1. This review covers 64 species and one sub species of terrestrial mammal known, or believed, to breed in Britain. It includes those feral species that have persisted as breeding populations for at least fifteen years, but excludes the cetaceans.

 

2. For each species there is an assessment of the current status, historical and recent changes in numbers, population trends and population threats. For most species these assessments are based on subjective rather than objective criteria because there are few species for which there are long-term population data.

 

3. For each species except Nathusius' pipistrelle a pre-breeding population estimate was calculated to provide a base-line against which to measure future changes. Estimates are provided for Great Britain as a whole and separately for England, Scotland and Wales. The results are summarised in Table 14. The minimum aim was to achieve a population estimate with an accuracy to within an order of magnitude, but most are thought to be more accurate. A code is used to identify the level of confidence for each estimate. For ten species the estimate was graded 1 (most reliable), for ten it was graded 2, for 20 it was graded 3, for 19 it was graded 4, and for five the estimate was graded 5.

 

4. Several problems were highlighted in the course of making these estimates. First, there are very few species for which an estimate of total population size was available; for most, population size was calculated either by estimating their abundance relative to other species and/or by multiplying the amount of suitable habitat by known population density estimates for those habitats. However, even this approach proved problematical for many species, either because density estimates were not available from Britain, or because these estimates were only available for a limited number of habitat types. Furthermore, most population sizes calculated in this way will tend to be over-estimates, because density estimates so derived are invariably based on a limited number of studies in some of the more suitable habitats for that particular species.

 

5. The review highlights the paucity of population data for many species of mammal, and density data are few or non-existent even for a number of common and/or widespread species such as the hedgehog, house mouse and common rat. Thus, further field studies are needed to improve our knowledge of the distribution and density of most species; only then will it be possible to refine many of the population estimates.

 

6. Absolute numbers of mammals are perhaps less important than trends in population size and degree of population fragmentation. The known or believed changes in British mammal populations over the last thirty years are summarised: two species have become extinct; eight are known, or believed, to have undergone substantial increases in range and/or numbers; nine have undergone some increase; for 23, population size was believed to have remained approximately stable; nine have undergone small declines in range and/or numbers; and for 14 there have been substantial declines. The species known, or believed, to be increasing in numbers include several already, or potentially, damaging to agriculture or silviculture, such as the rabbit, red deer, sika deer, roe deer and muntjac and several species which are of conservation concern and which previously had been reduced to low levels, e.g. otter and polecat.

 

7. The population estimates for mammals are compared to those for other vertebrates. Few species of mammal are as rare as the species of bird listed on Schedule 1 of the Wildlife and Countryside Act 1981. However, mammals are less mobile, and minimum viable populations are likely to be larger. The commonest species of mammal have population sizes an order of magnitude larger than those of the commonest species of bird; the same relationship applies for the rarest species of mammals and birds.

 

8. There are a number of population threats faced by British mammals. Of the 65 mammals included in the review, seven are known, or believed, to be threatened by competitors, seven by climate change and/or adverse weather conditions, four by disease, seven by population fragmentation or isolation, 31 by habitat changes, five by inter-breeding, 18 by deliberate killing, 25 by pesticides, pollution or poisoning, four by predation and seven by road deaths.

 

9. The conservation status and legal protection afforded to all species of mammal in Britain are summarised. See Table 15.

 

10. The conservation status of each species is discussed from a European perspective. On this basis, most of the species of mammal that are rare in Britain have larger populations in Europe. The insectivores are generally about as common in Britain as in the rest of their European range; other than the Bechstein's and barbastelle bats, which are rare throughout Europe, the British populations of bats contribute only a small proportion of the European population; of the lagomorphs, the brown hare and rabbit populations are important in a European context; of the rodents, the grey squirrel and field vole populations constitute a significant proportion of the total European population; of the carnivores and pinnipeds, the otter, badger, common seal and grey seal populations are important in a European context; of the artiodactyls, the red, sika, fallow, muntjac and Chinese water deer populations are important and the populations of Soay sheep and wild goats that are of ancient origin are of particular interest because the populations in Britain are unique and constitute about half the ancient feral caprines in Europe.

 

11. The monitoring of endangered wild mammal populations is now a statutory responsibility under European Union legislation. This review identifies those species of particular conservation concern from both a British and a European perspective and provides a basis on which to develop a comprehensive monitoring scheme for British mammals.