evolutionary histories of taxa. With the addition of concepts from geogenomics, phylogeography can be profoundly changed, whereby the addition of an explicit historical/geological context into sampling designs will help illuminate complex evolutionary and biogeographic patterns.

– Leilton Luna & Alexandre Aleixo

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The thermophilous grasshopper Dericorys carthagonovae. Photograph by Francisco Rodríguez.

Ortego, J.,  González-Serna, M. J.,  Noguerales, V., &  Cordero, P. J. (2022).  Genomic inferences in a thermophilous grasshopper provide insights into the biogeographic connections between northern African and southern European arid-dwelling faunas. Journal of Biogeography,  49,  1696– 1710. https://doi.org/10.1111/jbi.14267

In my opinion, an important limitation is the difficulty to integrate geological information into process-based phylogeographic inference, which is particularly challenging when ancient events are involved. Geology and phylogeography illuminate each other, but what is probably needed is more active collaboration between researchers of the two disciplines. Among others, this could help to consider more carefully uncertainty surrounding geological reconstructions when interpreting phylogeographic evidence (e.g., dating of events).

Something I find very exciting are the discrepancies between geology/geography and phylogenomic evidence, as such findings can provide important counterintuitive insights into disregarded phenomena governing species distributions and geographical diversification. For instance, mounting biogeographical evidence – including our study published in this special issue – suggests that the colonization of semiarid areas of Iberia by thermophilous organisms of African origin most likely took place from the central Maghreb region (Algeria or Tunisia). This indicates that the exchange of terrestrial organisms between Iberia and Africa did not exclusively take place across the strait of Gibraltar (i.e., through the shortest geographical distance) and suggests that long-distance overseas dispersal might be much more common than previously thought.

Understanding the processes shaping species distributions and their spatial patterns of genomic variation requires the effective integration of multiple disciplines and, as such, geogenomics will be instrumental to the advance of phylogeography. However, organismal traits (i.e., dispersal capacity, interspecific interactions, ecology, etc.) must be also considered to formulate and test phylogeographic hypotheses, as not all taxa are expected to respond in the same way to a shared abiotic background and only certain geological events (e.g., Quaternary climatic oscillations) but not others (e.g., landmass/ocean configuration) might explain their demographic and evolutionary trajectories depending on species-specific attributes. Phylogeographic studies on geophilic organisms with low dispersal capacity might particularly benefit from a “geogenomic approach” and provide insights that, in turn, could contribute to refine geological hypotheses, particularly when developed within a multi-species comparative framework.

– Joaquín Ortego

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Additional information:
Virtual issue on Geogenomics: https://onlinelibrary.wiley.com/doi/toc/10.1111/(ISSN)1365-2699.Geogenomics

REFERENCES
[1] Baker, P. A., Fritz, S. C., Dick, C. W., Eckert, A. J., Horton, B. K., Manzoni, S., … & Battisti, D. S. (2014). The emerging field of geogenomics: constraining geological problems with genetic data. Earth-Science Reviews, 135, 38-47.
[2] Dawson, M.N., Ribas, C.C., Dolby, G.A. and Fritz, S.C. (2022), Geogenomics: Toward synthesis. Journal of Biogeography, 49, 1657-1661.
[3] Leugger, F., Broquet, T., Karger, D. N., Rioux, D., Buzan, E., Corlatti, L., … & Pellissier, L. (2022). Dispersal and habitat dynamics shape the genetic structure of the Northern chamois in the Alps. Journal of Biogeography 49,  1848– 1861. https://doi.org/10.1111/jbi.14363
[4] Elizangela dos Santos Barbosa, W., Ferreira, M., de Deus Schultz, E., Willians Luna, L., Orsi Laranjeiras, T., Aleixo, A., & Cherem Ribas, C. Habitat association constrains population history in two sympatric ovenbirds along Amazonian floodplains. Journal of Biogeography, 49, 1683-1695.
[5] Luna, L. W., Ribas, C. C., & Aleixo, A. (2021). Genomic differentiation with gene flow in a widespread Amazonian floodplain‐specialist bird species. Journal of Biogeography, 49, 1670-1682.
[6] Ortego, J., González‐Serna, M. J., Noguerales, V., & Cordero, P. J. (2022). Genomic inferences in a thermophilous grasshopper provide insights into the biogeographic connections between northern African and southern European arid‐dwelling faunas. Journal of Biogeography, 49, 1696-1710.

