The dynamic niche

A0216_L_raniformisI suspect I’m one of many for which the predictability of species distributions was a gateway drug to ecology. My journey began with an extraordinary biogeographic transition point, delimited by the sheer slopes of the Plenty Gorge on Melbourne’s northern outskirts. East of the river – where I grew up – was a world of Yellow Gum, Red Box, Wattle and Cassinia. Undulating mile upon undulating mile of it, built on sedimentary soils laid down during the Ordovician. The plains west of the Plenty River were a very different landscape. Volcanic epochs transformed this country, laying deep basalt clays over the underlying sedimentary rock. Kangaroo and Wallaby Grass, Spear Grass and Poa grew here in prolific stretches prior to European settlement, and gave rise to a markedly different faunal assemblage to that east of the Gorge.

As a young fellow, this biogeographic divide enabled an idea to crystallise in my mind. If I could grasp the geological, climatic or floristic affiliations of species, I could always find them. Little Whip Snake? West of the river, on cracking clay soils under exfoliating basalt. Weasel Skink? Rotting leaf litter or timber in damp gullies east of the river. Jacky Dragon? Cassinia and Acacia dominated heathlands on the steeper banks of the Gorge, where protruding mudstone reefs dominate. These were rules, as good as the laws of thermodynamics – learn them, and you have each species pegged.

But what if you don’t? What if these rules can change? What if much of what you have learnt about a species can go out the window in a metaphoric blink of the eye? Welcome to the concept of the ‘dynamic niche’, where biotic shifts can drastically reshape the distribution, habitat affiliations, demography and even evolutionary trajectory of species.

Over recent decades, amphibian ecologists have been confronted with rapid niche shifts of this kind. The driver has been the emergence and spread of the pandemic lineage of chytrid fungus. The headline grabbing impacts of this pathogen have been wholesale extinction events and drastic population declines. But another, more subtle phenomenon has also emerged – changes in the ecology of many frogs, some of which now appear quite irreversible. Our most recent paper, led by Ben Scheele, is an attempt to describe these shifts, to synthesise the patterns therein, and to guide conservation responses for species that appear permanently changed by their interactions with this pathogen.

Two frog species from Australia’s south-east are useful exemplars. The first is my beloved Growling Grass Frog, a species that was both abundant and widespread prior to the arrival of chytrid to Australia’s east coast in the late 1970s. Through the 1980s and 1990s, the species experienced drastic population declines, collapsing out of the areas that were environmentally most suitable for the pathogen – colder and/or wetter parts of the range, such as higher-elevations, frost-ridden plains and wetter foothills (see figure below). Habitat affiliations of the frog also appear to have narrowed, to wetlands that are relatively warm and/or slightly salt-affected (conditions in which the pathogenicity and virulence of chytrid is reduced). No longer can they persist in the cooler, fresher sites in which they once thrived, nor those that dry out with any frequency. The Growling Grass Frog we know today is a different beast to the frog we knew prior to 1978.


Changes in the distribution and realised niche of the Growling Grass Frog due to chytrid. The map shows records of the species up to 1980 (orange) and subsequent to 2000 (green), broadly representing the pre- and post-chytrid distribution. The plots show the change in the elevational and thermal niche of the species, resulting from its distributional contraction.  Figure 2 from Scheele et al. (2019), Biological Conservation 236, 52-59. © Elsevier.

Likewise, the Alpine Tree Frog has been changed dramatically by the arrival of chytrid. A once fairly long-lived species (up to 7 years), today it rarely lives beyond 2 years of age. Early reproduction is now imperative, and there is evidence the heavy selection for early reproduction is altering the life history traits of this species. Habitat affiliations are also shifting, because consistent recruitment is now utterly vital for persistence at the population level (with adult survival so low, even a single year of reproductive failure can spell doom for isolated populations). As a result, the species is dropping out of the ephemeral wetlands systems that were once prime breeding habitat, and it is increasingly affiliated with permanent or near-permanent wetlands.

As well as highlighting the ecological impacts of chytrid, these two species are useful for demonstrating the shift that conservation managers must make in their approach to species with endemic chytrid infections. Take re-introduction to former haunts, which is often proposed as an approach to conserve these frogs and many others. Such initiatives will fail miserably if chytrid has the upper hand at the target locations. Indeed, we’ve seen this for another Australian hylid for which re-introduction has been attempted numerous times – the Green and Golden Bell Frog. Most re-introductions have failed, often as the frogs were put into habitat with characteristics that they used to be affiliated with. Ultimately, these re-introductions failed because we, as amphibian ecologists, failed to recognise the new world order for these frogs.

