Okavango Delta: at least 11 wildlife species are in serious decline

According to an article published in the online edition of the South African Mail & Guardian, the wildlife population in the Okavango Delta in Botswana is under serious thread.
The Botswana Department for Wildlife and National Parks and Michael Chase and Kelly Landen of Elephants Without Borders found out that at least 11 species in the Delta are in serious decline. The scientists of Elephants Without Borders compared aerial surveys dating back to 1993 with recent surveys undertaken by them and found out that:

• wildebeest numbers have shrunk by 90%
• giraffe numbers have shrunk by 65%
• tsessebe numbers have shrunk by 83%
• lechwe numbers have shrunk by 58%
• zebra numbers have shrunk by 53%.
• warthog numbers have shrunk by about 80%
• roan antelope numbers have shrunk by about 80%
• ostrich numbers have shrunk by about 80%

Michael Chase and Kelly Landen also found out that from 1999 to the present the running average for total species decline is about 61%.
It appears unclear what exactly is causing the catastrophic decline, but it seems that the shrinking numbers of wildlife in the Okavango Delta is a result of "a complex mixture of local causes", including human encroachment, game fences and poaching. Another factor could be fire. Much of the area surveyed in the late dry season of 2010 was burning.