Differences in dispersal abilities and habitat specialization determine the postglacial range expansion of three high-elevation plants

Above: Steep limestone cliffs in the Pre-Pyrenees, a glacial refugium for the study species.

When I first visited the Pyrenees as a child from the Mediterranean lowlands, I got fascinated by the accordion-like folded landscape, the green and dense meadows, and the amount of unknown colorful flowers which painted the entire picture. In the coming years, I began to understand what processes and conditions produced that landscape and vegetation, and the original fascination slowly turned into questions. Inspired by classical and more recent literature about glacial refugia for plants from the European Alps, the question started to burn inside me: what about the Pyrenees? What happened in these mountains that create an almost perfect border between the Mediterranean and the template climate? As claimed in some classical studies, did mountain plants in the Pyrenees have more possibilities of surviving the cold stages than their counterparts in the Alps? And looking in the opposite temporal direction, what possibilities do they have to cope with climate change in the near future? To date, the lack of a critical mass of molecular data for Pyrenean mountain plants does not allow us to answer these questions, and that is the reason why we started the project in which our publication is framed.

Cover image article: (Free to read online for a year.)
Carnicero, P., Wessely, J., Moser, D., Font, X., Dullinger, S. & Schönswetter, P. (2022). Postglacial range expansion of high-elevation plants is restricted by dispersal ability and habitat specialization. Journal of Biogeography, 49,  1739– 1752. https://doi.org/10.1111/jbi. 

In this project entitled Conserving intraspecific diversity in a warmer world, we aimed at tackling the above mentioned questions and to understand the impact of past and current changes in the diversity within species. Nine species endemic to the Pyrenees were selected for this purpose, and in this first publication of the project, we present the results for three species with preference for carbonate bedrock. In the general trend of searching for individual examples, which could be a proxy for a wider group of ecologically similar species, I expected these three species to be good models to explain what happened to other species with similar bedrock preference. Nothing further from the truth. Our results showed two different responses to the end of the cold stage: one expanded rapidly, while the other two showed postglacial stasis; they remained in the areas where they survived the cold stages, with only a few colonizations of areas previously under the ice. Why?

When we were seeking an answer to this incongruent pattern, we went back to basic questions that a biologist should never forget: where and how do our study species live, disperse, reproduce, etc.? And these indeed brought us to a reasonable explanation. Salix pyrenaica, the species, which rapidly expanded and colonized previously glaciated areas, is a very good disperser and often able to find a place to grow provided a carbonate bedrock. In a way, we can call Salix pyrenaica a generalist of carbonate bedrocks. This ecological trait allowed it to rapidly and efficiently explore the newly available terrain after glaciation, by dispersing many fruits, and a considerable amount of them finding a proper spot to grow. Instead, Cirsium glabrum and Silene borderei showed a much more static picture. The areas where, according to our genetic and climatic data, they survived the cold stages are almost a perfect match to the areas where they occur nowadays. In contrast, they barely occur in potentially suitable areas that were under the glaciers during the ice ages. Silene borderei has no particular adaptations for long-distance dispersal, but Cirsium glabrum does. While it is true that Silene borderei indeed showed a more static postglacial history than Cirsium glabrum, something still has to explain why both expanded so little after the glaciations as compared to Salix pyrenaica. In contrast to the latter, Cirsium glabrum and Silene borderei are microsite-specialists in terms of habitat selection, which imposes a severe drawback in terms of colonization of new areas: even if dispersing seeds efficiently, only a very small portion of them will find a suitable habitat to grow, rendering the colonization of new areas an extremely slow process.