For more on the topic, you can find the paper here (or email me for a copy if you can’t get through the paywall). It is a thought-provoking, challenging and perhaps even controversial concept, but one we must consider for effective amphibian conservation in the post-chytrid era. Nevertheless, the parallels with invasive species impacts and wildlife diseases more generally are clear, and so we hope the paper will prove of interest beyond the realms of amphibian conservation. Niche shifts are happening across the globe, and we must be alert to them if our conservation initiatives are to remain effective.

Lessons in biogeography (from the back seat of a Datsun station wagon)

Road trips of my youth rarely entailed music. My siblings and I cruised the highways of south-eastern Australia mostly in silence. Melbourne to Bright – silence. Melbourne to Barmah – silence. Melbourne to Canberra – silence. Melbourne to Mildura – silence.

It wasn’t enforced – I didn’t grow up in some sort of ‘Footloose’ inspired cult family in which the revelry of music was considered sacrilege. My father, who was invariably driving, merely preferred quiet contemplation and the joy of the drive. Or at least that’s what I presume; the truth is I’ve never actually asked.

Whatever my father’s motivations, the outcome for me was profound. Our family holidays were invariably north, away from Melbourne to more xeric climes. We wandered the Hume, the Calder, the Northern and the Newel, seeking out rivers and lakes across the great expanse of the Murray-Darling Basin. From the cool mountain streams in the upper catchment, to the languid lower reaches of the Murray and Darling Rivers themselves. The thing is, as you cruise through these landscapes in silence, you take more in. You notice the change from White Box to Grey; the first appearance of Buloke; the disappearance of Cassinia and the appearance of chenopods. You absorb the landscape, subtly and osmotically. As the kilometres fall away, the great tapestry of biogeography is revealed.

Take Triodia. My first sighting of this extraordinary grass remains etched on my mind. Triodia – or Spinifex as it is more commonly known – is perhaps the singular success story of the Australian Outback. It occurs over literally millions of hectares of the most arid and desolate terrain Australia has to offer. My fascination with this grass is not (I must admit) with it per se, but rather with the animals it houses. For Triodia hummocks are a remarkable store of lizards. You may know that Australia’s deserts have some of the highest densities of lizards in the world, both in terms of abundance and species diversity. You may not know that Triodia is a key driver of this, at least according to some. Its hummocks provide seemingly perfectly designed lizard lairs, complete with a protective shroud of spiny foliage, cool and stable microclimates and an abundance of juicy invertebrate prey.

It was somewhere around 20 km north of Ouyen in the Victorian Mallee where I caught my first glimpse of Triodia. We’d been driving up the Calder Hwy for hours – probably five by that stage – on our way to Mildura. I’d been watching the vegetation change from tall forest to Box-Ironbark to Mallee, through various shades that entailed evermore diminutive Eucalypts. Now we were approaching proper sand dunes, and I knew that Triodia was on the cards. Then, sure enough, on cresting a dune of previously unrivalled proportions, there it was. A hummock. A Spinifex hummock! In the flesh, whizzing past my window. I craned my neck and watched it until it disappeared from view.

Whipping back, I looked for more. It took time, perhaps another 5 km, and then I spotted another. Then another and another, until they were carpeting the ground. We’d hit Hattah-Kulkyne National Park and the blessed things were everywhere.

To this day, I think about that sole hummock, a good 5 km from its nearest brethren. Was it the most southerly hummock in that region? If it wasn’t, then such a hummock must have existed, out there, somewhere. And if that is true, then it is also true of every species in the world. All must have one individual, at any one point in time, that is the most far flung member of its species. One intrepid individual pushing the limits of the species’ range. What drives these boundaries? What holds them in place? Is it thermal tolerances, or geological affinities, perhaps a deep-seated fear of frosts or some mutualist that a species just can’t do without?

Ultimately, what I learned on those long, silent drives with my father (and all those that have followed since) is that this intrigue is never ending. If one ever achieved immortality, they still couldn’t solve the riddle for all species. What’s more, ecosystems are forever changing and species distributions along with them. The search for biogeographic truths is endless, and will always be so.

