Threats to platypus populations are widespread across their range and synergistic. Increases in agriculture and urbanisation have led to clearing of vegetation, reducing riparian vegetation and instream organic matter
Threats to platypus populations are widespread across their range and synergistic (Bino et al. 2019). Increases in agriculture and urbanisation have led to clearing vegetation, reducing riparian vegetation and instream organic matter (Bradshaw, 2012; Evans, 2016). While platypuses are known to occur in waterways in cleared agricultural land (Lunney et al. 1998), land clearing is considered a significant threat to their habitat (Bino et al. 2019). Across the distribution of platypus, 31.5% of sub-catchments have had more than a 50% reduction in tree cover (trees 10-30 m) since European colonisation, and 18.4% of these have had a >70% reduction (Figure 20a).
Excluding the South Australian Gulf with only one sub-catchment, the Murray-Darling Basin had the greatest average proportion of tree cover loss (0.49±0.32 sd; Table 9), as was the case for the state of NSW (0.36±0.33 sd). Unrestricted livestock access to rivers has caused further degradation of riverbanks through trampling (Lunney et al. 2004). Bank erosion can significantly increase without riparian vegetation for stability, increasing sedimentation and turbidity, further degrading platypus habitat (Figure 1b).
Figure 1. a) Proportion of cleared vegetation across sub-catchments within the current platypus IUCN distribution (GeoscienceAustralia 2003b; GeoscienceAustralia 2003a) and b) the weighted average of tons of erosion per year for each sub-catchment within the current IUCN distribution for platypus (Grill et al. 2019).
Platypuses occupy urban and peri-urban environments, but declines and localised extinctions in heavily urbanised areas suggest they are sensitive to urbanisation. Platypus distribution is limited by catchment imperviousness in urban areas. They have disappeared from Melbourne’s CBD, now rarely sighted within 15 km of the city (Serena & Pettigrove 2005). However, both nest-building behaviour by a gravid female and mating activity was observed and filmed in the Yarra River at Templestowe Lower (15-16 km from the city centre) as recently as September 2020 (M. Serena, pers. comm.).
A newspaper report from the Kerang New Times in 1908 (Figure 2) indicates that on the Prince’s Bridge on the Yarra River in Melbourne CBD, 22 platypuses were captured, highlighting this decline.
Platypuses have also disappeared from the metropolitan areas of Sydney and Wollongong (Grant 1998). Urban streams have high flow variability, with extended periods of reduced baseflows and increased magnitude and frequency of high flows (Bino et al. 2019). High flow events in urban environments may increase foraging energetics (Bethge 2002) and reduce recruitment (Bino et al. 2015; Serena & Grant 2017). Decreased baseflows also reduce habitat quality and increase predation risks.
Urbanisation is also associated with increased water pollution, including litter entanglement and roadkill (Serena & Williams 1998; Serena & Williams 2010), and high concentrations of pharmaceuticals in the diet of platypuses (Richmond et al. 2018).
Figure 2. Newspaper cutting from the Kerang New Times, August 1908.
Water resource development, including the building of dams and extraction of water, poses a significant threat to platypuses. The distribution of the platypus overlaps significantly with Australia’s most regulated rivers, with dams being present in 40.8% of sub-catchments in which platypuses have been recorded (Figure 3a). Of these, 14% have more than four dams present within the sub-catchment (Figure 3b). Large dams also contribute to fragmentation between basins and rivers (Figure 3c).
The river basins most heavily impacted by dams (excluding the South Australian Gulf) were the Murray-Darling Basin (45.3% of sub-catchments have dams) and Tasmania (100%). Tasmania and Victoria were the two states most affected by dams, with dams being present in 100% and 41.3% of sub-catchments, respectively.
