The semi-aquatic platypus (Ornithorhynchus anatinus), along with echidnas, belong to the order Monotremata, the most species-poor (n = 5) and most basal branch of mammals, which diverged from marsupials and eutherians 187 Mya¹. Platypuses have a unique combination of features, including oviparity, venomous spurs in males, electroreception used to locate freshwater macroinvertebrates, biofluorescent pelage, and multiple sex chromosomes (five pairs instead of one^2,3,4). The uniqueness and rarity of platypus features (sensu Pavoine et al.⁵) and its evolutionary distinctiveness⁶ make it arguably one of the most irreplaceable mammals existing today.

The platypus is currently listed as ‘Near Threatened’ by the International Union for Conservation of Nature (IUCN⁷), ‘Endangered’ in South Australia (National Parks and Wildlife Act 1972) and ‘Vulnerable’ in Victoria⁸.

There is increasing evidence of larger numbers of platypuses in historical times⁹ and ongoing declines and extinctions of local populations ^2,10,11. Declines have been driven by multiple and synergistic threats, including river regulation, loss and modification of habitats, climate change, pollution, by-catch mortality and predation by invasive species^2,9,10,11. Continued declines due to current and future climate change are predicted as a result of increased frequency and severity of droughts ^2,12,13, as well as elevated temperature conditions which could lead to the loss of more than 30% of suitable habitat by 2070 ^12,14.

Threats to freshwater ecosystems are commonly synergistic and are intensified by the construction of major dams that can have immediate and long-term impacts ¹⁵. Nearly half of the world’s river discharge is impacted by flow regulation and fragmentation ¹⁶. Dams pose a major threat to global freshwater biodiversity ¹⁷. Large dams form major barriers for aquatic organisms, limiting critical ecological processes, such as fish migration ¹⁸. Water impoundments behind major dams form wind-exposed, deep, and standing (lentic) ecosystems which can offer little resources for flow-dependant species¹⁹. In Australia, dams are one of the more serious threats for platypus conservation, given their potential broad impact on habitat^2,12,20. Major dams are widespread across much of the platypus’ distribution, where as many as 77% (383 out of 495) of the Australian major dams (wall height >10 m; ancold.org.au) coincide within the regions where platypuses occur (Fig. 1a; see also Bino et al.¹¹). Immediate adverse effects of major dams extend over large areas both upstream and downstream. Below major dams, altered natural flow regimes, including changing of the timing of flows and important reduction in flow volumes have been found to significantly impact platypus abundances and demographics²¹. Conditions below and above major dams represent poor foraging and burrowing habitat for platypuses, given lower productivity of macroinvertebrate prey species ^{10,22,23,24,25}.