Australia’s species need an independent champion

img_1196Last week myself and colleagues from Charles Sturt University, Deakin University and the University of Queensland wrote a piece for The Conversation arguing that the role of Australia’s Threatened Species Commissioner should be strengthened and made independent from government. It is a timely piece – the first Commissioner recently stepped down and his replacement is currently being sought. It represents an important moment to reflect on the successes and failings of the role so far, and ways it could better serve Australia’s diverse and precious biodiversity.

You can read the article in full here.

New paper: Can habitat management mitigate disease impacts on threatened amphibians?

swabbingHow does one control a rapacious pathogen? If it were an infectious agent of humans, we would have much in our armoury. We could isolate the stricken, and slow the pathogens spread. We could search for the vector and extinguish it. We could take antibodies from the immune and treat the susceptible with their serum. Or we could disseminate doses of powerful antibiotics or vaccines, and lead the pathogen down the path to functional extinction.

But what if the pathogen targets wildlife? In that case, our armoury is much diminished. So much so that the outcome of wildlife-pathogen interactions in nature are almost always determined by natural mechanisms; death of the vulnerable and, failing complete extinction, either survival and proliferation of the immune or the recovered, or persistence of susceptible populations in refugia, away from reservoir hosts or in regions outside the pathogen’s environmental hitting zone.

In our latest paper, just out in Conservation Letters, we assess the degree to which knowledge of environmental refugia can be used to mitigate the impacts of perhaps the worst wildlife pathogen of modern times – the amphibian chytrid fungus. Chytrid emerged as a major pathogen of amphibians late last century, for reasons unknown. It spread across the globe, facilitated by us, and decimated frogs and toads as it went. The toll is difficult to quantify (and continues to mount), but at least 200 species are now thought to have either succumbed completely to chytridiomycosis, or suffered significant population declines.


Chytrid sporangium, CSIRO

Despite this, chytrid is not invincible. In fact, it has some key environmental frailties – its pathogenicity falls sharply at warmer temperatures, and it cannot tolerate acidic or alkaline environments, nor those which are somewhat saline. In short, it has several environmental Achilles heels.

We set out to assess the degree to which these weaknesses could be harnessed to bolster amphibian population viability. For small metapopulations of our focal species, the threatened Growling Grass Frog, we used simulations to understand how slight increases to wetland water temperatures and salinity (achieved by reducing wetland shading, increasing wetland size and depth, and tapping groundwater) could reduce pathogen prevalence and increase rates of frog population persistence. In addition, we assessed the degree to which strategic creation of warm and slighty saline wetlands (<10,000 µS/cm) could enhance metapopulation viability. The work builds on our 2015 paper which demonstrated that environmental refugia, metapopulation size and connectivity are important determinants of the persistence of Growling Grass Frog populations afflicted by chytrid.

So, what did we find? Three things in particular. First, our simulations suggest that habitat management to mitigate chytrid impacts will be most effective in climates where hosts are already less susceptible to the disease; that is, within climatic refugia where disease impacts are already curtailed. Second, our work suggests that creating new wetlands with refugial properties may be substantially more effective than manipulating existing habitat, in part because altering existing habitat will be constrained by other environmental considerations. Third, increasing metapopulation size and connectivity through strategic habitat creation can greatly reduce extinction risk, because dense-clusters of wetlands are much more likely to enable a balance between the opposing forces of population extinction and (re)colonisation.

Our work is one of very few to assess the effectiveness of habitat-based management levers for controlling wildlife disease. The results are encouraging. The next step is to test the effectiveness of habitat-based management of chytrid in the field. We need well-designed, statistically rigorous experiments replicated across multiple taxa to understand the scale and breadth of its effectiveness, its practicality under varying contexts, and the cost-benefit ratio relative to other potential control options.


A pool on the Merri Creek recently choked by invasive Phragmites, Willows and Hawthorn. Once an important breeding site for Growling Grass Frogs, the species is no longer found here

But in the interim, our work provides clear guidance to managers of Growling Grass Frogs in southern Australia, and its sister-taxa the Green and Golden Bell Frog and Yellow-spotted Bell Frog. The niche of these frogs has narrowed in the wake of chytrid. Today, they require dense networks of wetlands that receive copious sun, and they can benefit from slightly saline environments. If shading from riparian trees and invasive emergent vegetation is prevalent in the systems you manage, thin it out or remove it all together. If wetlands are sparse, small and shallow, seek to build adjacent wetlands which are large, deep and unshaded, providing not just disease refugia, but bolstering metapopulation size and connectivity as well. And if you have access to slightly saline ground water (<8,000 µS/cm), consider sinking bores and feeding some wetlands with this water source.

