2.2 The Scottish populations of more than one third of native land mammal species are thought to be in decline [Figure 1]. These include the red squirrel and water vole, both of which have shown a progressive decline in population and range size throughout Great Britain, and are consequently Biodiversity Action Plan (BAP) priority species. Species whose population trends are particularly poorly known include Scotland's eight bat species, two of which, the common and soprano pipistrelle, have only recently been recognised as separate 'cryptic' species.
Figure 1. Recent, likely population changes in a) native and b) non-native land mammal species. These figures are based largely on subjective assessments and should be treated with caution.
2.3 Pollution and habitat loss are considered to be the commonest threats to Scottish native land mammals¹ [Figure 2]. Forms of pollution have been cited as a threat to 11 species, either through poisoning, or through their effects on food supplies. Nonetheless, habitat fragmentation and loss, particularly the disappearance of woodlands, hedgerows, grassland and marginal land, probably constitute a more serious threat to mammals. One form of habitat loss affecting bat species is a recent reduction in access to buildings, on which some species depend for roosting or nursery sites.
2.4 At least seven land mammals are considered to be threatened by introduced species or sub-species. In combination with habitat fragmentation, predation by American mink has brought about a substantial decline in the water vole population. Changes in agricultural practices have also lead to declines, for example, through overgrazing (affecting the mountain hare, pygmy shrew and field vole) and stubble burning (affecting the wood mouse), which still continues in parts of Scotland.
2.5 Some 35% of Scotland's land mammals are non-native. They include long-established species such as the rabbit, brown hare and Orkney vole, as well as more recent arrivals like the muntjac and sika. Trends in introduced land mammals are broadly similar to those of native species, although proportionately few non-native species are thought to be declining, and a slightly higher proportion are of unknown status. Declining non-native species include the brown hare, a BAP priority species whose numbers have dropped substantially since the early 1960s.
2.6 At least two Biodiversity Action Plan priority mammal species in Scotland are partially threatened by non-native species.
• The red squirrel is now largely confined to Scotland and Ireland as a result of habitat reduction and fragmentation, and competition with the grey squirrel. Red squirrels are usually displaced within 15 years of the arrival of grey squirrels, appearing to suffer competitive exclusion by a species better adapted to fragmented British woodlands.
• A national survey in 1989-90 failed to find signs of water voles in 67% of previously- occupied sites. A further survey in 1996-98 indicates that this trend had continued, with a further loss of 67.5% of remaining sites. The species' decline has been attributed to several factors, including mink predation, habitat loss, fragmentation, disturbance and pollution.
Figure 2. The number of native mammal species considered subject to various forms of pressure. Note that bat species have been excluded, as threats to individual bat species are poorly documented.
¹ Excluding bats, for which threats to individual species are poorly documented.
