Australia and New Guinea, collectively referred to as the Sahul, share many biotic elements; but could not be more different in terms of climate and geological history. Australia as a landmass is relatively geologically stable, does not have much in the way of mountains and is predominantly covered in arid deserts. In contrast, New Guinea is relatively young in geological terms, has extensive mountains ranges across the entire island and is largely covered in tropical rainforests. From a biogeographical perspective these landmasses present very different opportunities and challenges to their prospective biotic assemblages, and yet, there are some lineages that have diversified throughout both regions. Agamids, also referred to as ‘dragons’, have proliferated in arid Australia represented by over 100 species and have been relatively well studied. With 20 recognised species, Melanesian agamids are much less diverse, have received much less scientific attention, and yet have been shown to be closely related to the Australian clade. In a paper published in the Biological Journal of the Linnean Society, we examine the evolution, biogeography and ecological diversity of Melanesian agamids. We tested whether they originated on the Australian Craton or proto-Papuan islands to the north. Also, we examined to what extent mountains have played a role in Melanesian agamid diversification and tested the evolutionary trends and origins of agamid ecological diversity.
In addition to the 11 recognised Melanesian agamids sampled, we found further monophyletic divergent lineages in four taxa. This included two lineages within Hypsilurus modestus, and three within H. magnus, H. papuensis and Lophosaurus dilophus. We found little evidence for micro-endemism in Melanesia with most taxa being widespread and low genetic divergence across the Central Cordillera suggesting these mountains have only recently become a barrier to dispersal. We identified clear evidence of past over-water dispersal in Hypsilurus agamids, with H. modestus lineages dispersing between New Britain, New Ireland and northeast New Guinea. Also, the restricted range of H. schoedei and disjunct distribution of H. longii indicates insular extinction events may have occurred. We identify the Australian Craton as the ancestral area of Lophosaurus and all Australo-Papuan agamids. However, Hypsilurus were clearly able to disperse over water and show deep divergences in the Melanesia region. The distribution of extant Hypsilurus lineages suggests a history of allopatric speciation on former islands with some now accreted onto New Guinea. We infer from the biome evolution analysis that Australo-Papuan agamids were associated with rainforests which date back to the early Miocene, but also that agamids had similarly long histories in other biomes. This indicates that while the rainforest biome might well be ancestral for many Australian radiations, semi-arid or seasonal environments also have a long history in Sahul. Finally, we note the disparity in ecological diversity within Australo-Papuan agamids. Despite their long history in rainforest regions there is a complete lack of Melanesian terrestrial or semi-terrestrial agamids compared with the Australian assemblage. This reflects fundamental differences in how these lizards diversify in dry versus wet habitats and suggests adverse environmental conditions, such as reduced solar radiation, as well as biotic interactions, such as competition or predation, form a barrier to ecological diversification in rainforest biomes.
Author: Oliver Tallowin
Uncovering the evolutionary trajectories of species assemblages can provide fascinating insights into the past environmental and geological processes, as well as the biological traits, that have led to present day diversity patterns. Furthermore, time-calibrated phylogenies can shed light on the historical sequence and timing of speciation events which, in turn, can be used to complement geological models aimed at reconstructing the formation of the earth. In our paper published in Molecular Phylogenetics and Evolution, we focus on the Melanesian radiation of bent-toed geckos (Cyrtodactylus), a clade occurring throughout New Guinea and adjacent islands, and Australia’s tropical northeast. We examine the sequence and timing of diversification in Australo-Papuan Cyrtodactylus and investigated three biogeographic scenarios. Firstly, did Cyrtodactylus diversification originate on the Australian Craton or former proto-Papuan islands to the north. Secondly, does Australo-Papuan Cyrtodactylus diversity correlate with distinct geological regions and to what degree do they exhibit infra-regional clustering. Lastly, to what extent did New Guinea mountain uplift impact Cyrtodactylus diversification and if so when did this occur.
Author: Oliver Tallowin
We tend to think of ways to categorize animals either by shape (4 legs? 2 legs? no legs?, thin? Fat? big headed? small headed?) or phylogenetic affinities (reptiles? lesser animals? Snakes? Skinks?). But what about ecology?
The many aspects of a species ecological niche are generally quantified singly, or we refer to some abstract “multidimensional niche”, meaning we give up trying to characterize it before we even started. In a paper published recently in the Journal of Biogeography GARDians, led by Enav Vidan and Jonathan Belmaker, tried to define, and numerate, the main types of lizards out there – as reflected in their ecology.
We have selected four traits that we felt define much of the fundamental axes of ecological variation seen in lizards: 1. use of space / microhabitat preference, that defines where in the environment a lizard is active (on the ground? In trees or rocks? Under ground? In water?); 2. Activity times: species active in the same place, or even on the same branch or piece of ground, can segregate their use of the environment by dividing the temporal niche. Furthermore, being diurnal or nocturnal (or being cathemeral and enjoy both ‘worlds’) has strong implication on thermal biology and hence on metabolism and rates in which lizards take up resources and exchange them with the environment; 3. Diet: is a species insectivorous/carnivorous, as most lizards are? Or do they predominantly feed on plant matter (these guys seem to even take more leaves and plant parts with lower energy content)? Or do they in fact use both plants and animals (and then probably energy richer plant parts such as berries or sap)? This is directly related to the way a lizard affects its environment and may also influence its position across the sit and wait-active foraging continuum (no use waiting for plants); 4. Body size – while not an ecological trait per se size nonetheless strongly influences a host of ecological processes, from the degrees of metabolism and energy flow, to the types of available foods – and potential predators.