Do let us know how you get on.

2016 Herping in review

Between 2013 and 2015, I was lucky enough to embark on an ecological odyssey to the UK. With backing from a Victorian Postdoctoral Research Fellowship, I set off for the University of York and spent a happy two years learning at the knee of Prof. Chris Thomas. Through daily chats with Chris, and the other great folk that called York’s J2 lab home, I gathered a sense of the incredible biodiversity data sets that UK ecologists have at their disposal. The British populace, I soon realised, are just as fanatical about collecting biodiversity data as they are about train spotting, building model aeroplanes and tracking down obscure antiques. From immense observational data sets, to comprehensive, statistically-rigorous monitoring programs, the Brits produce masses of species occurrence and abundance data every year. I was hugely impressed; not just with the British fervor for good, solid data, but the end products too – great ecological science and perhaps an unrivaled capacity to monitor the country’s biodiversity.

Returning to Australia, I had a new found sense of the importance of maintaining records of the species I see in my travels. Specifically, time-stamped occurrence data, the sorts of which are vital to producing species distribution maps and models, and which, in the long-term, can provide insights into population declines, range shifts or even invasions. I’ve been diligently keeping these records ever since, with annual uploads to the Victorian Biodiversity Atlas and the Atlas of Living Australia.

This post will be the first of an annual series sharing those records with you. My intentions are two-fold: i) to share a snippet of the wonderful reptiles and frogs I’ve had the pleasure of meeting each year, and; ii) to encourage you to keep these records too, and to submit them regularly to either a state or national biodiversity atlas.

So, what of 2016? In all, I managed 126 records of 53 species, with observations from the tropical forests of North Queensland, through southern Queensland, New South Wales and into the hills and plains of Victoria. I met numerous new species, including Scrub Python, Jungle Carpet Python, New England Tree Frog, Striped Burrowing Frog, Golden Crowned Snake, Cascade Tree Frog, Sudell’s Frog, Rugose Toadlet and Red-eyed Tree Frog. The full species list can be found in the table that follows, with further details here. I also had the opportunity to photograph many of the species I encountered this year, having finally managed to save enough pennies for a decent digital camera. I’ll leave off with a few of the images I captured (you can head over to my Flickr page if you would like to see more).

Until next year then. Here’s hoping 2017 is just as rewarding…

Species list:

Species Common Name
Amalosia lesueurii Lesueur’s Velvet Gecko
Amphibolurus muricatus Jacky Dragon
Boiga irregularis Brown Tree Snake
Cacophis squamulosus Golden Crowned Snake
Carlia tetradactyla Southern Rainbow Skink
Chelodina longicollis Common Long-necked Turtle
Christinus marmoratus Marbled Gecko
Concinnia martini Martin’s Skink
Crinia parinsignifera Plains Froglet
Crinia signifera Common Froglet
Cryptoblepharus pannosus Ragged Snake-eyed Skink
Ctenotus spaldingi Robust Skink
Ctenotus taeniolatus Copper-tailed Skink
Cyclorana alboguttata Striped Burrowing Frog
Delma impar Striped Legless Lizard
Diporiphora australis Tommy Roundhead
Egernia striolata Tree Skink
Emydura macquarii Murray River Turtle
Eulamprus quoyii Eastern Water Skink
Geocrinia victoriana Victorian Smooth Froglet
Chelonia mydas Green Sea Turtle
Hemidactylus frenatus Asian House Gecko
Hemiergis decresiensis Three-toed Skink
Intellagama lesueurii Eastern Water Dragon
Intellagama lesueurii howittii Gippsland Water Dragon
Lampropholis guichenoti Garden Skink
Land Mullet Bellatorias major
Lerista bouganvilli Bouganville’s Skink
Limnodynastes dumerilii Banjo Frog
Limnodynastes peronii Striped Marsh Frog
Limnodynastes tasmaniensis Spotted Marsh Frog
Liopholis modesta Eastern Ranges Rock Skink
Litoria aurea Green and Golden Bell Frog
Litoria caerulea Green Tree Frog
Litoria chloris Red-eyed Tree Frog
Litoria ewingii Southern Brown Tree Frog
Litoria fallax Eastern Dwarf Tree Frog
Litoria pearsoniana Cascade Tree Frog
Litoria peronii Peron’s Tree Frog
Litoria subglandulosa New England Tree Frog
Litoria verreauxii Whistling Tree Frog
Mixophyse fasciolatus Great Barred Frog
Morethia boulengeri Boulenger’s Skink
Neobatrachus sudelli Common Spade-foot Toad
Parasuta flagellum Little Whip Snake
Pogona barbata Eastern Bearded Dragon
Pseudechis porphyriacus Red-bellied Black Snake
Pseudomoia pagenstecheri Tussock Skink
Pseudonaja textilis Common Brown Snake
Strophurus intermedius Eastern Spiny-tailed Gecko
Tiliqua scincoides Eastern Blue-tongue
Tropidechis carinatus Rough-scaled Snake
Uperoleia rugosa Rugose Toadlet



