Contrary to what you might imagine, our understanding of the changing fortunes of Britain’s mammal populations is far from complete. In fact, it turns out that we have very little knowledge of just what has been happening to many familiar mammal species over the last two decades. This lack of information contrasts with what we know about birds, primarily because mammals tend to be more secretive than birds and, therefore, that much more difficult to monitor at the population level. Additionally, many of our mammal species are nocturnal in habits or occupy habitats that make them hard to observe.
The information that we do have tends to come from periodic national surveys, but these are usually expensive to run and so only tend to take place once every few decades. Annual monitoring, which can be key to understanding how and why populations change, is limited to just a handful of species, a number of which are actually monitored by birdwatchers participating in national surveys, like the weekly BTO Garden Birdwatch or the annual BTO/JNCC/RSPB Breeding Bird Survey. These birdwatchers have been kind enough to take on some additional recording of other taxa. Such surveys have underlined the decline in hedgehog populations nationally and, more recently, have flagged up a worrying decline in our rabbit population, which fell by 48% over the period 1995 to 2012. It has also been possible to detect regional declines in some of the more widespread species. For example, the fox population in England fell by 27% over the same period.
Some species have increased in number, with one group – the deer – showing a particularly pronounced increase in their populations. Living in Norfolk, we will all be familiar with the increasing numbers and distribution of muntjac, an introduced species that became established here after escaping from private collections. Nationally, its numbers have increased by a staggering 191% since 1995. Less dramatic, but still significant, increases have been charted for red deer (up 71%), fallow deer (up 89%) and roe deer (up 60%). Such increases have already had an impact on habitats and the other species that use them, underlining the importance of collecting reliable information on a regular basis to support conservation action.