Susanna Bryceson has recently completed her PhD at La Trobe University, Melbourne. She is an ecologist primarily researching the migration of C4 grasses to Australia and their effect on the resident flora and fauna with which they had not co-evolved. Here, Susanna shares her experience and exciting findings.

Susanna Bryceson at savanna fire research site near Darwin, Northern Territory Australia.

Institute. La Trobe University, Australia.

Academic life stage. Recent PhD completion.

Major research themes. I’m interested in the interface of Australian ancient plants and animals, with taxa immigrating into the country (particularly through Southeast Asia) over the past few million years, and how interactions between them—and potentially with humans—shaped what we see in Australia today.

Current study system. My study system is the Australian continent and my focal species are the grasses, particularly those using C4-type photosynthesis. In a global context, the Australian flora and fauna stand alone, having evolved in isolation over tens of millions of years. In Gondwana, the Australian landmass was at the ‘end of the road’ in the east, and today it has a similar position in relation to the main source of immigrant taxa, Southeast Asia.

Recent JBI paper. Bryceson, S. R., Hemming, K. T., Duncan, R. P., & Morgan, J. W. (2023). The contemporary distribution of grasses in Australia: A process of immigration, dispersal and shifting dominance. Journal of Biogeography, 50(9): 1639-1652. https://doi.org/10.1111/jbi.14676

Motivation behind this paper. Today, grass-driven fire is a prominent feature in many parts of Australia. My recent research found that Australian ecosystems evolved largely free of grasses until the past few million years, unlike on other continents which had massing herds of grazing animals co-evolving with grasses over tens of millions of years, forming complex savanna ecosystems of plants and animals. With Peter Linder’s words about Africa ringing in my ears, that ‘C4 grasses likely created an orgy of extinction’, my mission was to work out how C4 grasses that originated on other continents could become so prominent in Australia. I started thinking about the consequences of immigrant grasses entering a woodland, where they would be bulkier and taller than the existing ground cover. For example, a fire in a tall, bulky grass would potentially affect more of the neighbouring vegetation than a flaming small plant with fine leaves. I wanted to find out where these grasses speciated and spread, particularly the tall species.

Key methodologies. Earlier phylogenetic research showed that not all grasses were the same and that C4 grasses inherited the morphology of their C3 ancestors (Erika Edwards et al, 2010). To get a picture of the progress of grasses in Australia, we combined morphology (height), phylogenetic lineage, photosynthetic type, endemism and migration period. The grass species were categorised as endemic or shared (with other continents) and using environmental models and the combined analysis enabled us to track their distribution across time. We gathered data on the heights of all Australian grass species and linked this to the distribution of C3 and C4 lineages over about 30 million years.

Susanna Bryceson in tall C4 grass covering a previously clear track through savanna country, Northern Australia.

Unexpected challenges. One of our aims was to create a map showing grass heights across Australia. We thought that by combining location data for each species with their height data, that would be straightforward, but the maps we produced were baffling and didn’t represent what we had observed in the field. We realised that the problem arose with the height data: in the various floras we consulted, the height of a species was often expressed as a range, e.g. 50-80 cm. We applied the tallest height to all occurrences of that species across the country but what this could not account for was that generally, the height of C4 grasses in the cooler south was at the very low end of the catalogued range. Without more detailed adjustments to represent local grass heights, producing a map that made sense was impossible.

We were also stunned by the way the endemic and shared species of the different phylogenetic types were distributed and how, when informed by the environmental modelling, they could describe a picture of dispersal (or not!) through the dry interior of the country, when most of the ancient plants and animals were retreating to more mesic regions. Matched up with time of arrival in the country, we could also see patterns in which some grass types had spread before the aridification in the inland but had retreated into a disjunct and relictual pattern, another type had been favoured by aridification and was widespread inland, and a further group showing that the recent arrivals had barely spread from the north coast.