Significant alterations to flow regimes, including the timing and temperature of flows, can significantly impact platypus abundances downstream of these regulatory structures (Hawke et al. 2020). It is also probable that large dams impede platypus dispersal over water and land, potentially reducing genetic diversity and breeding capabilities and increasing the risk of extinction (Furlan et al. 2012). A recent study currently in progress (Mijangos et al. unpublished data) investigated genetic estimates of exchange per generation collected from platypuses in three catchments (Border Rivers, Snowy Rivers, Upper Murray Rivers). Within each catchment, platypus samples were collected from above and below large dams as well as to an adjacent free flowing river. Preliminary results indicated that the dams restrict lifetime dispersal of platypus.
This restricted dispersal is expected to have both short-term and medium-term impacts. In the short term, reduced dispersal will limit the ability of one part of the river to recolonize another part that has experienced adverse effects. In the medium term, dividing the river into two separate populations that must be smaller than the entire pre-dam population is expected to lead to loss of genetic diversity, which in turn reduces survival and breeding, as well reducing the ability of populations to respond to environmental change (Frankham et al. 2002; Allendorf & Luikart 2007). In addition, a range of water resource development projects involving the building of dams and diversion of water are planned that intersect with the current distribution of platypus (Dungowan Dam proposal).
Figure 3. a) The height of dam walls across eastern Australian, b) the number of dams in sub-catchments within the current IUCN distribution of platypus (Australian Government 1990) and c) the weighted average percent fragmentation (Grill et al. 2019) in each sub-catchment within the current IUCN distribution for platypus.
Platypuses are susceptible to predation by red foxes (Vuples vuples) and dogs (Canis familiaris) (Grant & Fanning 2007), with anecdotal evidence of predation by feral cat (Felis catus). Enclosed traps (e.g. Opera house style), which are used to capture fish and crustaceans, frequently drown platypuses that become trapped inside and cannot escape. In Victoria, where mortality was tracked and could be assigned, 56% of 186 platypus mortalities (1980–2009) were caused by drowning in illegal nets or enclosed traps (Serena and Williams 2010a). The Victorian Fisheries Authority announced a state-wide ban on enclosed traps in 2019, but they can still be used in private waters in NSW and QLD and are still used illegally in some public waters where platypuses occur. The nature of platypus foraging also makes them particularly susceptible to entanglement around their neck and torso by plastic, fishing line, and rubber bands.
Platypuses are infected by a number of pathogenic organisms (Munday et al. 1998), but the major disease impacting morbidity and mortality is caused by the fungal infection Mucor amphibiorum, currently restricted to some Tasmanian populations. The disease has a low-level, ongoing impact, with little evidence of declines in infected areas (Gust & Griffiths 2011). The prevalence of the disease was initially high (mean of surveys between 1994 and 2005; 0.295), but has since decreased (0.071), although early surveys targeted diseased areas. The disease is still present in some populations (Connolly 2009; Gust et al. 2009).
Climate change is impacting platypuses by reducing suitable habitats across their range, affecting distribution and abundances. During the recent (2017-2019) extreme drought across much of eastern Australia (in some areas the worst in over 120 years of records; BOM Webinar 18 July 2019), many incidences of platypus distress and mortality were reported in the media as well as through private communications with WIRES, zoos, and platypus conservation groups.
Reductions to river flows due to increased dry periods and increases in temperature are predicted to have a significant impact on the future survival of the species in its northern extent (Klamt et al. 2011). Drying of streams and refuge pools will increase overland movements that make platypuses more susceptible to predation and air temperatures in excess of their upper thermal tolerance of over 30°C (Robinson 1954). Increases in drought frequency and severity are predicted to reduce the total population abundance of platypuses by up to 73% within the next 50 years (Bino et al. 2020). Increasing human water demands during drought conditions will increase stress on water sources with regulation of rivers with dams likely exacerbating these impacts (Klamt et al. 2011).