Schematic representing the impact of habitat specialization in range expansion. In the left panel, a generalist like Salix pyrenaica showing multiple successful dispersal events. In the right panel, for a micro-site specialist like Cirsium glabrum, most of the dispersal events will not succeed.

How close do these results bring us to inferring where the glacial refugia for high-elevation species in the Pyrenees actually were? Despite the uncertainties conferred by the above mentioned results, we are in the position of formulating some statements and, of course, new questions. On the one hand, Salix pyrenaica shows the expected behavior of contraction-expansion of glacial refugia: survival of a harsh period in refugia, and posterior expansion from there. Unfortunately, this expansion seems to have been so fast and involved a high degree of exchange of genes between expanding populations, that the signal for identifying the refugia in detail is too weak. In this case, we can state that there were at least two refugia in the east and the west. Instead, for Cirsium glabrum and Silene borderei we can clearly state that the eastern and western Pre-Pyrenees played an important role for the survival of the species during the ice age. The Pre-Pyrenees have already been suggested as a main refugium for species with carbonate bedrock preference, and our results confirm this.

In conclusion, our results show that species-specific ecological traits play a major role in the recent history of the studied species. Thus, for species like Cirsium glabrum and Silene borderei, refugia remained their permanentdistributions, while others, like Salix pyrenaica, started remarkable range expansions from the refugia. Ecological traits therefore strongly impact which species have narrow or widespread distributions. Future molecular data coming from our and other projects will provide further evidence to confirm, reject or reformulate the general applicability of the hypotheses formulated here.

Written by:
Pau Carnicero, PhD, Department of Botany, University of Innsbruck

Additional information:
@CarniceroPau
https://www.uibk.ac.at/botany/staff/scientific_staff/carnicero-campmany_pau.html.en

Acknowledgements
I would like to thank Peter Schönswetter for his relevant feedback on this blog post.

Danish islands help to disentangle how plant dispersal characteristics shape species richness patterns.

Above: The Danish coastline with the island Hjelm in the background. © Anders Sanchez Barfod.

Suppose you hear the names Galapagos, Hawaiian or Canary Islands. In that case, I am sure you have a picture in mind right away. These islands are well known in general and very popular in island biogeography. We have seen pictures and documentaries or read evolutionary and ecological studies about these islands. But have you every head of islands like Anholt, Egholm, Fur or Hjelm? These are islands scattered along the Danish coast, and I would like to convince you that such islands have great potential to help us solve pending questions in island biogeography.

Editors’ Choice article: (Free to read online for two years.)
Walentowitz, A., Troiano, C., Christiansen, J. B., Steinbauer, M. J., & Barfod, A. S. (2022). Plant dispersal characteristics shape the relationship of diversity with area and isolation. Journal of Biogeography, 49, 1599–1608. https://doi.org/10.1111/jbi.14454

Denmark has more than 1400 immensely diverse islands. Some are home to Danes while others are uninhabited; they are located in the North Sea, Baltic Sea or in bights and fjords and differ in size. My co-authors Prof. Dr Manuel Steinbauer (University of Bayreuth) and Assoc. Prof. Dr Anders Sanchez Barfod (Aarhus University) were intrigued by these islands and their flora. They raised the question: can the way plants disperse to islands help explain the species richness patterns on these islands?

Indeed, they can. For example, a plant like the common oak tree (Quercus robur) has heavy acorns dispersed by mammals or birds (zoochore dispersal). The chances are high for an acorn to reach and get established on a larger island. The transporting animal was looking for a spot with enough resources for survival and therefore decided to head for a larger island. For plants dispersed by water (hydrochore dispersal) like the common cordgrass (Spartina anglica), chances are high that these are adapted to habitats associated with coastal areas. Now simple mathematics can help to explain why larger islands have proportionally fewer hydrochore plant species than smaller islands: If the coastline doubles, island area roughly quadruples.

It was most astonishing to unveil such relationships between plant dispersal and island characteristics despite the human impact on plant communities on Danish islands for centuries. These patterns seem so fundamental and robust that anthropogenic encroachment could not overwrite them. However, if we additionally include human impact in our models, we can explain plant species diversity on islands even better.

