Australasian Science: Australia's authority on science since 1938

Cryptic Clues to Species Diversity

The clawless gecko  actually consists of ten or more species.

The clawless gecko (Crenadactylus ocellatus) actually consists of ten or more different species.

By Paul Oliver

Genetic research is revealing how much we have seriously underestimated species diversity in many Australian vertebrate groups.

It surprises many people that the vast majority of species have not yet been discovered, let alone scientifically named. An even greater surprise might be that large numbers of new species remain undocumented in supposedly well-known, highly conspicuous groups such as fish, frogs, reptiles and mammals. Even in a developed and comparatively rich country such as Australia, research over the past decade has added nearly 100 new species of terrestrial vertebrates, including new species in such obvious and seemingly well-known groups as possums, rats, venomous snakes, turtles and even wallabies.

While in some cases these new species are found during expeditions to remote areas, increasingly they are being found within supposedly widespread species that turn out to be complexes of outwardly similar but genetically divergent and evolutionarily independent species. These problematic groups are known as cryptic species complexes, and while this phenomenon has been known for many years, the extent and scale of the problem has not yet been assessed comprehensively in many Australian groups.

To understand the scale of unrecognised diversity, over the past decade a number of different research groups have been targeting this problem in the Australian gecko lizards. With more than 160 recognised species, the Australian gecko fauna is already one of the most diverse in the world, and recent research has also revealed that it includes some of the most ancient components of the Australian lizard fauna, with the initial radiation of some unique Australian groups dating back to around the extinction of the dinosaurs.

To assess levels of cryptic diversity within Australian geckos we have been using genetic information to assess both evolutionary divergence (time of separation) and levels of gene flow (evidence of reproductive isolation) between different populations of geckos. Our results have been nothing short of remarkable.

In the two groups our lab has focused upon we have more than doubled estimates of overall species diversity. From 13 recognised species of stone geckos (Diplodactylus) we estimate that there are probably around 30 species, all very genetically distinct from each other.

In an even more remarkable case, the single previously-recognised species of clawless gecko (Crenadactylus) is actually at least 10 different species scattered across isolated pockets of western and northern Australia. Crenadactylus represents an ancient evolutionary radiation (group of related evolutionary lineages) that is as old as diverse and iconic groups such as all living kangaroos and wallabies or the Australian dragon lizards.

Ongoing research on other groups of Australian gecko is producing similar results, particularly within the dtellas (Gehyra), the velvet geckos (Oedura) and another genus without a common name (Heteronotia). In at least six cases, five or more species have been confounded within a single supposedly widespread species.

Based on work over the past 10 years, scientists have already revealed compelling evidence for well over 60 new species of Australian geckos. Ongoing assessment of other poorly known taxa is likely to reveal many more.

A major collaborative effort involving museums and universities from across Australia and overseas is now underway to describe this wealth of new species. Eight species have been formally described in the past couple of years, and descriptions of many more are in preparation.

Specimens of many of these unrecognised species have been photographed, studied and collected by scientists and naturalists for many years, but because morphological variation is either low or has little relevance to species boundaries, their distinctiveness from other species has only been recognised through the application of genetic techniques. Indeed, a large number of these new species are found in very close proximity to major capital centres.

For instance, our work has identified four unrecognised species (three of which have now been named) within a short drive of Adelaide. In a few cases some species found in suburban backyards remained undescribed until recently.

As scientists we find these results both exciting and concerning. Exciting because it suggests we may now have the tools to identify and describe all Australian vertebrates over the coming decades. But it is also worrying because rapid anthropogenic climate change and other environmental pressures mean that many species we now consider widespread and secure will actually comprise complexes of multiple species, each with much more restricted ranges and greater vulnerability.

For example, at least ten of the new geckos we have discovered appear to have relatively small ranges, and some of them are concentrated in areas of considerable anthropogenic disturbance, such as mining, agriculture or urban development. The status and range of these newly identified species will need to be assessed carefully.

This abundance of unrecognised species also has important ramifications for ecological studies and for mapping bio­diversity. For instance, studies of thermal behaviour in a species of the east Australian gecko (Diplodactylus vittatus) reported exceptional variability, but it is now clear that these studies actually included two quite separate species. Many other studies of the ecology of widespread species could be confounded in this way.

The range of recognised species is also one of the main means by which scientists map the distribution of biodiversity, assess levels of endemism and decide priority areas for conservation. Unfortunately it appears increasingly likely that many cryptic species with restricted ranges (especially across northern Australia) are missing from these analyses, suggesting that important and localised hotspots of high biodiversity and potential priority areas for conservation may have been overlooked.

Our work has also revealed that for large parts of Australia we lack sufficient specimens in museum collections to properly assess species diversity and distribution. While no one enjoys preserving animals, the collection of material from across the range of species is essential for properly understanding species diversity, distributions and many aspects of ecology.

Many new gecko species from northern Australia are only known from a limited number of individuals, the majority without associated tissue samples for DNA analysis. Even more worryingly, and quite surprisingly, many species in densely settled and heavily developed eastern Australia are also inadequately sampled to assess their species diversity and distribution.

For instance, the nominal “species” Diplodactylus vittatus is found over a huge area of eastern Australia, but the small number (10) of genetic samples we analysed demonstrated that there are at least two species in this area, and that their ranges may overlap. Both species show enormous variability in colour and pattern that does not correlate with species boundaries. Without genetic material from much of their range we are currently unable to assign most specimens to correct species or even check the possibility that other unrecognised cryptic species may be present.

Our ongoing systematic assessment of gecko diversity clearly demonstrates that we have a long way to go before we have a full inventory of the gecko fauna of Australia, let alone a complete understanding of the ecology and conservation status of individual species. This problem is likely to be similarly acute in many other Australian vertebrate groups, especially among other relatively species-rich groups such as other lizards, frogs and fishes. These include some of the most diverse elements of the Australian vertebrate biota and, based on results to date, it seems not unreasonable to assume that ongoing studies using both genetics and morphology will reveal hundreds of unrecognised vertebrate species.

And while vertebrates are comparatively well-known, the scale of unrecognised diversity among other animal and plant groups can only be guessed at the moment, but is clearly far greater than within the vertebrates.

From suburban backyards to the most isolated deserts, Australia remains a major frontier for biodiversity research. There is an ongoing and critical need to support research to complete a species inventory of our diverse and unique biota.

Paul Oliver recently completed his PhD on the evolution, systematics and diversity of Australian geckos, and currently works at the South Australian Museum.