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Platypus populations impacted by large river dams are more vulnerable to threats, genetic study finds
The platypus is possibly the most irreplaceable mammal existing today. They have a unique combination of characteristics, including egg-laying despite being mammals, venomous spurs in males, electroreception for locating prey, biofluorescent fur, multiple sex chromosomes, and the longest evolutionary history in mammals.
Platypuses are a threatened species in some Australian states and their conservation is of concern more broadly, due to known decline in their populations.
A new study published in the international journal Communications Biology examined the genetic makeup of platypuses in free-flowing and nearby rivers with large dams in New South Wales. These included the free-flowing Ovens River, along with the dammed Mitta Mitta River, and the free-flowing Tenterfield Creek, along with the nearby Severn River regulated by a large dam.
The study found that large dams are significant barriers to platypus movements. This was reflected in greater genetic differentiation between platypuses above and below large dams compared to rivers without dams. Importantly, this genetic differentiation increased over time since the dam was built, reflecting the long-term impacts of the dam.
"We extracted the DNA from the blood collected by our Platypus Conservation Initiative researchers at UNSW. By using thousands of molecular markers, we were able to identify a strong signal indicating that genetic differentiation increased rapidly between platypuses below and above these large dams,” said lead author Dr Luis Mijangos, a former UNSW PhD student who is now at the University of Canberra.
Professor Richard Kingsford, Director of the UNSW Centre for Ecosystem Science and one of the paper’s authors, said, “This is a profound result with significant implications for platypus conservation.
“We've long suspected that prey can restrict platypus movements, but this is the ‘smoking gun’. These animals just can't get around big dams."
This movement restriction of platypuses separated by large dams means there is limited or no gene flow between groups, making these separate populations increasingly vulnerable to threats. There is increased possibility of inbreeding depression, loss of adaptive genetic variation, failure to recolonise areas where local extinctions have occurred, and failure to disperse to areas with more suitable conditions.
"We know that platypuses are declining in many parts of their range in eastern Australia, affected by many threats. This study identifies one of the main threats to this iconic species,” said Dr Gilad Bino, leader of the Platypus Conservation Initiative at UNSW Sydney and another author of the study.
“There is still much we don’t know about the ecology of the platypus, but given its international status as a monotreme, it is increasingly vital that we understand and manage the threats to this unique species.”
The authors recommend that water conservation and management planning should consider alternative approaches to large dams. These could include storing water in off-river reservoirs and implementing strategies to reduce the effects of dams, such as the artificial relocation of individual platypuses between groups above and below dams, or the construction of passage structures that increase dispersal.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Dr Melody Serena is a Conservation Biologist at the Australian Platypus Conservancy
Research has shown that platypus can disperse competently across land between neighbouring rivers:
- A genetic study found that 11% of the animals in neighbouring river systems in New South Wales were first-generation migrants that had travelled overland between the two systems.
- Populations occupying rivers on different sides of the Great Dividing Range remain genetically similar, implying that gene flow occurs across the Divide.
In addition, platypus can circumvent natural waterfalls up to 30+ metres high and manmade weir walls up to at least 10 metres high.
The new study by Mijangos et al. is therefore mainly of interest in indicating that a different rule may apply when a platypus encounters a very large weir (more than 70 metres high) as compared to either small weirs or natural barriers.
However, the good news is that the study also confirmed that inbreeding has not yet actually increased due to restricted movement – platypus populations on both sides of study weirs remain genetically diverse.
Two important questions remain:
- How much do weir walls in the unstudied size range (between 10 and 70 metres) restrict platypus dispersal?
- Apart from installing fishways, what steps can be taken to improve platypus passage around weirs in general?
Euan Ritchie is a Professor of Wildlife Ecology and Conservation at Deakin University
Few animals are as peculiar and globally iconic as the platypus. Tragically, the work of Luis Mijangos and colleagues shows that this egg-laying, venomous mammal, may be suffering reduced genetic diversity due to large dams impeding the movement and dispersal of individuals.
Given Australia’s already abysmal record for conserving mammals, this study is yet another siren that must be heard and acted upon. Australia’s freshwater ecosystems and species are vital for people, communities, and the environment more broadly. Maintaining functioning and healthy aquatic systems for platypus, by removing barriers or installing structures that allow them to move around, through or over barriers, will likely benefit other aquatic life too.
Many aquatic and semi-aquatic wildlife species are already under pressure due to reduced water flows, pollutants, climate change, invasive species and other threats. We must do everything we can to maintain healthy and functioning ecosystems that allow animals to live in and move through landscapes.
Jack Ashby is Assistant Director of the University of Cambridge’s Museum of Zoology, and author of Platypus Matters: The Extraordinary Story of Australian Mammals
A key challenge with platypuses is that people don’t necessarily expect to see them, because they are so elusive. This can make it easy to miss when their populations have declined. In reality, we now know that platypus numbers are far lower than historical records show they were in the past.
This new study adds incredibly valuable information to the list of threats that platypuses are facing. Knowing the roles that dams play in keeping populations apart, with all the risks that this brings to their long-term survival, highlights the need for land-managers to take action to maintain platypus diversity around dams.
Platypuses have evolved an astonishing range of adaptations for living their lives in Australian freshwater habitats. This includes the ability to hunt with their eyes and ears closed underwater, by detecting the electrical impulses given off by their prey’s muscles – a skill unparalleled by other mammals. Their hands act like Swiss Army Knives, with different foldaway tools for digging, swimming and walking; and the males are the world’s only known seasonally venomous animals – a feature they use for competing over mates during the breeding season.
Any threat that puts this amazing mammal at risk should be decreased as far as possible, and this paper provides options for mitigating the risks posed by dams.
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