While working with Danish Islands, I was highly impressed by two former botanists and scientists, who massively shaped this study, although I never had the chance to meet them. Eric Wessberg was a Danish botanist who inventoried numerous islands jointly with his team. His plant records built the baseline of our study. Such data are essential for the studies we conduct as island biogeographers. I was also profoundly impressed by the theories and thoughts developed by Alvar Palmgren, a Finnish botanist who lived from the late 19^th until the mid-20^th century. Back in his day, he developed first thoughts on how plant dispersal characteristics shape richness patterns on islands long before the discipline of island biogeography was established. Alvar Palmgren seems to suffer a bit from what I call the “Alfred Russel Wallace syndrome”, as his genius work is little remembered. Alfred Russel Wallace developed theories about evolution at the same time as Charles Darwin. Still, his counterpart is better known to be the founder of evolutionary biology. With our study, we hope to not only put Danish islands onto the map of island biogeographical research but also value the work of Eric Wessberg and Alvar Palmgren.

I learned from this study that numerous islands are out there whose potential for island biogeography has not been explored yet. We should continue to be on the lookout for such islands that help answer questions on how island biodiversity is being shaped by natural and anthropogenic forces.

Written by:
Anna Walentowitz, M.Sc., PhD candidate
University of Bayreuth
Department of Biogeography
Universitaetsstrasse 30 95447 Bayreuth

Additional information:
Twitter: https://twitter.com/ArchipelagoAnna

The central role of the Amazon River in the evolution of Western Atlantic reef fishes.

Above: Amazon River mouth, where the plume of freshwater and sediment reaches the Atlantic Ocean. Photo by Coordenação-Geral de Observação da Terra/INPE.

The magnitude of the Amazon River, by far the largest river of the world, can be illustrated by its mean annual water discharge – 2.5 times greater than the combined discharges of the world’s second and third largest rivers (Congo and Orinoco rivers, respectively). Actually, the 300 million liters released by the Amazon River every second in the northern portion of South America represent about 20% of the freshwater input of all rivers of the world into the ocean. The amount of freshwater and sediments is so impressive that the physicochemical properties of a huge area of the Atlantic are modified, forming the most important biogeographic filter responsible for the faunal discontinuity between the Caribbean and Brazilian Provinces – the Amazon-Orinoco plume, which is an extensive layer of fresh muddy water about 30 meters deep that extends for about 200 km into the sea.

Editors’ choice article: (Free to read online for two years.)
Araujo, G. S., Rocha, L. A., Lastrucci, N. S., Luiz, O. J., Di Dario, F., & Floeter, S. R. (2022). The Amazon-Orinoco Barrier as a driver of reef-fish speciation in the Western Atlantic through time. Journal of Biogeography, 49, 1407-1419. https://doi.org/10.1111/jbi.14398 

The effectiveness of the Amazon-Orinoco plume as a biogeographic barrier varied over time according to sea-level height: during high sea-level stands, like in modern times, the less dense freshwater plume floats high above salt water on the continental shelf, allowing marine organisms to use the relatively extensive “normal” marine layer below the plume as a corridor between the Caribbean and Brazilian provinces. However, during low sea-level periods in geological time, the continental shelf is shallower, and as a consequence this corridor is practically closed by the huge freshwater plume. It has been proposed that adult body size of fishes is important for the ability to cross the Amazon-Orinoco barrier today. However, the Amazon River has a very long and complex history, and just about 9.4 million years ago the amount of freshwater and sediment discharge in the Atlantic was much lower (about 24 times) than what we see today.

With that in mind, the question we addressed in this paper is: considering the evolution of the Amazon River itself and the dramatic increase in freshwater and sediment discharge throughout its history, what is the influence of the Amazon-Orinoco Barrier on the diversification of the Western Atlantic reef fish fauna over time?

