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.

New paper: Structure and fragmentation of growling grass frog metapopulations.

PlotJust prior to Christmas, Conservation Genetics delivered us a present in the form of acceptance and rapid fire publication of a study we commenced way back in 2004. The publication process never ceases to amaze me. After years of toil collecting data, several more years of painstaking lab work, and finally the rigors of a multi-author drafting process, this paper took just weeks to go from being accepted, through proofing to online-early publication. In fact, to my astonishment, we received proofs a mere five days after the paper was accepted, two days of which were a weekend!

But back to the paper itself. In 2004 I was a shiny new PhD student, champing at the bit to decode the population biology of the growling grass frog. I began a mark-recapture study on the species north of Melbourne, and diligently went about obtaining tissue samples from each frog I captured for a later, prospective, genetics study. Two years and 800 frogs later, the opportunity arose to get that work underway. Josh Hale had just started his own PhD on the conservation genetics of frogs in urbanising landscapes, and was keen to collaborate on the growler project. Josh’s first task was to develop a microsatellite library for the species. He emerged from the lab a year later, gasping for air and 10 kilos lighter, bearing nine new microsatellite loci for our little green friends. You can read about those in another paper, found here. Josh then set about genotyping ~200 growlers from three population clusters that I sampled during my mark-recapture work. These clusters, which I prefer to call metapopulations, were distributed along the Merri Creek at roughly 5 km intervals. Each included pools along the creek, plus neighbouring wetlands such as quarries, swamps and farm dams. Josh used his microsatellite data to assess the pattern of population subdivision in this area, and to test the effect of geographic distance and urban barriers on genetic distance.

So what did we find? In short, remarkable levels of genetic sub-division. Growlers have long been thought to be highly dispersive; vagabonds that wander the landscape in search of optimal conditions. On the contrary, our genetic work indicates that while individual frogs may be able to undertake significant journeys, the majority either can’t or don’t. Josh’s work found strong genetic sub-divisions between each of the wetland clusters I sampled, and even some sub-division within population clusters. The latter is especially interesting because the distances involved are small – 2 km max. Thus, in line with our previous occupancy and mark-recapture work (described here), Josh’s genetic work suggests that the migration rates of growlers are low and strongly distance-limited.

Another important outcome was the fact that populations separated by housing estates, industrial estates or dual-carriage roads displayed relatively high genetic distances. We can infer from this that urbanisation does indeed fragment populations of growlers; something we’ve long suspected but lacked any specific evidence of. It’s not a surprising result – imagine yourself as a little green frog attempting to cross a dual-carriage highway. But it is a vital piece of the puzzle for our understanding of the effects of urbanisation on this species, and for mitigating those effects.

Where to from here? Well, now that you mention it, our molecular work continues. Claire Keely, a PhD student in the QAEG, is working on the broader genetic structure of growlers around Melbourne, the fine-scale determinants of gene flow, and the efficacy of alternate tissue sampling techniques. You can read more about Claire’s great work here. As for Josh and I, we have one more paper to go from our collaboration. It’s a gooden too – a comparison of contemporary genetic diversity in growler populations from the Merri with a now extinct population from the adjacent Plenty River catchment. Think ancient DNA techniques and pickled frogs collected way back in the 1960s. Oh yes, there’s an exciting blog post in that. Stay tuned…

Not so tough guy

Having a Whistling Tree Frog for breakfast. Photo: Claire Keely

When I first started working on Growling Grass Frogs, I thought of them as the tough guy of the Victorian frog fauna. I already knew them to be a big, voracious predator that would happily scoff just about anything that would fit into their mouths (including each other). But I also thought of them as ‘tough’ in an ecological sense. They seemed pretty general in their habitat requirements, I’d read reports of them being master dispersers, and I presumed them to be long-lived and super fecund. But after a decade of working on Growlers, I’m starting to wise up to the fact that they aren’t so tough after all.

So, from what frailties does the poor old Growler suffer? Here’s the key ones, as I see it:

