New research shows many mammals are disappearing from the landscape in northern Australia.For the past year and a half, the Australian National University study has been underway into how many mammals remain in the Top End.Researcher Mark Ziembicki says the findings show the region's environment is not as pristine as the public thinks it is."Talking particularly about the critical weight range mammals which are medium size animals," he said."Things like possums, bandicoots, quolls ... they have been declining."The last 10 to 15 years, we've had the opportunity of doing a widespread surveys and certainly those places where we have had widespread monitoring like Kakadu [and] Kapalga research through CSIRO research."They have indicated there are significant declines through many species."