Outside of Arnold Cooke and Lynne Farrell's comprehensive study of water deer in Cambridgeshire, we have relatively few data on their population and density across their feral range in Britain. Most farmland populations are managed in some way, such that the density is probably more a result of desired numbers than the carrying capacity. Where direct management of the deer is lower, such as in fenland nature reserves, populations fluctuate according to land management practices. Dr. Cooke gave a nice example of how habitat management can have significant ramifications in his 2019/20 Deer Surveillance Report for Woodwalton Fen:
“The population increased [from 2006] up until 2010/11 because of the presence of elephant grass on Mormon land for several years, but then decreased even more dramatically due to loss of the crop and to inclement weather conditions in some years. I suspect that the population is currently at the highest level the landscape can sustain, but could change in the longer term if there were major management changes in the reserve or on the farmland – and could decrease in the short term if there was extreme weather.”
While we know from the above study, Gérard Dubost's data collected at Branféré Zoological Park, and census information from Whipsnade that populations can build very rapidly, doubling every 1.27 years in good conditions, variations in land management may help explain how water deer populations have failed to persist in some spots, disappearing a few years after their initial colonisation. This is presumably part of the explanation for water deer showing the lowest expansion of any of our introduced species since 1972 that Adrian Ward noted in his 2005 review.
It is important to note that density estimates are highly dependent on the context of habitat, season and disturbance. Within any given area there will be pockets that attract and support higher numbers of water deer than others, while biotic and abiotic factors may serve either to concentrate or dissipate deer. The abundance of water deer on an undisturbed spring grass crop, for example, has the potential to be substantially higher than that in nearby woodland, or in the same area during the autumn when there is harvesting and/or gamebird shooting taking place. In Buckinghamshire, for example, Sharon Scott has observed that the number of water deer on her farm increases in the weeks when neighbouring estates are undertaking organised shoots. Similarly, densities may be higher in nature reserves than on surrounding farmland, particularly if that farmland is used to pasture livestock. Additionally, the time of year has a considerable impact on how many deer are likely to be found in any given area, with higher concentrations often observed during the winter rut. Consequently, the following values are intended to give examples of some of the densities that have been recorded in England, and should not be presupposed to apply to habitat elsewhere, suitable for extrapolation to a county or national level, or necessarily reflect the current situation.
The highest densities recorded in England were seen at Whipsnade during the early 2000s, when the population stood at an estimated 2.4 per hectare. During the winter of 2019/20, Arnold Cooke estimated the water deer population on the Woodwalton Fen reserve to be around 0.8 per ha, although the presence of elephant grass (Miscanthus sp.) on an adjacent farm for several years had resulted in higher estimated densities, just over one deer per hectare, until its removal in 2011. In their 2008 chapter, Cooke and Lynne Farrell give average densities at both Woodwalton and on the Norfolk Broads of around 0.1 per ha, while in Muntjac and Water Deer Cooke notes that on one area of farmland in Bedfordshire the density was maintained at 0.09 per ha. Higher densities have been estimated based on counts by the Bedfordshire Natural History Society, suggesting between 0.2 and 0.5 deer per hectare during the early 1990s. No formal data exist, but I suspect some of the game estates in the Woburn region may substantially exceed this. I know of no comparable data for neighbouring Buckinghamshire or for Essex or Suffolk.
In terms of overall numbers, in 2005, the Tracking Mammals Partnership, in their UK Mammals: Species status and population trends report, gave the water deer population in England at fewer than 2,000. According to the Non-native Species Secretariat, a UK governmental body responsible for documenting and tracking non-native species, there were an estimated 1,500 living wild in Britain in 2004, the population having increased to 4,000 individuals by 2009. In the Mammal Society's Britain's Mammals 2018, the population range given for water deer was between 200 and 143,000, with an anticipated, but highly uncertain, estimate of 3,600. These figures came primarily from an analysis conducted by a team of biologists at the Animal and Plant Health Agency, led by Simon Croft, published in 2017. Unfortunately, they were based on old density data for water deer and the 3,600 covers only arable and improved and rough grassland, as wetland density information was curiously absent from the analysis. More recently, Arnold Cooke told me that he thought England's wild population was “closer to 10,000 than it is to 1,000 or 100,000” and very tentatively suggested the number was probably 7,000 to 8,000 animals. In Muntjac and Water deer, Cooke provided his updated opinion that the population is likely in the range of 5,000 to 10,000.
If we consider the appraisal that there were “no more than 5,000 water deer in China” that East China Normal University biologist Min Chen gave Richard Fautley in 2011 to be the current (or close to the current) situation, the UK's population would seem to be of conservation significance. Indeed, Fautley in his Ph.D. thesis, and Rory Putman and colleagues in their 2021 paper to the Zoological Journal of the Linnean Society, proposed that the 7,000 deer in Cooke's 2010 estimate, coupled with about 500 animals in captivity in Britain, probably accounted around 40% of the global population.
In my view, the number currently in captivity here in Britain is probably substantially fewer than 500. Whipsnade's official figures suggest 30 or so free-ranging water deer in their collection, but for argument's sake let's assume that's an underestimate and say there are 50. We can only guess at Woburn's number as they don't release their official census data, but in 2014 one of their park managers told me they aim to maintain about 30 in the park. Houton Hall is also an unknown, but it's parkland akin to Woburn and probably in the same ballpark, while Watatunga have only two individuals on their reserve. In my opinion, therefore, Britain's captive population is unlikely to exceed 200 animals, and is probably much closer to 150.
If we accept Min Chen's estimate for China and Cooke's lower 2019 estimate for England at face value, the UK population (wild and captive) might contribute at least half of the animals found both here and in China. If the huge recent guestimate of at least half a million in South Korea that was recently extrapolated by the Ministry for the Environment in 2018 is correct, the UK's contingent of water deer represents perhaps closer to one-percent of the global total. While substantially lower than the estimates suggested previously, it should not be taken to mean that England's population is of no conservation significance - quite the contrary, given that we may have a genetic lineage now extinct in the wild in China. (I appreciate there are also water deer kept in captivity in Russia and Asia, and in the wild in Russia and North Korea, and these will further reduce our percentage, but I have no accurate data on these populations.)