  1. Living hard and dying young: If by some quirk of reincarnation you wake up as a Growler tomorrow, don’t be too concerned, because it’s not going to last very long. Despite being one of the largest frogs to grace our good state, it appears that Growlers might be one of the shortest lived. Growler tads metamorphose quickly, taking around 2-3 months to complete the journey. And that’s some journey physiologically – we’re talking growing from a 5 mm grey wriggler to a 110 mm monster taddy, before growing lungs, sprouting legs and losing a tail in the space of a few weeks. And they don’t slow down once they’re out of the water – they hit the ground running, as it were. I once marked a Growler shortly after it metamorphosed and recaptured it at adult size 54 days later. That’s some seriously fast growth by anyone’s measure! With this rapid growth comes early maturation. It appears that most Growlers are able to reproduce in the first breeding season after the one in which they metamorphosed. I’ve recorded this for males, and observed females to reach the required body size within that time frame. It’s just as well they mature so rapidly, because most Growlers are probably dead before their first birthday roles around. Using mark-recapture data collected across northern Melbourne, I’ve estimated that survival rates across a single breeding season may be 13% or lower.
  2. Habitat specificity: As I said, Growlers are superficially quite general in their habitat requirements. They turn up in a raft of wetlands; from slow-flowing sections of streams, to artificial lakes, old quarries and storm-water wetlands. What’s more, they can often be found in some pretty horrible spots. But the thing is, just because you find a few Growlers in a particular wetland doesn’t mean they are breeding there, nor does it suggest they can persist there for long. Growlers are highly aquatic, and do best in wetlands that permanent or semi-permanent. But they need more than just long hydroperiods – they require diverse aquatic vegetation, and are sensitive to high turbidity, low water temperatures, water pollution and fish predation. When Growler’s turn up at ephemeral wetlands that have recently filled, it’s probably not because they’ve been hunkering down under a rock waiting for the rain (as many of the other resident frogs are likely to have done), it’s because a few have scooted over from a neighbouring wetland that supported a population during the dry. The same is true of Growlers you might find in storm-water wetlands and the like – in many cases I’d bet my bottom dollar that these frogs came from a neighbouring, high quality wetland rather than the putrefying, trolley-ridden waters that ripple before you.
  3. Relatively poor dispersal capabilities: From the last point, you might be thinking that Growlers are indeed quite the vagabond, roaming widely in search of new opportunities. But my experience is that only a small proportion of Growlers move about, and that the distances these frogs travel are relatively small. During my PhD I marked ~800 Growlers across 19 wetlands in northern Melbourne. Of the 131 that I recaptured, the longest recorded dispersal distance was only 430 m, and the majority stayed pretty close to the spot where I first found them. Now I acknowledge that inferring dispersal rates from mark-recapture has its problems, but this concurs with our occupancy and genetic data. Both suggest that populations separated by more than about 1 km have relatively little interaction with each other. This contrasts markedly with some of the frogs with which Growlers co-occur. Take the Brown Tree Frog and the Common Froglet. Recent genetic work to the east of Melbourne has shown that populations of these frogs display considerable gene flow even when separated by distances of 8 km. It’s quite embarrassing really – here are a couple of frogs that aren’t even half the size of your average Growler, and yet they apparently give Growlers a hiding on the dispersal front.

Clearly, these traits haven’t driven Growlers into decline on their own – the species was abundant historically, and I many of these traits are hard-wired. Instead, these traits represent frailties in the Growler’s ability to cope with the environmental conditions we have thrust upon. My view is that the occurrence of Growlers was probably always quite dynamic at a regional scale. Individual populations would go through booms and busts; the busts would result from wetland drying, changes in wetland condition, or stochastic events that jointly killed off adults and tads, and the booms from colonisation of wetlands that recently filled, or became suitable for some other reason (changes in veg structure etc.). Given enough wetlands in close proximity, and some spatial asynchrony in the local conditions, these systems would have been pretty robust. But the spatial and temporal dependence inherent to such systems (called ‘metapopulations’) gives them an Achilles heel when it comes to human-induced environmental change. Put simply, metapopulations are doubly sensitive to habitat loss, degradation and fragmentation. In the first instance these processes reduce metapopulation size and connectivity, which increases the likelihood that all populations will simultaneously go bust by chance. However, habitat loss and degradation also reduces the diversity of patch conditions, which increases the chance of synchronous local extinctions during widespread perturbations (like drought). It’s no coincidence, in my view, that Growlers disappeared very rapidly from many places across their former range when two big environmental perturbations coalesced in the late 1970s: a severe drought that affected much of south-eastern Australia, and the importation of the renowned frog-killing fungus, Batrachochytrium dendrobatidis. For years we’d being setting Growlers up for a fall through wetland destruction, degradation and fragmentation, and they fell hard when drought and disease arrived in tandem.

So what does this all mean for the conservation of Growlers in contemporary landscapes? For me, it highlights just how important wetland density, diversity and connectivity are for the persistence of this species. It remains commonplace, sadly, for the wetlands upon which this species relies to be destroyed or degraded to make way for urban and agricultural development. Moreover, we continue to isolate wetlands by placing roads and other major barriers between them. History suggests that there is only so much of this that Growlers can withstand. If we don’t learn this lesson soon, we’ll just keep setting the poor old Growler up for a fall, and fall it will when the next perturbation comes along.

 Further reading:

Hale, J.M., 2010. Human-induced changes in the population genetic structure of frogs in south-eastern Australia. PhD thesis. University of Melbourne, Melbourne.

Hamer A. J., Lane S. J. & Mahony M. J. (2010) Using probabilistic models to investigate the disappearance of a widespread frog-species complex in high-altitude regions of south-eastern Australia. Animal Conservation 13, 275-85.

Heard, G.W., Scroggie, M.P., and Malone, B.S. (2012). The life history and decline of the threatened Australian frog, Litoria raniformis. Austral Ecology 37, 276-284

Heard, G.W., Scroggie, M.P., and Malone, B.S. (2012). Classical metapopulation theory as a useful paradigm for the conservation of an endangered amphibian. Biological Conservation 148, 156–166.