To date, the declines of smaller and medium-sized mammals, such as quolls, bandicoots, and native rodents, have been mainly documented in monsoonal forests and woodlands across Australia's top end -- such as those at Kakadu National Park, where the declines have been best studied.
Additional research -- including interviews of Aboriginal communities in remote areas of northern Australia -- have shown the declines to be widespread in nature, extending over many thousands of kilometers.
Now, recent research -- which has yet to be published -- suggests that the declines might even extend to northeastern Australia, to the wet tropical region of far north Queensland.
In this area, live-trapping and camera-trap studies by Sandra Abell-Davis of James Cook University suggest that the tropical bettong, an endangered wallaby-like marsupial, may also be declining sharply.
Abell-Davis studied three areas that had formerly been live-trapped for the bettongs, using identical trapping methods. She found that, on average, bettong numbers had fallen by more than 80%.
Abell-Davis emphasizes that her findings, while alarming, are still provisional. She wants to trap more extensively for the bettong and use novel genetic analyses, to see if its numbers have fallen elsewhere in the region. In fact, she is looking for volunteers to help with this important field work.
A number of possible drivers have been suggested for northern Australia's mammal declines, with feral cats and altered fire regimes being among the leading suspects.
Other suggested possibilities include foreign pathogens, changing rainfall regimes, overgrazing, foxes, and the introduced cane toad, which produces toxins deadly enough to kill virtually anything that eats it.
Whatever is going on, it raises frightening prospects for an Australian continent that has already suffered massive extinctions of its native mammals.