 Two sites were chosen to study the effects of cane toads on the northern quoll , one of which was at Mary River ranger station , which is located in the southern region of Kakadu National Park . The other site was located at the north end of the park . In addition to these two sites , there was a third site located at the East Alligator ranger station , and this site was used as a control site , where the cane toads would not interact with the northern quoll population . Monitoring of the quoll population began at the Mary River ranger station using radio tracking in 2002 , months before the first cane toads arrived at the site . After the arrival of the cane toads , the population of northern quolls in the Mary River site plummeted between October and December 2002 , and by March 2003 , the northern quoll appeared to be extinct in this section of the park , as there were no northern quolls caught in the trapping trips in the following two months . In contrast , the population of northern quolls in the control site at the East Alligator ranger station remained relatively constant , not showing any symptoms of declining . The evidence from the Kakadu National Park is compelling not only because of the timing of the population of northern quolls plummeting just months after the arrival of the cane toad , but also because in the Mary River region 31 % of mortalities within the quoll population were attributed to lethal toxic ingestion , as there were no signs of disease , parasite infestation , or any other obvious changes at the site that could have caused such a rapid decline . The most obvious piece of evidence which supports the hypothesis that the invasion of the cane toads caused the local extinction of the northern quoll is that the closely monitored population of the control group , in the absence of cane toads , showed no signs of decline . 
