An urban evolutionary island
In central Melbourne, the Yarra River flows, brown and busy. It seems an unlikely place for one of nature's most reclusive animals, the platypus (Ornithorhynchus anatinus). Yet, a small population persists here, separated from their upstream relatives. Genetic studies reveal these urban platypuses are an isolated group, cut off by weirs and other man-made barriers. This isolation has created a distinct genetic cluster, effectively a landlocked island in the middle of a metropolis.
This population faces pressures unknown to their counterparts in cleaner, more remote streams. Urban stormwater runoff introduces pollutants and causes sudden, high-velocity flows that can destroy riverbank burrows and wash away their food sources. A 2018 study found that 48% of the Yarra catchment's waterways provide 'very poor' habitat conditions for the platypus. The quality of the vegetation on the riverbanks, important for stable burrows where they raise their young (called puggles), is also in poor condition in the more urbanized lower reaches of the river. Despite these challenges, the platypuses' continued presence shows an adaptation to city life. Their numbers in the lower, urbanized sections of the Yarra have declined, and they are now considered a vulnerable species in Victoria.