New paper: Connectivity over a disease risk gradient enables recovery of rainforest frogs

ejytbgcu8aaey1wCold, wet, exhausted. It’s 2AM and you’re 1000 m up a rainforest ravine in the Wet Tropics. You’re finally horizontal, but your hammock – saturated and starting to smell of mildew – offers little prospect of sleep. Outside, in the pitch black, the stream you’ve been relentlessly climbing cascades over boulders that it has been wearing down for millennia. But over the white noise of water on granite, you can hear a frog calling “wreek wreek wreek”. It’s the Common Mistfrog, and you’ve just detected the species high atop the jungle-clad Mt Lewis. In the process, you’ve confirmed that this species is winning the battle against chytridiomycosis, reclaiming a footing in the upland rainforests from which it disappeared two decades earlier.

This tale – which sounds straight out of the choose your own adventure novels I devoured as a boy – was Sara Bell’s reality in 2013. Sara, then a postdoctoral fellow at James Cook University, was on an audacious mission to climb five rainforest streams across the Wet Tropics in search of frogs. Under the guidance of Lee Berger and Lee Skerratt, Sara was seeking to document the contemporary elevational range of four species in particular, each of which had each been knocked out of the uplands when chytrid fungus invaded during the early 1990s. Sara’s study sought to assess the degree of recovery in these species. Were they still stuck at low elevations, in the warmer environments in which chytrid is both less prevalent and less virulent? Or had they managed to recolonise environments higher up, where the cooler conditions better suit chytrid and constrain the immune system of their ecothermic hosts?

Our latest paper answers these questions, and several others. In 2016, I came onboard Sara and ‘The Lees’ ambitious project, being called upon to help crunch the resulting data. Sara had amassed not just records of where frogs were and were not detected on her epic elevational transects, but with the assistance of several crazy-brave assistants, had swabbed frogs for chytrid up and down the mountains, and placed out air and water temperature loggers to monitor the microclimate at each 200 m elevation step. My job was to quantify the patterns of infection prevalence and intensity, link these to the microclimatic variation across the elevational gradient, and assess the correspondence between chytrid prevalence and the contemporary distribution of each frog species. With such a hard-won and significant dataset already amassed, I was an easy recruit.

sara-on-creek-1  2013_05_22-19

Sara risking life and limb for science. Waterfalls be damned!

What Sara’s data reveal is that three species appear to be winning the battle against chytrid. The Common Mistfrog (Litoria rheocola) was found up to 1000 m above sea level, the Waterfall Frog (L. nannotis) was found up to 800 m asl and the Green-eyed Tree Frog (L. serrata) was found up to 1200 m asl. Each was previously known from up to or above 1200 m, but dropped out of high-elevation monitoring sites as chytrid swept through in the 1990s. Hence, the data suggest recolonisation is well underway in some locations, which in turn suggests that these three species have developed some resistance to chytridiomycosis. The additional data Sara collected give us insights into the extent to which this has occurred. They show that chytrid still appears to constrain the distribution of two of these species – L. rheocola and L. nannotis – with a strong negative relationship between mean monthly temperature and chytrid prevalence, and a negative relationship between chytrid prevalence and the probability of occurrence. Hence, chytrid prevalence increases as elevation increases (due to cooler temperatures at higher elevations), and the highest elevations are still not quite attainable for these species as a result. In turn, we can surmise that whatever mechanisms of resistance these frogs are evolving, they have not yet fully overcome the environmental determinants of chytrid virulence that produced the elevational decline in the first place. The frogs are still struggling in chytrid’s environmental hitting zone, and recovery is a work in process.

But what about the third species that has shown signs of recovery, L. serrata? Well, Sara’s data suggest this species is effectively now reached ‘chytrid normal’ (to steal a COVID analogy). This species displayed a high prevalence of infections (47%) and there was no attenuation of infection loads, meaning frogs were just as likely to have heavy infections as light infections. Both are patterns we see in species that are asymptomatic to chytrid – lots of individuals are infected and some have high burdens, but chytrid has lost its killing power. Indeed, we found that the probability of occurrence of L. serrata actually increased with predicted chytrid prevalence, with the host and the pathogen sharing a similar preference for higher, cooler and wetter rainforest. Chytrid can no longer control this species’ distribution, and its been freed to reclaim the deepest, darkest jungle that it always preferred.

2013_05_23 (1)Rainforest glamping? Not at 1000 m up a ravine, with blood oozing from myriad leech bites, battered shins and little more than re-hydrated porridge awaiting for breakfast.

Now, observant readers are wondering about the fourth species on Sara’s list of targets. Well, the data suggest that this species – the incredible Australian Lacelid (Litoria dayi) – remains in chytrid’s thrall. Sara didn’t detect the species above 400 m asl – under half its known elevational range pre-chytrid. Furthermore, only two individuals swabbed had the pathogen, and infection loads were low in these two frogs. In short, L. dayi is persisting in lowland refuges away from chytrid’s full impact. It does not appear to have gained any tolerance for the pathogen, and remains stuck in lowland environments where the likelihood of infection – and particularly getting a fatal infection – is low.

So, what does this all mean for our understanding of chytrid impacts in Australia, and management of frogs threatened by it? First, our study provides further evidence that some frogs that took a strong initial hit from chytrid are starting to recover, as has been witnessed in two other Australian species, one of which is the threatened Fleay’s Barred Frog (Mixophyes fleayi). Second, our study shows that environmental refuges – areas in which the prevalence and/or virulence of chytrid is relatively low – have been crucial to the persistence of these rainforest frogs, as has habitat connectivity, which has facilitated recolonisation as resistance has developed. This reliance on environmental refuges and habitat connectivity mirrors the situation for my beloved Growling Grass Frog (Litoria raniformis) and its sister-species, the Green and Golden Bell Frog (Litoria aurea). Third, our study reveals that some species will remain restricted to environmental refuges, even while other species recover, and this may lead to both a permanent change to the species’ niche and a heightened conservation focus on protecting these crucial refuges. Fourth, the work lays the ground-work for attempts to hasten recovery of the target frogs. For example, among Sara’s target species, translocation to unoccupied sites could be pursued for L. dayi and L. nannotis, both of which were not detected at streams from which they were historically known – they appear to have been knocked out entirely by chytrid along these streams, and have no capacity to naturally recolonise them, being separated from remnant populations by cleared land. Sara’s work has shown that these frogs could survive in lowland sections of these streams, and L. nannotis could push back up into the uplands, particularly if the founding individuals were taken from recolonised upland sites, where some form of resistance is likely to have developed.

Chytrid impacts remain a key challenge for Australian frog conservation, with some species still losing the battle. The Southern Corroboree Frog and Baw Baw Frog, for example, remain in dire straits. However, positive news is accruing, and Sara’s study adds to this story. The work shows that natural recovery is underway for some frogs hit hard by chytridiomycosis, and that there are tangible options to both facilitate recovery and shore-up refuge populations. Gone are the days of hand-wringing about our incapacity to tackle this threat. Sara’s study is one of a several to show that when we gain an understanding of the pathogen, and host responses to it, management actions are revealed. We now find ourselves in a real-world ‘choose your own adventure’ – we need to experiment with alternate management actions, and find the combination that produces the happy ending we all so desperately want for these frogs.

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