The carnivorous diet and substantial food requirements of the platypus limit the number of individuals that can exist in a given area. For example, platypus population density in the Shoalhaven River upstream of Braidwood in New South Wales has been estimated to be 12.4-14.5 animals per kilometre of channel (reviewed in Serena and Grant 2017). Elsewhere, platypus population density has been estimated to be 1.2-2.1 animals per kilometre of channel in five suburban or outer suburban creeks near Melbourne, and from 2.0-3.6 animals per kilometre of channel in two relatively pristine creeks located in Flinders Chase National Park on Kangaroo Island (reviewed in Serena and Grant 2017).
According to the IUCN Red List, between 30,000 and 300,000 platypus are thought to have lived in Australia in 2016. The high degree of uncertainty in overall population size reflects the following facts:
First, platypus mark-recapture studies (currently the only reliable technique for estimating how many platypus occur in a given area) are logistically demanding to carry out, and can only be effectively conducted in a fairly limited range of platypus habitats. This in turn has restricted the number of water courses where such studies have been conducted.
Second, like most species living in the wild, platypus numbers are expected to fluctuate through time. Numbers will drop if mortality rises, breeding declines and/or less habitat or food is available (for example, when surface flow declines during drought), and rise again if these factors improve (for example, when surface flow increases with higher rainfall). Based on what is known about platypus fertility and age at maturity, a population that has lost many members during an extended drought should in theory be able to approximately quadruple in size in as few as 5 years once wetter conditions reliably resume.
Based on three accounts in 19th century Australian newspapers of 10 to 18 platypus being shot or speared “in a few hours”, “on an expedition” or “in one day”, Hawke et al. (2019) concluded that “historic (sic) accounts of platypus numbers during the 19th century far exceeded contemporary numbers”. In response, Carrick et al. (2019) noted that similar numbers of platypus are still sometimes captured in a few hours of mark-release survey effort by researchers today. For example, 10 to 13 animals have been recorded in mesh nets set in single pools of two different rivers in New South Wales, and 21 animals have been recorded in fyke nets set along 1.5 kilometres of the Buchan River in Victoria. Thus, although the early newspaper articles are of some interest, they do not convincingly demonstrate that platypus numbers have declined severely over time. In fact, if anything they appear to provide evidence that platypus abundance in high quality river habitats is much the same now as in the mid- to late 1800s.
Photo courtesy of M. Chalk
LITERATURE CITED
Carrick F, Goldney D, Grant T, Serena M and Temple-Smith P (2019) Limitations on the use of historical and database sources to identify changes in distributon and abundance. Response to A silent demise: Historical insights into poulation changes of the iconic platypus (Ornithorhychus anatinus). Global Ecology and Conservation 20, e00777.
Hawke T, Bino G and Kingsford RT (2019) A silent demise: Historical insights into poulation changes of the iconic platypus (Ornithorhychus anatinus). Global Ecology and Conservation 20, e00720.
Serena M and Grant TR (2017) Effect of flow on platypus (Ornithorhynchus anatinus) reproduction and related population processes in the upper Shoalhaven River. Australian Journal of Zoology 65, 130-139.