Abstract from BTO Research Report No 404:

Stuart E. Newson & David G. Noble (2005)

The production of population trends for UK mammals using BBS mammal data: 1995-2003 update

1. The BTO/RSPB/JNCC Breeding Bird Survey (BBS) was expanded in 1995 to record mammals as well as birds. This was the first multi-species, annual mammal survey to be carried out in the UK. It focuses on large-sized easily identifiable species, although observers record any mammal species seen or known to be present. In this report we update Newson & Noble (2005) to generate estimates of population change for 1995-2003.

2. Annual indices of relative abundance are produced at a national scale for nine mammal species for 1995-2003 - Brown Hare, Mountain/Irish Hare, Rabbit, Grey Squirrel, Red Fox, Red Deer, Fallow Deer, Roe Deer and Reeves’s Muntjac. Comparing the abundance of these species in 2003 relative to 1995, Grey Squirrel and Roe Deer were significantly higher in 2003, whilst Rabbit, Mountain/Irish Hare, Red Fox, Red Deer and Fallow Deer were significantly lower. It is important to interpret change in abundance between 1995 and 2003 in relation to the underlying trend over this period.

3. Where data were sufficient, regional indices of relative abundance were produced for the nine English Government Office Regions (GOR) and the four countries that constitute the UK. In total indices of relative abundance could be produced for five mammal species (Brown Hare, Rabbit, Grey Squirrel, Red Fox and Roe Deer) for two or more regions. Additionally, data were sufficient to produce trends for Red Deer in Scotland and for Fallow Deer and Reeves’s Muntjac in England.

4. Population trends are produced for government Environmental Zones for the most commonly sighted species. Environmental Zones are categories of landscapes found in the UK from the lowlands of the south and east, to the uplands and mountains of the north and west. The resolution of these analyses is at the 1 km square level, and hence this approach is comparable with other mammal surveys associated with the Tracking Mammals Partnership, such as the BTO/MS Winter Mammal Monitoring.

5. There are six mammal species (Badger, Mole, Hedgehog, Brown Rat, Stoat and Weasel) for which there were insufficient count data to produce indices of abundance, but for which observers collected a large amount of information on presence/absence from field signs, dead animals or local knowledge. These data were used to examine their change in presence/absence on BBS squares over time. As discussed in previous work (e.g. Newson & Noble 2005) interpreting the data from the first few years may be difficult because they may reflect increasing awareness by the observer of the presence of a particular species. With existing data, it is not possible to assess the significance of this potential bias. However, since 2002 observers have recorded the criteria that they used for reporting presence (live animals, field signs, dead animals, local knowledge of presence from that season or live animals seen on additional visits), which should aid interpretation in the future. We present information on the change in presence on BBS squares of these six species from 1996 to 2003 and discuss reasons why caution is needed in interpreting these trends. Using geostatistical methods trialed in Newson & Noble (2005), we examine finer scale spatial patterns in relative abundance for two mammal species Grey Squirrel and Roe Deer by interpolating maps of relative abundance for 1995 and 2003, and producing maps of change for these species between years. CEH landcover data is used to improve the model fit. Because these analyses are time-consuming for the analyst as well as computationally, it is suggested that we continue to produce maps for two species per year as we have done here, until methodology for automating maps of this type can be developed.

6. Data for a large proportion of mammal species recorded by the BBS are insufficient to calculate robust indices of relative abundance or occurrence. However, these data still provide important information on the distribution of species, which in many cases are not properly monitored by any existing scheme. For most of these species, it would not be useful to produce annual maps of distribution, but distribution maps of species presence over intervals of perhaps five or ten-year blocks might be considered as more data are collected. There is also the potential for combining these data with those from other surveys and perhaps with incidental records through the National Biodiversity Network to provide a better understanding of species distribution and if temporal data were available, identify changes in distribution over time.

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