保全：オーストラリアの大規模な河川ダムがカモノハシの長期的な生存を脅かしている

Major river dams — human-made dams taller than ten metres — may fragment and isolate platypus populations and threaten the species’ long-term survival, according to a study published this week in Communications Biology.

The platypus is currently classed as Near Threatened by the International Union for Conservation of Nature Red List of Threatened Species. Platypuses spend most of their time in or near water and face threats from foxes, cats and dogs when moving over land. Although they have previously been reported climbing around small dams, the impact of large dams on the movements of individual platypuses and their populations has not been examined.

Luis Mijangos and colleagues studied the impact of major dams on platypus populations by comparing DNA collected from 274 platypuses living around nine rivers in the Australian states of New South Wales and Victoria. Five rivers — the Mitta-Mitta, Snowy, Eucumbene, Nepean, Severn — contained a major dam measuring between 85 and 180 metres in height and four — the Ovens, Thredbo, Wingecarribee and Tenterfield — were undammed.

The authors found that the genetic differences within platypus populations separated by river dams was between four and 20-fold higher than within populations living near neighbouring undammed rivers. Among populations living near the dammed Mitta-Mitta, Nepean and Severn rivers, the authors observed the greatest genetic differences within the platypus population separated by the Nepean river dam. The genetic differences within some populations separated by major dams were similar to the differences between populations living near separate rivers. For example, the genetic differences within the population separated by the Nepean river dam was greater than the differences between populations living near the dammed Severn river and the unconnected and undammed Tenterfield river. By estimating the number of platypus generations born since each major dam was completed (based on each generation being an average of approximately 7.9 years), the authors found that the genetic differences within populations separated by major dams increased with every generation.

Together, the findings indicate that major river dams may be leading to the fragmentation of wild platypus populations by acting as barriers to platypus movements and population mixing within river systems. The authors suggest that continued genetic isolation of platypus populations could threaten the species’ long-term survival by increasing the risk of inbreeding, which can make populations more vulnerable to threats such as disease outbreaks. Strategies that promote platypus population mixing, such as structures to help platypuses to climb dams or moving individuals between populations should be considered, they add.