The impact of a decade of urbanisation on a semi-aquatic mammal in a subtropical freshwater ecosystem

Abstract Context Urbanisation replaces vegetation with impervious cover, impeding water filtration and increasing runoff contamination. These changes contribute to the ‘urban stream syndrome’—a suite of negative impacts on freshwater ecosystems and associated species. Objectives Assessed how urbanisation, measured using landscape metrics related to variations in impervious cover (‘imperviousness’) impacted platypus ( Ornithorhynchus anatinus ) occurrence over 11 years in southeast Queensland, Australia. Methods Leveraging citizen science data (477 annual platypus observations, 67 sites across five catchments, 2013–2023) and satellite imagery, urbanisation was quantified using three remotely-sensed metrics: Normalised Difference Vegetation Index (NDVI), Normalised Difference Built-up Index (NDBI), and urban land cover. Five scenarios based on temporal variation in platypus occurrence and imperviousness were modelled for each metric. Results All metrics showed imperviousness negatively impacted platypus occurrence, with the strongest effect observed for increased NDBI. Occurrence declined in highly urban, sparsely vegetated areas over the study period, indicating habitat selection preference for healthy waterways and a tolerance threshold to the accumulating effects of urban stream syndrome through time. Conclusions These findings occurred despite minimal variation to the catchment landscape over the 11-year study, indicating platypus response was due to prolonged urban exposure rather than land-use change. This study supports concerns that platypus declines are being driven by urbanisation. It also presents a widely applicable approach for catchment managers to dynamically assess urban impacts in freshwater ecosystems using remote-sensing metrics and long-term distribution data collected by citizen scientists. To mitigate freshwater degradation and localised extinction risk of platypus, policy recommendations include riparian buffer protection (> 30 m) and water-sensitive urban design.