How 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.
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.
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 (<10,000 µS/cm), consider sinking bores and feeding some wetlands with this water source. Do let us know how you get on.