Faded out: What environments did Australian frog populations disappear from due to disease?

By looking at historical and recent frog records across Australia, including from FrogID, we reveal how Australian frog distributions have changed in response to the introduction of a deadly pathogen.

Globally, biodiversity is in trouble, with hundreds of species already extinct and others threatened with extinction. Disappearance can be swift for many species – but for others, it’s a gradual process where they disappear from parts of their range and persist in smaller, fragmented patches of habitat. For conservation purposes, it’s important to understand not just the geographic location of lost populations, but also whether species disappear from particular environments, such as cooler or wetter places. Although this is much harder to figure out, it is vital to understand, because if a species no longer persists in much of its former environmental space (or niche), it may be more susceptible than we think to other threats.

Our recent study sought to understand how the geographic range and environmental niche of frog species across eastern Australia have changed since the arrival of the amphibian chytrid fungus (Batrachochytrium dendrobatidis) on the continent. Thought to be introduced in the late 1970s, this fungus is responsible for the potentially deadly disease, chytridiomycosis, which is likely to have driven 7 Australian frogs to extinction and caused dramatic declines in dozens of others. Using the combined data, we then examined changes in the geographic area and environmental space occupied by each species after the amphibian chytrid fungus invaded Australia.

We focused on 55 frog species from eastern Australia, categorising each as either affected by the amphibian chytrid fungus or not. We obtained occurrence records of Australia’s frog species from several sources, including the Australian Museum’s FrogID database. As we needed precise records of each species to understand the environmental conditions at each site, we checked these in great detail. Thousands of recent records obtained by people recording the calls of frogs on their smart phones via the FrogID project were vital, particularly as they were so precise due to the in-built GPS of the phones. We then examined changes in the geographic area (extent of occurrence) of each species and their environmental space, particularly temperature, rainfall and elevation.

We found that the frog species most impacted by the amphibian chytrid fungus disappeared from colder, drier areas with greater temperature variability and/or from areas at higher elevation. This fits with previous research that has shown that frogs at cooler and higher elevations tended to be hardest hit – but this was the first time it had been shown across so many frog species.

Interestingly, we also found that frogs in eastern Australia experienced greater reductions in their environmental space (i.e., their niche breadth, the range of environmental conditions across which different populations are found) compared to their reduction in geographic extent after they were impacted by the amphibian chytrid fungus emergence. This was particularly true for the frog species most impacted by the amphibian chytrid fungus.

Our findings have important consequences for Australian frog species and biodiversity in general. They demonstrate that the degree of impact the amphibian chytrid fungus has on susceptible frog species is strongly mediated by the environment and that some species can persist in environmental refuges from the pathogen. If we understand the characteristics of these refuges, we can focus our conservation efforts upon them. This is especially important, as having a much narrower set of environmental conditions to call home may well increase a species’ vulnerability to other threatening processes such as climate change. Moving forward in our conservation management of threatened species we should consider not only shrinking geographic distributions, but also shrinking environmental space.