It is often commented upon that water deer have spread much more slowly than other introduced cervids, such as sika or muntjac, and this certainly seems to be the case in Britain, although the pace appears to have increased in recent years. In 2008, Alistair Ward and colleagues calculated that the average annual increase in water deer distribution since their introduction had been only 2%, but an astonishing 22% of that was between 2002 and 2007. Indeed, the number of 10 km (6 mile) squares occupied by water deer had increased from 23 in 1972 to 54 in 1998 and 187 in the most recent (2016) analysis, although 54 squares where they were recorded in 2007 or 2011 didn't yield records in 2016, while 38 were recorded for the first time in 2016.
Some of this increase will be down to the coarse scale over which the country is divided for the analysis, giving the impression that the range is continuous when it's not, and that more people know what water deer are and report their sightings now than in the 1970s and 80s. Equally, however, the deer appear to have prospered in a large area of suitable habitat in coastal and Broadland Norfolk and Suffolk that has allowed them to colonise rapidly. In his 2019 book, Arnold Cooke notes the situation is slightly different in Buckinghamshire, where animals appear to be settling more quickly and expanding their core distribution south-west. Cooke suggests this may reflect densities on farmland being higher than in East Anglia, making small populations less likely to die out or move on. Overall, the 2016 data point to a general expansion westward from their core range (i.e., East Anglia and the Home Counties) into western and northern Oxfordshire and eastern Gloucestershire.
In an article to Sporting Rifle in 2014, Paul Childerley considered that in Bedfordshire “keepered ground has been the key to the CWD's survival and subsequent establishment in the wild” owing to fewer foxes and their night patrols limiting poaching and coursing. Indeed, in their 2009 book Wild Deer in Britain, Roy Harris and Ken Duff proposed that:
“Another possible explanation for their relative lack of success as wild animals is that they suffer crippling losses to predators.”
Based on data collected by Arnold Cooke and Lynne Farrell, water deer also seem to have a more specific habitat requirement and a more restricted diet than other species. Furthermore, based on my observations and those of both Richard Champion at Whipsnade and Gérard Dubost Branféré Zoological Park in France, they are much more sensitive to disturbance than muntjac.