Distribution and numbers

Change in distribution

To assess platypus distribution, we collated 16,797 distributional data points from the Atlas of living Australia state atlas databases (ACT Wildlife Atlas Records; BioNet Atlas of NSW Wildlife; Tasmania Natural Values Atlas; Victorian Biodiversity Atlas; WildNet Queensland Wildlife Data; Biological Databases of South Australia), iNaturalist, museum records (Victorian Museum, Queensland Museum, The Australian Museum and the  Smithsonian National Museum of Natural History), trove, and platypusSPOT. For analysis, each distributional data point was assigned to a sub-catchment (HydroBASIN Level 7 sub-catchment; Lehner et al. (2008)). We assessed presence in a sub-catchment based on records, assuming no sightings were indicative of the absence of platypuses while acknowledging problems associated with false positives and false negatives.

Over the last 250 years (1770-2020), platypuses have been recorded in 288 sub-catchments (885,096 km2) across eastern Australia (Figure 2a). There are 412 sub-catchments (1,103,410 km2) within the current IUCN distribution where platypuses potentially occur (Figure 2b). However, the current IUCN distribution for platypuses fails to incorporate areas where platypuses have recently (since 1990) been recorded. We provide an updated distribution for the platypus (Figure 1) which considers hydrologic features by including sub-catchments (HydroBASIN Level 6 sub-catchment; Lehner et al. (2008)) with recent (>=1990) platypus records and those which fall within the currently used distribution and also include three sub-catchments along the Murray River which connects the proposed distribution (two of which have had sightings between 1970 and 1980).

There are increasing reports of localized declines and extinctions of platypuses, but these are inconsistent across the range and difficult to quantify due to scarcity and uncertainty of empirical historical data, particularly estimates of abundances. Relying on reported observations, we quantify changes in platypus distribution using all records (1770-2020) and those over the last three generations (30 years; 1990-2020). We classify data into four periods: <1990, 1990-1999, 2000-2009, 2010-2020 (Figure 3). Increases in the number of sub-catchments over time are attributable to increased reporting rather than an increase in the distribution of the platypus (Table 1). Despite new areas with more recent platypus sightings, platypus sightings have also declined from several sub-catchments. To assess overall change, we calculated the number of sub-catchments within each period, which didn't contain any platypus records in subsequent periods.

Since 1770, platypuses have been recorded in a total of 288 sub-catchments (885,096 km2; Table 1). By 1990, there were 20 sub-catchments that no longer had platypus records in the subsequent periods, representing a distribution loss of 42,418 km2 (Table 1). Between 1990 and 1999, 17 sub-catchments lost record continuity (49,355 km2) and between 2000 and 2009, there were an additional 62 sub-catchments that didn't record a platypus in the subsequent period (150,564 km2). The total potential distribution loss of platypuses since the 1990-1999 period was 199,919 km2, representing a 22.6% decline in the total area that platypuses have been reported.

As platypuses are restricted to waterbodies, we estimated change in the length of river occupied by platypuses across this distribution. Rivers were defined to start at every pixel where the accumulated upstream catchment area exceeds 10 km2, or where the long-term average natural discharge exceeds 100 litres per second (Grill et al. 2019). We assumed that if a platypus was recorded within a sub-catchment, it could occupy all rivers within that sub-catchment. We calculated the length of rivers for each sub-catchment and compared changes since 1990 (Table 1), which indicate a decline of 22.3% in the length of rivers occupied by platypuses in the 1990-1999 period. Given increased fragmentation of rivers due to in-stream barriers, land-use change, and some evidence of localized declines in platypus numbers, the assumption that they occupy all rivers in a given sub-catchment with a record likely underestimates declines in length of occupied rivers.

Discerning between extinction of a species in an area and inappropriate survey effort for detection can be supported with probabilistic models which consider record continuity (Bino et al. 2020), supporting improved monitoring efforts. A lower number of records in the 2010-2020 period compared to the 2000-2009 period may account for some declines. However, given increased accessibility of reporting and records in 18 additional sub-catchments where platypuses had not been reported, it is possible that platypuses no longer occupy the sub-catchments they were not reported since 2010, but this warrants further investigation. Estimated declines may be greater but cannot be quantified given the significant knowledge gaps (i.e., areas without any platypus observations) across much of the species potential range (Figure 2b). Some sightings in South Australia (Yorke Peninsula and Fleurieu Peninsula) and western Victoria were possibly false identifications, given platypuses are assumed to no longer occupy these areas, warranting systematic confirmation. The inclusion of likely false positive data in this analysis may mask further declines in some sub-catchments.