Major results. This paper lowers its focus to the ground level of vegetation. Grasses are such a ubiquitous element of many ecotypes that they are easily overlooked in favour of larger or more showy plant forms—trees, shrubs and flowering curios—when describing ecosystem patterns. From an ecological perspective, the tendency of grasses to create dry fuel brings the potential of fire into the ecosystems they infiltrate, making them potent factors in landscape function and, potentially, transformation. This paper shows the transformative power of grass as an evolutionary force, a migrator, an invader, and an opportunist. Over 20 million years, grasses gradually moved into Australian ecosystems, with that migration becoming a surge as the continent drew closer to Southeast Asia, culminating in the arrival of Andropogoneae grasses in the far north where their presence or absence dictates the fire regimes of the northern savannas today.

Tall grasses of the Asian-origin Androgooneae tribe have transformed landscapes across Australia. This species, Themeda triandra Forssk, is now distributed across the country to far southeast and southwestern regions.

Next steps for this research. Some of the world’s most flammable grasses have their origins in Asia, dominating the savannas which ranged through to Sunda (the former Southeast Asian landmass). But it wasn’t just the grasses that migrated, it was a system: many trees and shrubs from ancient Sundanese savannas are also present in northern Australia and as such, they are adapted to more frequent fire. I’ll be looking in more detail at Australia’s savannas and analysing the distributions of plants of different origins (either ancient Australian or Sundanese) in relation to fire frequency.

If you could study any organism on Earth, what would it be? I am intrigued by the Eriachne genus, the only Australian-origin C4 grass. This speciose, finely-statured group seems to have a low-key role in ecosystems, being able to grow in harsh sun as well as in shady and long unburned places. Where it is the dominant grass, it probably means that fire is infrequent.

Species of Eriachneae R. Br., Australia’s only local-origin C4 grass tribe, are commonly the only grass found in rocky areas and fire refuges.

Is there anything else you would like to tell us about yourself or your featured research? This paper was five years in the making. I met my collaborator, Kyle Hemming, at a conference, surprised that he was also looking at grass distribution and our very different approaches turned out to be a great match. We kept adding new dimensions to the analysis but as this expanded, I realised a ‘prequel’ was needed, spawning my paper that described Australasia’s grasses in a global context (Bryceson & Morgan, 2022). Once the global picture was clarified, work on the Australian distributions paper resumed with many further iterations, thanks to Kyle’s endless patience. It was intriguing to see the patterns being revealed.

Nahla Lucchini is a PhD candidate at the Centro de Investigação em Biodiversidade e Recursos Genéticos (CIBIO) in Portugal. She is an evolutionary ecologist with a special focus on the biogeographical dynamics of reptiles. Here, Nahla shares her recent work on climatic adaptation and diversification in European vipers.

Nahla sampling vipers in Morocco

Personal links. Twitter | ResearchGate

Institute. CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, InBIO Laboratório Associado, Universidade do Porto, Vairão, Portugal

Academic life stage. PhD Candidate

Major research themes. Evolutionary ecology; Biogeography; Herpetology

Current study system. For my PhD, I am working with European Vipers (genus Vipera) – a monophyletic group of small- to medium-sized viviparous, venomous snakes. The genus includes three main parapatric clades and 27 lineages that diversified around the Mediterranean Basin and adjacent areas, some of which exhibit cold-adapted physiology, while others are considered warm-adapted. Being ectotherms, they have a low dispersal ability compared to other vertebrates, which makes them highly vulnerable to climatic changes. All these traits make this group of vipers a unique system to study diversification dynamics in relation to niche evolution, particularly in one of the world’s biodiversity hotspots, the Mediterranean Basin.

