Arid springs: the hidden evolutionary cradles of Outback Australia
Deep in the blazing heat of the Australian desert, a few centimetres of water sits unobtrusively on a flat clay pan. There’s nothing around for miles but big skies, the merciless sun, cracked clay and weathered red dirt, and certainly nothing to tell you that the unimpressive puddle you’re standing next to is actually one of the great wonders of the natural world.
Out here, water is life, but in this case, that’s an understatement. What makes this puddle special is that it’s no ordinary puddle, but a spring, meaning that the water here has flowed up from an artesian source deep underground, making it a permanent puddle. Drought is the norm here in arid and semi-arid Australia—while standing here by the springs on Edgbaston Reserve in Central Queensland we’re in the middle of yet another one that’s stretching the resolve of the tough local graziers who run cattle on the surrounding properties.
But there’s always water in the springs. And, amazingly, if you kick off your shoes, walk carefully out into the spring and peer into the shallow water, you’ll find fish. And not just any fish—these fish, red-finned blue-eyes (Scaturiginichthys vermeilipinnis), are Australia’s most endangered fish, because they are found nowhere but this one former station. The red-finned blue-eye’s global distribution is less than 8000 hectares and it is confined to only four springs within the Edgbaston Reserve site, making the amount of habitat it can occupy little more than about 4000 square metres.
The sheer improbability of fish living in this arid environment is astounding. Temperatures regularly soar over 47 degrees Celsius during the summer, and plummet to near-freezing overnight in winter. The spring is only a few centimetres deep, so there is little protection from these large temperature fluctuations. Although you’re technically on a floodplain, there’s no other water anywhere remotely close to you. It’s hard to imagine a worse place for a fish to live.
Yet despite this, if you look more closely into the spring around you, you’ll find that the eye-catching red-finned blue-eyes are the tip of the iceberg, and that many of the species—plants, fish and invertebrates such as crustaceans and snails—are unique to the springs. How did all these aquatic species get here, and why aren’t they found anywhere else? The answer is that you’re looking at an example of an evolutionary highway, where isolated environments and the perfect combination of stability and environmental variance create conditions that favour rapid speciation.
Why so many aquatic species in the desert?
To understand the species diversity here, you first need to know why there is permanent water. Below the dry and sweltering desert expanse lies a vast ancient aquifer that is slowly transporting water deep underground from the wetter coastal regions, through the heart of the driest parts of the Australian desert. This aquifer, known as the Great Artesian Basin (or GAB), is one of the world’s largest and one of the few that still maintains strong pressure. The extent of the Basin is huge—it covers about a quarter of the Australian mainland, including large areas of Queensland, the Northern Territory, New South Wales and South Australia. Unlike many other aquifers that require pumping to bring groundwater to the surface, groundwater in the GAB is trapped under pressure (artesian). In some locations, the underground pressure is so great that unrestrained bores will shoot water 5 metres vertically above the ground when drilling penetrates the rocks confining water within the aquifer.
Springs are located at points in the landscape where this pressure naturally forces water to the surface along weak points in the rocks that overlay the aquifer. These springs range from expansive pools of warm water deep enough to swim in (e.g. Dalhousie springs, Witjira National Park, South Australia), to bubbling pools no bigger than a fish-pond (e.g. Blanche Cup, Wabmakdarbu National Park, South Australia) and shallow emerald-green meadows no deeper than your ankles (e.g. Edgbaston and Elizabeth springs, Central Queensland). Despite this diversity in form, their connection to the GAB ensures these springs are constantly refilling with water, making them one of the only permanent sources of water in the Australian arid zone.
That explains why there’s water here, but why are there so many endemic species? To answer this question you must think not only about the springs as they are now, but also as they would have been in the past. What is now Australia’s arid interior has witnessed a lot of change over geological time. The area was once covered in moist semi-tropical forests, which slowly gave way to the grasslands, scrub and deserts we now know. As the landscape became drier, rivers and streams that were once permanent dried up. Species that relied on these water sources were left ‘stranded’ in what little water was left. However, the species that found themselves in springs not only persisted there, they underwent a phenomenal amount of diversification, speciation and change, and this resulted in the vast array of animals and plants we see today. Rather than just being a ‘museum of past diversity’, GAB springs also act as ‘evolutionary cradles’ that appear to see greater numbers of species appear through time than other ecosystems. For example, in one family of snails scientists currently think there are 34 species but new species are still being discovered. The species from Edgbaston in the north are completely different to the species in the springs surrounding Kati Thanda (Lake Eyre), which are different again to the species at Dalhousie.
