Difficulties in reliably quantifying platypus abundance compromise estimation of population size and its trends (Gust and Griffiths 2009). There has been no robust assessment of the population size of the Platypus either nationwide or for the key states in the species’ geographic range. Lunney et al. (2008) stated that ‘It is a common species’, but noted that ‘in general there is a surprising lack of knowledge about its abundance’. Carrick et al. (2008) considered it ‘common (though rarely abundant)’ but regarded it as ‘potentially vulnerable due to its specialised dependence on established water bodies for food and shelter’. Grant and Temple-Smith (2003) noted that the current and future conservation status of the Platypus is not easily predicted since their abundance is not readily measured. Since abundance is difficult to determine, population trends are generally poorly defined and the impacts of particular threats have been suggested and inferred, but rarely rigorously demonstrated (Gust and Griffiths 2009).
However, there have been some estimates of the number of individuals or of density for (parts of) individual rivers and for some areas. For Kangaroo Island, the size of the introduced subpopulation has been robustly estimated to be about 110 individuals (Furlan et al. 2012), with an effective population size of 11 individuals: this subpopulation was derived from a small number of individuals introduced between 1928 and the 1940s.
Platypus population densities in two reliably perennial rural streams in southern Victoria ranged from 1.3-2.1 subadults and adults per kilometre of channel (Serena 1994; Gardner and Serena 1995). In streams in high quality native forests at Lake Eildon National Park in central Victoria, densities were only 0.1-0.3 subadults and adults recorded per kilometre of perennial stream channel (Serena et al. 2001a, b). In a Tasmanian study of one catchment, catch rates were lower in headwaters than downstream reaches and in areas exposed to previous logging than in unmodified areas (Koch et al. 2006).
The extent of knowledge of population trends, and the trends themselves, have varied across its range. There is little available information on trends in Queensland. In New South Wales, Dickman (1994) noted that it had ‘declined in all regions’; and some studies have demonstrated local declines or even disappearances in recent decades (e.g. Grant and Denny 1991; Grant 1992, 1993, 1998; Rohweder 1992, Rohweder and Baverstock 1999; Lunney et al. 2008; Serena and Williams 2010d). A state-wide survey in 1987-88 concluded that its overall distribution had not changed significantly since the 1950s (Grant and Denny 1991; Grant 1992), and this pattern was broadly evident in some subsequent broad-scale assessment in 1994-96 (Grant et al. 2000). A more detailed and systematic assessment was undertaken in 2006, based on community knowledge (D. Lunney pers. comm. 2013). This concluded that there had been no sharp overall decline, but there were more locations showing decline than increase.
In Victoria, the Platypus is faring far less well, with substantial declines over recent decades. Mark-recapture studies carried out along the Wimmera River and its tributaries indicated that this system (conservatively estimated to have supported >1,500 Platypus at the time of European settlement) supported fewer than 200 individuals by the late 1990s, with numbers declining to <30 individuals by 2007 (Serena and Williams 2007b); juveniles have not been recorded in this system since 2006 (Griffiths and Weeks 2012). Capture frequency in replicated surveys in the Coliban River in central Victoria declined five-fold from 2001 to 2010, presumably due to drought (Williams 2010). It has been lost from several largely self-contained catchments, including the Cardinia Creek system in the mid-1980s (Serena and Williams 2004), the Curdies River system in the mid-1990s (Serena et al. 2002), and the Bass and Avoca River systems in the mid-2000s (Serena and Williams 2007a; M. Serena pers. comm. 2014). In the greater Melbourne area (across four hydrologically independent drainage basins) numerous subpopulations have contracted or become fragmented since 1995 as a by-product of urban or agricultural development and these are continuing to decline (Serena and Williams 2008a; Griffiths et al. 2012). Elsewhere in Victoria, both reduced flow regimes and severe flooding have contributed to the local depletion or disappearance of Platypus subpopulations (e.g. Serena and Williams 2007b, 2008b, 2010c).
There is little information on recent population trends in South Australia (Kangaroo Island). In Tasmania, trapping surveys in 2008-09 indicated that it remains widespread (Geragthy et al. 2011) and occupies a diverse array of water bodies (Gust et al. 2009). The lack of evidence for major reductions in abundance or distribution across Tasmania should be treated cautiously given the insensitivity of existing measures to detect change, the lack of systematic, rigorous monitoring for the species, and the possibility that significant localised declines occurred but went unnoticed (Gust and Griffiths 2010).