Hey frog! … A tale of climate, drought, and small poisonous frog

Comment by Joern: Today we have a guest post by Ben Scheele from The Australian National University. Ben is reporting on a recent paper we published (with several other authors) in the ESA open-access journal “Ecosphere”.

By Ben Scheele

Species response to climate change is receiving an ever increasing amount of research attention. However, understanding how species are likely to be affected by changes in climate is not a simple task.

One of the main challenges is that the most devastating effects of climate change are only likely to be felt decades into the future. To overcome this stumbling block many studies utilise broad-scale simulation modelling to predict the impacts of climate change across a range of species. While useful, these studies are often limited in providing local-scale insights for individual species. In light of this, an alternative, complementary approach is to study species’ responses to climate extremes. Understanding how species respond to climate extremes is likely to provide an insight into how they will be affected by long-term shifts in climate. In addition, there is a growing concern that increasingly intense climate extremes may be a more serious threat than gradual mean shifts, and therefore knowledge on how species respond to climate extremes is crucial in its own right.

In 2010, at the end of the longest and most severe drought on record in south-eastern Australia, we set out to investigate how the endangered northern corroboree frog had faired. The strikingly coloured, highly poisonous corroboree frogs are peculiar little amphibians; they rarely enter water, they build and territorially defend terrestrial nests, they ‘run’ instead of hopping and they respond with their threat call to a person yelling ‘hey frog’! They also possess several biological characteristics that make them particularly vulnerable to prolonged drought including low fecundity, reliance on ephemeral ponds for breeding (8 months larval development), small geographic range and previous declines associated with the disease chytridiomycosis.

To start with we used 13 years of annual monitoring data to classify sites as locally extinct or present. At locally extinct sites no frogs were recorded in the four years leading up to 2010. This cut-off was biologically relevant because the corroboree frog takes four years to reach sexual maturity and has a sexually mature adult lifespan of four years. During the breeding season each site was surveyed three times and an analysis of survey data confirmed very high detectability. After the surveys were completed we measured habitat variables to investigate whether population persistence during the climate extreme was related to breeding habitat characteristics. We focused on variables related to site wetness because suppressed breeding, egg desiccation and tadpole mortality from early pond drying have been identified as potential factors affecting recruitment. We then investigated if these variables were associated with corroboree frog presence or absence.

Our results clearly suggested a lack of water at extinction sites. Overall, 42% of monitored sites became locally extinct (with no recolonisation during 2011 or 2012). In the analysis extinct sites were characterised by fewer ponds, limited water and tree invasion. Although habitat measurements were only taken in 2010, significant difference in variables such as tree invasion indicate that extinct sites have experienced a prolong period of desiccation. At some sites, tree invasion was so thick that the previously tree-free wetland resembled a young forest! This prolonged desiccation is likely to have caused local extinctions through the gradual attrition of adults without the recruitment of juveniles. Finally, as field work came to a conclusion, the drought broke in late 2010. However, in a cruel twist of fate, record flooding during the 2012 breeding season prematurely flooded the corroboree frog’s terrestrial nest sites and resulted in egg mortality approaching 100% at some sites. Although, the impact of this flooding will only become apparent over time, given the small size of many of these populations it is likely to be significant.

With south-eastern Australia predicted to experience increases in drought intensity and frequency, the corroboree frog faces an uncertain future. Our results also suggest that, at least in the shorter term, intense climate extremes may have a more significant effect on species than gradual shifts in average temperature or precipitation. More broadly, there are many other amphibian species that use ephemeral or temporary ponds in regions predicted to experience increasingly intense and frequent droughts and our results indicate that increased frequency of failed recruitment is likely to be an emerging threat.

With climate change already affecting species such as the corroboree frog an important question is: Are there any possible mitigation strategies? Based on observations of corroboree frogs readily using ponds that originate from wombat diggings, one simple and potentially effective strategy could be to manually deepen breeding ponds. However, such a strategy would require careful examination to establish its effectiveness and off-target impacts. Ultimately, unless humanity can address the drivers of climate change, the corroboree frog is likely to be just one of countless species pushed towards extinction.

If you are interested in finding out more details about this research you can read the complete (open access) article published in Ecosphere here: http://www.esajournals.org/doi/pdf/10.1890/ES12-00108.1



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