These four traits obviously interact, and some combinations may be more or less common than others: small, diurnal, terrestrial insectivore is after all the first picture to pop to mind when the term ‘lizard’ is introduced (except for the diehard gecko lovers among us, and well, there are a few of us with this infatuation). But are there tiny nocturnal herbivores? Or huge nocturnal lizards? How common is a marine iguana (large, herbivorous, swimming diurnal beast) type lizard – do we remember it simply because it is exotic?
We used Archetypal Analysis to assign lizards to types – or archetypes. Archetypal Analysis is an unsupervised machine learning technique that seeks to find the number of clusters that create the smallest convex hull in an n-dimensional trait space, by using the extreme values rather than the centroid of the clusters. AA assigns, for each species, a vector of affinities to each archetype. Most species are probabilistically assigned to several archetypes, with the partial probabilities summing to one.
We found that the most common functional trait combinations are (1) diurnal, terrestrial, carnivores (20% of the species); (2) diurnal, scansorial, carnivores (16%); and (3) nocturnal, scansorial, carnivores (13%).
Lizards could be robustly classified into seven ecological “Archetypes”:
We then partitioned the global spatial patterns of lizard richness into these seven archetypes, and found that each shows pretty distinct patterns. Turns out Australia is the main hotspot for the herbivorous, nocturnal, fossorial, and terrestrial strategies – and for lizards in general. The Amazon basin is the main hotspot for the semi-aquatic, and scansorial strategies, whereas the large strategy has pan-tropical hotspots, especially in both the Amazon Basin and Northern Australia, but also in Africa, SE Asia and Mexico.
Surprisingly, we found that functional diversity peaks in areas with medium species richness and slowly decreases toward the speciose areas. This unexpected richness-functional diversity unimodal association is also revealed within the scansorial, large, and semi-aquatic strategies. The richness patterns of terrestrial, nocturnal, herbivorous, and fossorial strategies increase with species richness, but globally functional richness peaks in areas with medium species richness.
Thus increases in richness do not necessarily stem from increased functional diversity. Species diversification within specific strategies often dominates richness patterns. Our findings support the contention that it is important to consider different functional and ecological subgroups when studying richness patterns.
Author: Shai Meiri
In a recent publication in Global Ecology and Biogeography we explored the prevalence of nocturnality amongst Eurasian lizard species and tried to understand what drives these patterns.
Most animals – at least those that live above ground – are active either during the day or during the night. Being active at either time of day carries with it unique benefits and challenges, and thus particular adaptations. Because of this being nocturnal or diurnal is a trait that is pretty rigid amongst closely related species.
We found that nocturnal lizards have the highest species richness in the tropics and in deserts, and their richness decreases when they get closer to the North Pole. Nocturnal lizards are precluded altogether from the coldest regions inhabited by lizards – in high mountains and the highest latitudes.
Author: Enav Vidan
In our recent publication in the Journal of Biogeography, we assembled a comprehensive distribution map of all reptiles in Africa in order to quantify their geographical overlap with the other vertebrate groups, and to assess the environmental correlates underlying these patterns.
The latitudinal gradient of increasing biological diversity towards the equator is one of the best recognized patterns in biogeography, and has been acknowledged for some time. The naturalist, Alexander von Humboldt wrote of his travels over 200 hundred years ago, that as we approach the tropics, "the greater the variety of structure, form, colour, youth and vigor of organic life." A number of well-known hypotheses explaining this pervasive pattern of the increasing number of different species towards the equator have since proliferated. These include elevated ambient energy and precipitation, the number of different habitats or niches, higher plant productivity, and many more.
To create our geographic distribution map of reptiles in Africa, we obtained data from a variety of field-guides and atlases, museum databases, the primary literature, IUCN assessments, and maps based on expert knowledge of reptile species and the habitats they occupy. A challenging aspect of the project was to ensure that our maps remained current with respect to new species discoveries and taxonomic name changes (which are constantly being revised), and we also had to confirm the validity of type specimen identifications and localities, especially those referenced from obscure sources and archaic museum specimens. We used GIS software to digitize and overlay the maps of each individual African reptile species (1,601 species in total!) one on top of the other, which allowed us to count the number of species present in a given area - which we call “species richness”.
When we looked at which environmental predictors best explained these species richness maps we found that net primary productivity (the amount of photosynthetic activity by plants) and precipitation explain most of the variation in reptile and other vertebrates. This explains the clear latitudinal pattern seen in their respective maps, which reflects a strong correlation with plant productivity and rainfall as you move closer to the equator. But again, lizards are unique in that none of these environmental correlates explain their distributions. This is because lizards are well adapted to a wide range of habitats including the tropics as well as the harsh conditions of the desert where plant productivity and rainfall are low. We also showed that individual lizard species on average occupy smaller geographic distributions, reflecting their ability to occupy diverse niches.
Our findings that the distribution of lizard species in Africa is unique when compared to the other vertebrate groups now confirms a pattern that has been seen elsewhere in previous studies (i.e. Australia) and most recently by our paper on the global distribution of reptiles. This shows the importance of studying the diverse reptile groups distinctly instead of lumping them all together, and will have bearing on large-scale conservation efforts that do not represent all reptile groups.
Author: Amir Lewin
Mainly maintained by Shai Meiri and Uri Roll