The key to springs as evolutionary cradles is likely to lie in the isolation of the springs and the tough environmental conditions. The same principles that apply to famous examples of diversification across islands (e.g. Darwin’s finches) apply in springs, but in this case the springs are the ‘islands’ of wetland in a ‘sea’ of aridity. Because species like fish or snails die without water, populations of species in different springs are isolated from one another. This isolation means that there are extremely few immigrants and a limited choice of mates, making it more likely that mutations will be passed on to offspring. Over time, these accumulated mutations can make a spring population differ sufficiently from the source population that it becomes a new species. The harsh environment also helps to drive this speciation: the tough environmental conditions mean that individuals with certain characteristics survive to pass on their genes to the next generation. Only a limited set of genes are passed on in each generation, so individuals start to differ from the original population more rapidly than they would if all genes were equally likely. This accelerates evolutionary processes. The red-finned blue-eye is a good example of a species that has developed characteristics to suit the environmental conditions of the springs. All other species in the red-finned blue-eye’s family occur in coastal-draining catchments in northern Australia and Papua New Guinea, and most have long, flowing fins. The red-finned variety, existing as they do in water that is barely deeper than their bodies, have evolved short fins, almost certainly because long fins would be ecologically impractical.
Species of the springs
If you go looking in springs you will find their still, stable and permanent waters teeming with life. In each new region of springs you visit, you will find a completely different set of species unique to the area. There are at least 98 species that are only found in springs, and around 33 unique combinations of species in different parts of the Basin. Compared to other arid zone waterways, this diversity is astounding. There are eyeless shrimps whose ancestors lived in underground aquifers, dozens of different species of spring snails, adorable button grasses, beautiful but carnivorous bladderwort plants, and some of Australia’s (and the world’s) rarest fishes.
Of all the species that are only found in GAB springs, about 70% of them are snails. Each spring region is home to a separate lineage of species. Even the more closely-related snails found within a single cluster of springs look completely different and behave in very different ways. For example at the Edgbaston Reserve, six different species of snail within the family Tateinae live side-by-side. In some cases, diversification of species is happening so rapidly that you see it happening by comparing snails from adjacent springs. In the springs surrounding Kati-Thanda (Lake Eyre) Fonscochlea zeidleri, a small cream coloured spring snail, remains a single species but each isolated population has started to evolve along its own unique path. In time, each of these populations will become sufficiently different that we will call them different species, a perfect example of how isolation, time and the unique combination of environmental variability and stability make the springs evolutionary cradles.
In contrast, some species have adapted to life in the springs but did not diversify once they got there. Instead, these species are relics of the interior’s wetter past, surviving in the permanent water provided by the springs. For example, the red-finned blue-eye is the only species in its family to live in springs.
The Edgbaston goby (Chlamydogobius squamigenus), which shares the springs with red-finned blue-eye, and its relatives the Elizabeth Springs goby, the Finke goby and the Dalhousie goby (all in the genus Chlamydogobius) all persist in isolated populations in the outback. These species look very similar to one another and to another close relative called the desert goby (C. eremius), which is found throughout the arid areas of South Australia. All of these gobies probably evolved from a single species of goby which lived throughout Central Australia when the climate was much wetter. As the country dried out, the populations in places like Edgbaston and the other spring complexes became isolated, and over time have diverged enough – genetically – to be different species.
Threats, conservation and management
Although the species that live in springs have evolved to tolerate tough conditions, their existence is precarious. The spring communities are listed nationally as a threatened ecological community and many of the most diverse spring complexes are now reserved either as part of national parks or by private conservation groups (e.g. Bush Heritage Australia manages Edgbaston Reserve). Although reserving the springs is a step in the right direction, these unique Australian flora and fauna are by no means safe from extinction.
The biggest threat to the springs is groundwater extraction for human use. The existence of the springs depends on the pressure of the groundwater within the Great Artesian Basin staying high—if there’s not enough pressure to bring water to the surface then the springs will stop flowing and dry up. Drilling of free-flowing bores, predominantly for the pastoral industry, has reduced the groundwater pressure. Spring flows have reduced as a result—current estimates suggest that one in five spring complexes have stopped flowing since European settlement of the basin. Fortunately, substantial efforts were made to increase water use efficiency in the basin in the early 2000s. Most importantly, many of the free-flowing bores were capped to allow the groundwater pressure to stabilise. Although groundwater extraction for the pastoral industry may be manageable, extraction for the mining and coal seam gas industries are growing pressures and ones not yet fully understood.