Our results revealed a complex and dynamic barrier with temporally variable permeability. Almost all the cladogenetic events addressed in our meta-analysis that resulted in sister species or populations of reef fishes in the Caribbean and Brazilian Provinces can be associated with the emergence and development of the Amazon-Orinoco barrier. This panorama suggests that the Amazon River was likely the main promoter of diversification between the reef fish fauna of the Caribbean and Brazil. In other words, the main possible driver of speciation of marine species in the Western Atlantic was a river.

The results indicate that the maximum total length of ancestral populations of reef fishes was also a decisive feature in the ability to overcome the Amazon-Orinoco Barrier in the past, as it is nowadays. We observed a strong correlation between body size and dates of cladogenesis, with smaller body sizes primarily associated with older cladogenetic events. Older speciation events appear to be probably more associated with the onset and early development of the plume itself rather than with fluctuations in sea level, since only milder variations of sea level occurred between the Middle Miocene and early Pleistocene.

Examples of sister fish species found in Caribbean reefs (left bar) and Brazilian (right bar): first line: saddled blenny Malacoctenus triangulatus and rusty scaly blenny Malacoctenus zaluari, second: rainbow parrotfish Scarus guacamaia and greenbeak parrotfish Scarus trispinosus, third: redfin parrotfish Sparisoma rubripinne and gray parrotfish Sparisoma axillare. Photos by F. Krasovec, J.P. Krajewski, J. Lyle, C. Sampaio, and S.R. Floeter.

In the last 2.4 million years, the plume has become a more formidable barrier. More than half of the cladogenetic events detected in our study occurred only in this time-period, which is characterized by higher sedimentation rates and greater sea-level fluctuations. Of the 31 cladogenetic events recorded in the last 2.4 million years, 13 involved species larger than 50 cm, in contrast to only two events with species of similar sizes in the previous 7 million years. It seems therefore that these two factors together, higher sedimentation rates and greater sea-level fluctuations, fostered the isolation capacity of the Amazon-Orinoco plume, to the point that now if affects populations of larger fishes by restricting their dispersal ability, and therefore contributing to the differentiation of populations on both sides of the barrier.

Our data also revealed a substantial number of possible cryptic species still nor formally recognized as such. Surprisingly, 20% of the species addressed in the study that are reported to occur on both sides of the Amazon-Orinoco barrier have populations that are genetically distinct at the species level. We can therefore conjecture that there is still a surprisingly large hidden diversity of reef fishes awaiting formal recognition in the Caribbean and Brazilian provinces, despite recent advances in the taxonomy of western Atlantic fishes in general.

The importance of the Amazon-Orinoco plume as a biogeographic barrier began to be noticed in the 1970’s. Nearly half a century of research has shed light on the significant influence of the Amazon River on the marine environment. Despite recent advances, there is still a considerable lack of data on the distribution, taxonomy, and other basic aspects of the biology of Western Atlantic reef fish communities, especially in the Brazilian Province. The uniqueness of the Amazon plume region goes far beyond its role as a biogeographic barrier and main driver of diversity in the Western Atlantic over the last 9.4 million years. This region has a magnificent mesophotic reef system adapted to life under the influence of the plume, the Great Amazon Reef System. Today, we know only about 5% of this outstanding ecosystem, which is seriously threatened by the prospection and eventual exploitation of oil. Therefore, we draw attention to the urgent need for broad baseline studies to better know and preserve this fascinating and unique region of the world’s ocean.

Written by:
Gabriel Soares Araujo (1) & Fabio Di Dario (2)
1. PhD candidate, Programa de Pós-Graduação em Ciências Ambientais e Conservação, Universidade Federal do Rio de Janeiro, Instituto de Biodiversidade e Sustentabilidade – NUPEM/UFRJ, Macaé, Rio de Janeiro, Brazil.
2. Associate Professor, Universidade Federal do Rio de Janeiro, Instituto de Biodiversidade e Sustentabilidade – NUPEM/UFRJ, Macaé, Rio de Janeiro, Brazil

Additional information:
@SergioFloeter
@CoralReefFish