Recent JBI paper. Lucchini, N., Kaliontzopoulou, A., Lourdais, O., & Martínez-Freiría, F. (2023). Climatic adaptation explains responses to Pleistocene oscillations and diversification in European vipers. Journal of Biogeography, 00, 1–14. https://doi.org/10.1111/jbi.14694

Motivation behind this paper. Understanding how ecological factors promote species diversification is still a hot topic in ecology and evolutionary biology. In fact, the role of niche divergence and niche conservatism during speciation is not a minor matter, as it can impact species persistence and the colonization of new geographical areas. In the case of European vipers, although there are some regional studies on selected species addressing the evolutionary history or the role of climate during their diversification, no comprehensive study has yet investigated the eco-evolutionary dynamics of the whole genus Vipera. In this recent paper, we delimited the climatic requirements of each lineage to understand their responses to Pleistocene climatic oscillations and to identify the evolutionary mechanisms underlying their diversification.

Key methodologies. One major strength of our work was the combination of various advanced techniques, such as Ecological Niche Modeling, phylogenetic comparative methods, and niche overlap analyses. These techniques allowed us to investigate, for the first time, how climatic niche evolution or conservatism shaped the diversity of European vipers. By employing this integrative, multifaceted approach, we were able to demonstrate that climatic adaptation acted as a primary driver of diversification in this group, resulting in a complex pattern of niche divergence along the viper’s phylogeny.

Vipera latastei, Spain.

Unexpected challenges. Possibly the main challenge was compiling a representative set of occurrence data for each individual lineage, including the lesser-known ones, for which distribution records were not easy to find at all. During the process of data compilation, I ended up mining several papers written in different languages and alphabets that were not familiar to me, such as many papers in Cyrillic. That was definitely a struggle! However, validating the information found in the maps helped me learn about many different places across the globe.

Major results. We found that climatic adaptation was a key element driving diversification among European vipers. Cold-adapted and warm-adapted lineages presented similar climatic requirements, based on their geographical origins. Among the three main Vipera clades, lineages belonging to the Pelias clade can be considered cold-adapted, while those within Vipera 1 and Vipera 2 clades are generally warm-adapted. However, closely related lineages responded differently to the Pleistocene climatic oscillations, suggesting that other factors have defined their recent range dynamics. Additionally, the phylogenetic signal analysis suggested adaptation to some environmental conditions. All these results together suggest an intricate pattern of niche divergence along the phylogeny that favors a scenario of local adaptation rather than Phylogenetic Niche Conservatism.

Vipera monticola, Morocco

Next steps for this research. I’m currently working on the integration of complementary methodologies, such as the application of mechanistic modelling considering the eco-physiological tolerances of the different species, to corroborate (or not) the climatic specialization patterns that we detected in this work and develop an in-depth investigation of range dynamics.

If you could study any organism on Earth, what would it be? I’m definitely happy with reptiles and snakes specifically, but I’ve also been particularly fascinated by tropical habitats, their complexity and the abundance of intriguing organisms to explore. Mixing the two would be amazing! Moreover, the challenges posed by habitat loss, climate change and human interaction make the study of tropical reptiles extremely urgent. By comprehending their biology and ecology, we can better inform conservation efforts and ensure the preservation of these remarkable creatures and the delicate ecosystems they call home.

Vipera seoanei, Spain.

Ricardo Gaytán-Legaria is a PhD candidate at the UNAM – National Autonomous University of Mexico. He is broadly interested in the biogeography and evolution of different Mexican taxa, with emphasis in oaks species and other plants. Here, Ricardo shares his recent work on the phylogeography and niche breadh of Mexican oaks.

Ricardo Gaytán-Legaria during field work in a tropical forest.

Personal links. Researchgate | GoogleScholar

Institute. UNAM (National Autonomous University of Mexico).

Academic life stage. PhD candidate.

Major research themes. Biogeography; evolution; molecular ecology; biological conservation.

Current study system. The mountains of Mexico harbour the highest number of oak (Quercus) species of the American continent, which coexist under a wide variety of climatic conditions. Our work assesses the role of ecological niche in shaping their evolutionary patterns, in order to understand the mechanisms that make possible the coexistence of many oak species, as well as the processes that drove the group’s diversification and their phylogeographic implications.