We examined predicted changes to the platypus’ climatic niche (i.e., abiotic conditions) by developing a species distribution model which considered all observations of the platypus (1770-2020; 16,797 records). We modelled habitat suitability for platypuses using the Biodiversity & Climate Change Virtual Lab and the Maximum Entropy Species Distribution Modelling approach (Phillips & Dudik 2008). We considered four environmental variables of contemporary climate (Annual Mean Temperature, Max Temperature of Warmest Month, Annual Precipitation, Precipitation of Driest Quarter; 1976-2005 (Xu & Hutchinson 2013) at a scale of 1km. We then predicted future suitability under established Representative Concentration Pathways (RCPs) (Van Vuuren et al. 2011) of total radiative forcing produced by human greenhouse gas emissions resulting from different combinations of economic, demographic and institutional futures (IPCC 2018). We evaluated two future climate modes, the CSIRO Global Climate Model Mk 3.0 (GCM Mk3), (Vanderwal 2012) and the Hadley Centre Coupled Model version 3 (HadCM3), Gordon et al. 2000) to predict future climate conditions under RCP 2.4, 4.5, 6.0, and 8.5 for 2025 – 2065 at 10-year intervals. Current emissions are consistent with the RCP8.5 model (Schwalm et al. 2020).
Based on developed habitat suitability model and climate change emission scenarios, by 2055, platypus suitable climatic niche was predicted to contract between 24% (RCP 2.6) and 43% (RCP 8.5) under the HadCM3 model, or between 6% (RCP 2.6) and 17% (RCP 8.5) under GCM Mk3 model, (Figure 4) by 2055. Contraction mostly occurred in the northern and western regions of its range (Figure 4). Although significant uncertainties regarding future climate exist, models based on a species’ climatic niche likely underestimate impacts of climate change, which would increase drought frequencies and intensity (CSIRO and Bureau of Meteorology 2015) as well as impact meta-population dynamics as considered in the previous section.
Figure 4. Change in estimated climatically suitable area of under RCP 8.5 HadCM3 climate change scenarios between 2035-2065
Drought can cause rivers and creeks to reduce flows or cease flowing completely. Extended periods of drought, which dry up rivers and creeks, reduces available habitat for platypuses, decreasing foraging ability and increasing competition (Bino et al. 2019). This may force platypuses to move overland to disperse to refuge pools, where they become particularly vulnerable to predation by foxes and dogs (Grant & Fanning 2007).
Cease to flow duration has been shown to be significantly related to platypus abundance in the Melbourne region (Mitrovski 2008; Griffiths et al. 2019). Across the distribution of the platypus, river cease to flow days have been increasing in 85% of sub-catchments with available data (Figure 5). Current climate change projections indicate an increase in both drought frequency and severity, which will continue to put pressure on platypus populations.
Figure 5. Trends in cease to flow days (Australian Bureau of Meteorology 2020) for sub-catchments with available data within the current IUCN distribution for platypuses, where increasing indicates more cease to flow days.
Increased severity and occurrence of bushfires is also likely to significantly impact platypus populations due to loss of riparian vegetation and reduced water quality through deposition of ash and sediment into streams. Previous research suggests that fires that occurred in Gippsland (2006) (Serena & Williams 2008), western Victoria (Griffiths et al. 2015; Williams & Serena 2006), and near Melbourne (Bloink 2020; Armistead & Weeks 2009), had no impact on local platypus populations or their breeding. It is anticipated that in some areas, severe bushfires, in combination with drought and reduced water availability, will have a significant effect on platypuses.
The bushfires of 2019 and 2020 (Figure 27), which were preceded by a severe drought in many parts of the platypus’ range, have likely significantly impacted platypus populations in some areas. The timing of the fires may have also increased their impact, given they coincided with juvenile emergence in some regions (Grant et al. 2004), but this was likely also confounded by the drought (Serena & Grant 2017). To estimate the extent to which platypuses were exposed to bushfires, we used the predicted probability of occurrence derived from developed habitat suitability model using recent data (1990-2020). Following examination of model accuracy, we removed probabilities lower than P = 0.25. We then summed probabilities (cell size 250 m × 250 m) across all Australian bioregions that intersected with the predicted platypus distribution. Within each bioregion, we then calculated the sum of probabilities that overlapped with the extent of the recent bushfires (Environmental Resources Information Network 2020) and calculated their proportion from the sum of probabilities across the entire bioregion. We estimated that 13.56% of available platypus habitat was impacted during the 2019-20 bushfires (van Eeden LM et al., 2020).