New modeling shows how interrupted flows in Australia’s Murray River endanger frogs


Successive dry years increase the probability of extinction and this effect increases with: increasing dry duration, decreasing wetland size, and increased average frequency of dry years at the wetland. Plots are (a) local extinction during the second successive dry year, (b) local extinction during the third successive dry year, (c) local extinction during the fourth successive dry year, and (d) local extinction during the fifth successive dry year. Credit: Ecosphere (2023). DOI: 10.1002/ecs2.4379

Flooding in Australia’s Murray-Darling Basin is creating ideal breeding conditions for many native species that have evolved to take advantage of temporary flood conditions.

Australian scientists have now developed virtual models of the Murray River to reveal a crucial link between natural flooding and the extinction risk of endangered southern bell frogs (Litoria raniformis; also known as growling grass frogs).

Southern bell frogs are one of Australia’s 100 Priority Threatened Species. This endangered frog breeds during spring and summer when water levels increase in their wetlands. However, the natural flooding patterns in Australia’s largest river system have been negatively impacted by expansive river regulation that in some years, sees up to 60% of river water extracted for human use.

Flinders University Ph.D. Candidate and South Australian frog expert Rupert Mathwin and his colleagues have built computer simulations of Murray-Darling Basin wetlands filled with simulated southern bell frogs.

By changing the simulation from natural to regulated conditions, they show that modern conditions dramatically increase the extinction risk of these beloved frogs. Their findings are published in the journal Ecosphere.

Rupert Mathwin of Global Ecology at Flinders University and lead author of the study, says the data clearly indicate that successive dry years raise the probability of local extinction, and these effects are strongest in smaller wetlands. “Larger wetlands and those with more frequent inundation are less prone to these effects, although they are not immune to them entirely. The models present a warning. We have greatly modified the way the river behaves, and the modern river cannot support the long-term survival of southern bell frogs.

“Regulation has effectively locked these wetlands into a state of perpetual bust. The research clearly shows that river regulation has been a driver of historical declines in frog numbers. These effects are further compounded by interactive stressors such as disease, exotic species, and land clearance.”

A century of government water policies has reduced flows below the critical thresholds required for southern bell frogs to persist, but it has also created the means to save them.

The Millennium Drought shifted environmental thinking in Australia and created policies that give the Murray-Darling Basin ownership of a portion of flow. Government scientists use these flows (sometimes called environmental water) to create positive ecological outcomes. This includes pumping water into dry wetlands to create breeding opportunities for southern bell frogs.

Rupert Mathwin says, “The flows that inform our model include the Millennium Drought that occurred from 1996 to 2009. During that time, south-eastern Australia experienced a region-wide reduction in rainfall and river flows, which coupled with ongoing water extraction, resulted in seven or more successive dry years in all South Australian wetlands. Without environmental water, southern bell frogs would now be extinct in South Australia.”

Corey Bradshaw, Matthew Flinders Professor of Global Ecology and co-author of the study, says because large-scale flooding can no longer occur with sufficient frequency, proper management is the only way forward. “The few remaining frog populations that persist are at sites that are actively managed through regular inundation with environmental water applied at the right times for breeding. Our findings provide a blueprint for this process.”

Rupert Mathwin says that a single dry year is not a great risk to a healthy population, but managers should ensure that wetlands with southern bell frogs experience no more than two successive dry years to reduce the risk of local extinction. “Unfortunately, without human intervention in this heavily modified system, southern bell frogs will probably go extinct.”

More information:
Rupert Mathwin et al, Modeling the effects of water regulation on the population viability of a threatened amphibian, Ecosphere (2023). DOI: 10.1002/ecs2.4379

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Flinders University

New modeling shows how interrupted flows in Australia’s Murray River endanger frogs (2023, January 18)
retrieved 18 January 2023

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