Cascade Tree Frog (Litoria pearsoniana), Springbrook QLD


Eastern Brown Snake (Pseudonaja textilis), Redesdale VIC


Gippsland Water Dragon (Intellagama lesueurii howittii), Mallacoota VIC


Scrub Python (Morelia kinghorni), Tully QLD


Red-eyed Tree Frog (Litoria chloris), Mt Warning NSW


Lesueur’s Velvet Gecko (Amalosia lesueurii), Retreat NSW

New paper – After the epidemic: ongoing declines, stabilisations and recoveries in chytridiomycosis impacted amphibians

csiro_scienceimage_1392_scanning_electron_micrograph_of_chytrid_fungusWe humans live in perpetual fear of epidemics. Some nasty new bug emerging from the jungle, sweeping across humanity and knocking off millions of us in the process. Or perhaps an existing pathogen that mutates into a superbug capable of spreading like wildfire, transmitted by as little as a dirty look.

While sometimes bordering on irrational (cheers Hollywood), our fear of epidemics is well placed. We’ve had some doozies in the not too distant past. Take Spanish Flu – a disease that killed somewhere between 50 and 100 million people between 1918 and 1920, reducing the world population by up to 5%. What’s more, our spine-bearing brethren give us regular reminders of the ruinous power of pathogens. Examples include white-nose syndrome in bats, avian malaria, Parapoxvirus in squirrels and the recent implosion of Saiga populations on the Eurasian steppe.

In the wildlife realm however, one disease stands head-and-shoulders above the rest as a potent reminder of the destructive capacity of pathogens. Chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis, has killed literally millions of frogs across the globe over the last four decades, driving thousands of local extinctions and either major decline or global extinction for up to 200 species. A truly remarkable feat for a single pathogen. In Australia, chytrid hit in the late 1970’s, arriving first (we believe) in Brisbane, before heading north and south along the east coast, and skipping across to Western Australia and Tasmania. It left carnage in its wake. Frogs that were formally abundant and readily found simply disappeared. Apart from a few observed die-offs, numerous populations went up in a figurative puff of smoke. Seven species met their doom.

Our most recent paper reviews what happened next. Led by the inimitable Dr Ben Scheele, the paper draws together published and unpublished data to review the fate of Australian frogs impacted by chytridiomycosis following the initial epidemic. We detail the varying responses of these species, ranging from ongoing decline, to stabilisation and even recovery. Furthermore, the review draws together the known mechanisms underpinning these responses, which Australian and international herpetologists have steadily revealed over the last two decades.

The news is mixed. Chytridiomycosis remains the chief threat to several highly endangered frogs in Australia, such as the Southern Corroboree Frog and Baw Baw Frog, both of which may soon no longer persist in the wild. However, others have stabilised and some have even clawed back formerly occupied territory. Encouragingly, the latter may be on-route to full recovery. The review also highlights that we now know enough to trial management options for some species. For example, it may be possible to target reintroduction efforts to habitats with few reservoir hosts of chytrid, or we may be able to manipulate the environment in ways that gives susceptible frogs an epidemiological or demographic edge over the fungus (a topic on which my own research has focused in recent years).

With that, I commend the paper to you. As always, if you’d like to read the paper but can’t get through the paywall, drop me an email and I’ll send it through.

