In recent times we have experienced similarly large declines in mammals across the tropical savannas of northern Australia - an area that is sparsely inhabited and has generally undergone relatively limited human impacts. These declines have been best documented in the country's largest and best resourced national park (Kakadu National Park in the Northern Territory). No single factor explains these rapid and severe declines, rather it appears they may be due to several factors acting individually or in combination, and affect specific taxa and populations in different areas to different degrees. More info in a recently published paper here:

http://xrl.us/bjcatp