Recent JBI paper. Gaytán-Legaria, R., Oyama, K., Ruiz- Sánchez, E., & González- Rodríguez, A. (2023). The role of niche breadth in oak phylogeography: Quercus glaucoides as a study case. Journal of Biogeography, 50(9), 1653–1667. https://doi.org/10.1111/jbi.14612

Motivation behind this paper. Although several phylogeographical studies have been carried out in Mexican oaks, the eco-evolutionary processes that drive the distribution of genetic diversity in species exhibiting particular climatic affinities is still unclear. Accordingly, we explored and compared the spatial patterns of genetic diversity across species, in order to understand the role of geographical configuration, climatic changes, and adaptation to high temperatures and dry conditions in their diversification.

Key methodologies. As a baseline study system, we chose an oak species (Quercus glaucoides) occurring in unusual climatic conditions, i.e. high temperatures and a long dry season, for which we explored phylogeographic patterns. We then compared the distribution of the oak’s genetic diversity with other species that occur in temperate forests, in the Mexican highlands, associating the niche breadth of the species with the degree of population genetic differentiation and the amount of genetic diversity.

Mature tree of Quercus glaucoides from the transition zone to tropical dry forest in Teotitlan, Oaxaca, Mexico.

Unexpected challenges. At first, we expected a different phytogeographical pattern in this oak (Q. glaucoides) in comparison with oak species from temperate forests, but while analysing the data, we found a previously scarcely explored association between niche breadth and phylogeographic patterns. This made us realize about the need to focus on the importance of niche breadth in explaining the spatial patterns of genetic diversity.

Major results. We found that the niche breadth of Mexican oaks is negatively correlated with their degree of genetic differentiation. This suggests that species occurring in a wider variety of environments present less differentiation, probably as a result of higher connectivity among populations, while populations belonging to species exhibiting a narrower niche could be susceptible to longer periods of isolation. At the same time, we found a positive correlation of niche breadth with an important index of genetic diversity (observed heterozygosity), implying that niche breadth could be a crucial factor behind the evolution of Mexican oaks.

Acorn of Quercus glaucoides with a size of 9 to 16 mm of diameter, generally dispersed mainly by birds (Jays) and gravity.

Next steps for this research. At the current stage, we would like to generate more genetic data, preferentially using next-generation sequencing techniques, that could be comparable among species with different ecological characteristics, as this will help determine how robust is the association of niche breadth and genetic diversity patterns, not only in oak species, but also in other plant groups around Mexico. Additionally, it would be interesting to compare adaptive patterns among species with different niche breadths, since this will shed light on the processes driving plant species diversification and coexistence in a megadiverse country such as Mexico.

If you could study any organism on Earth, what would it be? I would like to study tropical trees and their mechanisms of adaptation, with emphasis in species from tropical dry forests that have evolved to cope with long periods of dry and warm conditions; these species are fascinating!

Aritra Biswas is a PhD student at the Indian Institute of Science. He is a evolutionary biologist with special focus on the processes that generate and maintain biological diversity. Here, Aritra shares his recent work on the biogeographical history of tarantulas.

Aritra Biswas in the field looking for tarantulas in the forests of the Western Ghats.

Personal links. Twitter | Instagram

Institute. Indian Institute of Science

Academic life stage. PhD student

Major research themes. I am broadly interested in studying biogeography and macroevolution. Specifically, I am interested in the processes that generate and maintain biological diversity and distribution over large timescales at both global and local levels.

Current study system. My PhD focuses on understanding the historical biogeography and macroevolutionary dynamics of tarantulas, which are spiders belonging to the family Theraphosidae. They represent one of the most diverse groups (~1060 species) of spiders, distributed worldwide. Tarantulas are portrayed as scary and harmful to humans through literature, movies, and ancient folklore. The most unique characteristic of tarantulas is that they have urticating hairs that can shoot towards the predator as a defence mechanism. They are the only arachnid group with this amazing ability. They come in all sorts of colours, and all tarantulas are venomous. The presence of all these traits make them an exciting model system for macroevolutionary studies. Apart from all these, there is a tarantula named after one of my favourite singers, Johnny Cash (Aphonopelma johnnycashi), making them a little bit more interesting (at least for me)!

