91É«ÇéƬ

Vegetation clearing

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).

River regulation

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).

Climate change

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.

Bushfires

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).