Latest paper: Manipulating wetland hydroperiod to improve occupancy rates by an endangered amphibian

IMG_1047_tweakedWorking on wetland biodiversity takes one to some rather odd places. In my case, it has meant copious hours at the back of industrial estates, in flooded quarry pits and dodging unsavory types around urban backwaters. Hardly Kakadu or the Okavango Delta. But even my haunts are glamorous compared to some. Take the Western Treatment Plant – 11,000 ha of sewage treatment ponds in Melbourne’s south-west that processes half of cities’ effluent (a daily contribution from ~2 million people). I can assure that the WTP looks and smells just like you imagine; hardly the sought of place that should fire the imagination of wetland ecologists.

And yet it does, because the WTP is wetland of global significance. It even sports a RAMSAR listing to prove it. Twitchers are particularly smitten with the place, flocking on mass to see Brolgas, Bitterns, Red-necked Avocets, Freckled Ducks, Pink-eared Ducks and even the critically-endangered Orange-bellied Parrot. For herpetologists like me, the attraction is frogs. As well as being home to squillions of Melbourne’s more common frogs, the WTP supports arguably the single largest remnant population of Growling Grass Frogs in the country.

Our latest paper focuses on keeping the resident Growler population booming. A collaboration with Andrew Hamer of the Australian Research Centre for Urban Ecology, Ecology Australia P/L and Will Steele of Melbourne Water, the paper uses monitoring data collected by EA over four seasons to update an existing model of the metapopulation dynamics of Growlers in Melbourne, and uses this model to predict how managing the hydroperiod of key wetlands could bolster occupancy rates. Furthermore, it includes a test of this management action in the field, assessing the change in occupancy rate at ponds that received top-up watering in the final year of the study. Encouragingly, occupancy rate increased among these ponds relative to those that didn’t receive top-up watering, suggesting that managing hydroperiod could increase both the range and abundance of Growlers at the WTP.

I’ll sign off here and let you peruse the paper for the juicy details (fire me an email if you can’t get through the paywall). If you’re interested in frogs, wetlands, management experiments, simulations or Bayesian statistics, you should like this one.

Here’s cheers to Parasite Ecology

IMG_1014_tweakedJust before Easter, my old friend Michael Scroggie alerted me to the fact that the good folks over at Parasite Ecology had posted a summary of our Ecology Letters paper on the role of chytridiomycosis in frog metapopulation dynamics. They really nailed the key messages of the paper, and even included a cheeky little cartoon that links our messages around migration to contemporary US politics. Nice!

Three new papers

A quick post to plug three papers that have hit the streets over recent months. The first is a case study of interspecific variation in the phenology of advertisement in frog communities. It was a fun paper to do, drawing together my own monitoring data from across northern Melbourne and coupling it with an extensive dataset collected by Stefano Canessa and Kirsten Parris across the same region in 2009. We built models of calling seasonality and within-season meteorological drivers for the resident frogs, and used these models to assess interspecific variation in peak calling periods. The paper is now published online over at Ecology and Evolution.

The second paper is the culmination of copious hard-work by Claire Keely, whose PhD I co-supervise. After 6 months trudging through the swamps of outer Melbourne searching for Growling Grass Frogs, followed by long hours in the lab genotyping hundreds of individuals, Claire has gained important data on the genetic structure and diversity of this endangered species around the city. It is great to see this work come to fruition. You can find out more about the paper and Claire’s PhD here. The paper itself is freely downloadable from Royal Society Open Science.

Third on the list is the first paper from Lucy Rose’s PhD. Lucy is interested in conservation decision making for freshwater systems in southern Australia, and has used our beloved Growler as a case study for integrating uncertainty in cost-effectiveness analyses. Lucy tackled the rather complex case of how Growlers will respond to urban growth in the Lockerbie Precinct north of Melbourne, working her way through the myriad of conservation options for the species to identify those that not only maximize cost-effectiveness, but minimize uncertainty due to demographic stochasticity and model imprecision. Lucy’s paper is currently online early at Conservation Biology.

New paper: ‘Refugia and connectivity sustain amphibian metapopulations afflicted by disease’

I had to wait until the ripe old age of 16 to see my first Growling Grass Frog. It was under a rock in a disused bluestone quarry in the Plenty Gorge Park, next to a spring-fed and slightly salty wetland. There were many Growlers dotted around the wetland that day; some taking shelter under rocks, others soaking up the sun in a patch of Bullrush. It struck me as odd at the time. I’d already spent an unhealthy proportion of my childhood knee-deep in Melbourne’s wetlands, but had not seen a one. So why were they doing so well in the old quarry? It’s taken me twenty years, but now I know.