Recent JBI paper. Biswas, A., Chaitanya, R., & Karanth, K. P. (2023). The tangled biogeographic history of tarantulas: An African centre of origin rules out the centrifugal model of speciation. Journal of Biogeography, 50(8), 1241–1351. https://doi.org/10.1111/jbi.14678

Motivation behind this paper. My initial plan was to focus on the origin and biogeography of Indian tarantulas. However, while surveying the literature, I found that the group’s overall historical biogeography is unclear. I found two studies suggesting two different centres of origin for these spiders. The first study by Opatova et al. (2020) proposed an African origin. In contrast, Foley et al. (2021) suggested a South American origin. These two scenarios are mutually exclusive and cannot be true simultaneously. Also, we identified problems with the fossil calibrations employed in the previous studies. Despite being an ancient group, tarantulas represent a fossil-poor lineage, with just two fossils available to date. None of the earlier studies have employed any of these fossils. That was the motivation behind performing an improvised molecular dating with reliable fossil calibrations and an elaborate historical biogeographic analysis which resulted in our recent paper.

Key methodologies. To infer historical biogeographic processes like dispersal and extinction, different programs or packages are used (like BioGeoBEARS or RASP). A dispersal multiplier matrix tells the program the probabilities of dispersal between different biogeographic units. These probability values are often subjective. We used plate tectonic reconstruction data integrated with the dispersal ability of the model organism to come up with an informed dispersal multiplier matrix which reduces the subjectivity significantly. Moreover, we have manipulated the probabilities of dispersals in the matrix to develop different matrix variants to explicitly test different geographic origins and dispersal scenarios not done in conventional historical biogeographic studies. Our work proposes an improvised pipeline for reconstructing biogeographic histories when plate tectonics are involved. Playing around with the dispersal multiplier matrix and testing different values of probabilities is essential and might produce a better solution than the program’s default settings.

Sampling tarantulas along roadside mud bunds

Unexpected challenges. We expected a South American origin since most of the extant tarantula diversity is found on this landmass. We were thus quite thrilled when African origin emerged as the most likely scenario!

Major results. The results show that South America, despite harbouring around 70% of the diversity of extant tarantulas, is not the centre of origin for this group. Africa is the most likely centre of origin (like many other groups). The movement of continental plates played a significant role in shaping the modern-day distribution of tarantulas as they reached Southeast Asia via the drifting Indian plate. Basically, tarantulas have boarded a ship called India from Africa to reach SE Asia and Australia. So, a combination of transoceanic dispersals and plate tectonics has generated the extant distribution patterns in this old group of spiders.

A Thrigmopoeus tarantula doing threat display and flashing its fangs.

Next steps for this research. The next step in this research is to investigate why tarantulas are so diverse in South America. This paper shows that the tremendous diversity of tarantulas in South America has resulted from a single dispersal followed by diversification. Now I am investigating what has caused such disproportionate diversity of tarantulas over the South American landmass. Stay tuned for the result! 

If you could study any organism on Earth, what would it be? I became interested in evolutionary biology mainly because of the “Jurassic Park” franchise. However, I have never wanted to be a palaeontologist. As a result, I have always been fascinated by extant giant carnivorous reptiles. If I get an opportunity, I would love to work on large constrictor snakes (pythons or anacondas) or crocodiles (the closest relative of dinosaurs). Especially, I am very much attracted to saltwater crocodiles and would love to work on them for my postdoctoral research.

Anything else to add? I had Arachnophobia during my childhood. I always used to run away whenever I came across any spider. It has been an exciting journey from being arachnophobic to choosing tarantulas as the model system for my PhD. Also, I saw a live tarantula for the first time only after a few months of submitting the first draft of this paper to the Journal of Biogeography. That was almost after spending one year into the PhD. So, yeah, I wrote a manuscript on the historical biogeography of my model organism without even seeing the animal ever!

Habitat of Western Ghats large burrowing tarantula