Can the common Western Pygmy-Possum be excluded from nest boxes to the benefit of the rare Little Pygmy-Possum in bushfire recovery?

The Black Summer bushfires of 2019–2020 that burnt half of Kangaroo Island, South Australia, raised concerns about the status of the rare Little Pygmy-Possum Cercartetus lepidus. A bushfire recovery nest box program revealed that only the Western Pygmy-Possum Cercartetus concinnus (9.2–10.2 g) occupied boxes with 20-mm entrances (902 cavities monitored on 13 properties) and preferred them (72.4% of observations at a Dudley West property) to those with 16-mm entrances (27.6%). This more common species may have outcompeted C. lepidus (8.4–9.0 g); therefore, we tested different nest box entrance sizes in an effort to exclude C. concinnus. A 1-mm difference in the entrance diameter of boxes may favour C. lepidus over C. concinnus. Boxes that accommodate C. concinnus may benefit this species to the detriment of C. lepidus. Very small differences in box entrance size could have implications for conservation, ecological research, genetic selection, and animal welfare.