On top of the threats to groundwater pressure, spring communities must also persist in the face of ongoing biological threats. Although the environment is tough, weeds and pest animals need to be controlled to preserve the precarious existence of the spring species. One of the worst localised impacts for small springs is trampling caused by large feral mammals like pigs and camels or by stock such as cattle. Introduced aquatic animals like cane toads and mosquitofish prey directly on spring fauna or compete with them for habitat and food. Managers and scientists at the red-finned blue-eye’s home (Edgbaston Reserve) are continuing to experiment with different approaches for managing mosquitofish—one of the world’s worst invasive fish, which is present in many larger springs and has been implicated in the declines of many Australian native fish populations—to help the native populations persist there.
What can I do to help?
There are many ways you can support research and conservation efforts that are working to ensure that the unique biodiversity of Australia’s desert springs is not lost. First, take the opportunity to visit some of the phenomenal reserves and parks that contain springs: Witjira and Wabma Kadarbu National Parks in South Australia, or Elizabeth Springs and Doongmabulla Mound Springs Reserves in Queensland. You can support or volunteer to assist on conservation projects through the National Parks or Bush Heritage Australia, as well as the Wangan Jagalingou Family Council’s work at Doongmabulla Mound Springs. And finally, if you’re living on the land, you can ensure that the groundwater pressure vital to maintaining springs is preserved by capping unused bores on your property.
This article is an edited version of the original, which was published in Wildlife Australia.
Publications and further reading
- Bush Heritage Australia’s Edgbaston Reserve
- Renee Rossini’s Spineless ecology research blog
- Nicol, S., Haynes, T. B., Fensham, R. and Kerezsy, A. (2015) Quantifying the impact of Gambusia holbrooki on the extinction risk of the critically endangered red-finned blue-eye. Ecosphere 6(3):41. http://onlinelibrary.wiley.com/doi/10.1890/ES14-00412.1/abstract
- Rossini, R.A., Fensham, R.J. and Walter, G.H. (2017) Spatiotemporal variance of environmental conditions in Australian artesian springs affects the distribution and abundance of six endemic snail species. Aquat Ecol. https://link.springer.com/article/10.1007/s10452-017-9633-4
- Rossini, R. A., Fensham, R. J. and Walter, G. H. (2015) Determining optimal sampling strategies for monitoring threatened endemic macro-invertebrates in Australia’s artesian springs. Marine and Freshwater Research 67: 653-665. http://www.publish.csiro.au/mf/mf15023
- Kerezsy, A. and Fensham, R. (2013) Conservation of the endangered red-finned blue-eye, Scaturiginichthys vermeilipinnis, and control of alien eastern gambusia, Gambusia holbrooki, in a spring wetland complex. Marine and Freshwater Research 64: 851-863. http://www.publish.csiro.au/MF/MF12236
June 15, 2017 at 5:22 pm
I would like to know if a study of plankton has been carried out, being the bottom of the aquatic food chain it would have to be there, could you advise me?
June 16, 2017 at 3:23 pm
In terms of plankton, I guess I conventionally think of things like Copepods and waterfleas. To my knowledge, there have been descriptions of these small organisms in springs at Edgbaston – Winston Ponder & team published a full survey from there in 2010 in which they report finding Cladocerans as well as calanoid Copepods. There’s also other small crustaceans like Amphipods – 12 species of which are endemic to GAB springs – and Ostracods – again lots of endemic species. The snails in this system are also generally small (<5mm) and are probably prey for the larger fishes and frogs. There's also plenty of aquatic insects and their larvae, again tasty morsels for the bigger predators.
When it comes to the snails especially, a large number of animals in this system are herbivorous/detritivorous – for example the majority of species endemic to springs are snails, and the majority of these snails are probably feeding on algal films and detritus. So these guys are probably functioning in that "bottom of the food chain" role as well. For example, in the few studies I know of looking at gut contents of invasive predators in GAB springs (Cane Toads and Gambusia), snails form a big part of the diet. This isn't that surprising, given that they can be very abundant (I've found over 200 individuals in a single 3 x 3cm sample), but is concerning as some of these snails only live in one or two springs and, in Edgbaston in the past for example, have never had to deal with predator numbers like they do now with Gambusia.