• To test for the possible role of such a component, we have divided the dataset into the five broad classes of vertebrates: mammal, bird, reptile, amphibian, and fish.

• There is mounting evidence that small mammal populations in northern Australia – a region that is considered to contain the largest area of intact tropical savanna left in the world – are in rapid decline (Woinarski et al., 2011).

• Australia supported a wide range of native mammal species that were once widespread and abundant (Bilney et al., 2009), a number of which are now either extinct or are rare and occupy less than 1–5% of their former range (Dickman, 2007).