Today, Ecology Letters published our latest paper on the spatial epidemiology of chytridiomycosis in remnant Growler populations. The paper – a culmination of 14 years of work – shows that the impact of chytridiomycosis on Growler populations is mediated by wetland microclimate and water chemistry, being considerably lower in warm and saline wetlands. We knew from previous work (on this system and others) that the prevalence and intensity of chytrid infections declines with increasing temperature and salinity (because chytrid is sensitive to both), but our new study is the first to demonstrate that these relationships have important implications for the persistence of frogs threatened by chytrid. Using 11 years of monitoring data, we’ve shown that populations of Growlers in warmer, saltier wetlands have a higher chance of persistence through time because the prevalence of infections is low. Moreover, we’ve shown that some metapopulations of Growlers are unlikely to survive without these warmer, saltier wetlands; that is, without their refuges from disease.

An example of a disease refuge for Growlers: a big, warm and relatively saline quarry wetland

An example of a disease refuge for Growlers: a big, warm and relatively saline quarry wetland

Now, by using ‘some’ in the last sentence, you might think I’m prevaricating. But let me explain, because this leads me on to the next main finding of the study (and, in due course, will take us back to the Plenty Gorge). Both theory and long-term empirical studies tell us that metapopulations are more robust if they are bigger and better connected. Our work shows that these two things also influence metapopulation persistence for Growlers afflicted by chytrid. In short, big, well-connected metapopulations that lack strong refugia can offset higher average rates of population extinction through rescue effects and recolonisations. Think of the extinction of a Growler population as a random event in time, but with a chance that varies between wetlands according to local chytrid prevalence and other factors that stress populations. If this situation prevails, extinctions will be somewhat asynchronous in space and time, and bigger, better connected metapopulations have two advantages. First, in these systems, there is a good chance that declining populations will be rescued by migrants from neighbouring populations before they bite the dust. Second, when a population does succumb, there is a good chance that neighbouring populations persist, which can fire off migrants to recolonise the now vacant wetland. Maybe next year the roles of the rescued and the rescuer will reverse, but the metapopulation will push on regardless.

Of course, for this to work, the benefits of migration need to outweigh the costs of pathogen transmission. Epidemiologists have been concerned about this for decades, using mathematical models to explore how to this trade-off might play out in nature. Our work suggests that for Growlers – and perhaps most frogs that are sensitive to chytrid – there isn’t much to worry about. Growlers just aren’t a major player in spreading the pathogen around. There are seven other frogs in our study area which do that job, and chytrid can travel in various other ways (e.g. though water courtesy of motile zoospores, by riding on the back of crustaceans, and possibly even hitching a ride on ducks feet!). Hence, there is no real trade-off between migration and disease spread for Growlers, and connectivity proves overwhelmingly beneficial.

And with that, back to the Plenty Gorge. What of those frogs today? Well, sadly, they are no more – Growlers haven’t been recorded in the park since 2008. Fundamentally, the local metapopulation was just too small and poorly connected. Populations persisted doggedly in two quarries after chytrid arrived thanks to their environmental leg up, but they blipped out one and then the other when severe drought added an extra layer of stress. Without re-enforcements from more fortunate neighbours, permanent extinction was the only possible outcome. And so it went.

A pool on the Merri Creek that could do with a trim. Lots of shade = cold water = higher chytrid prevalence.

But let’s not finish on doom-and-gloom, because this is not a doom-and-gloom story. In fact, it’s a very positive one, because what we now know about chytrid dynamics for threatened frogs is shovel-ready information (to use a bit of Australian political parlance). Think of the possibilities for enhancing wetlands to give them refugial properties, or creating new wetlands with the right characteristics. Warm wetlands in our study area are big and deep and have minimal shading from overhead canopy and emergent rushes. Slightly salty ones are subject to ground-water influxes. So, considering building a new wetland for Growlers? Well then, make sure it is big and deep, has minimal shading vegetation, and sink a bore to feed it with groundwater. While you are at it, lop the exotic Willows that have overrun the dam next door, and get the back-hoe in and clear out the Phragmites infestations from the adjoining creek. Excellent, excellent. What next? Well, it might be time to download our code and run some simulations. You can estimate the risk of extinction for your newly rejuvenated metapopulation, and test out further habitat management options for reducing that risk. You’ll have a ball, I promise.