Bovine TB: Research project summary
Project WM0310: Monitoring regional changes in badger numbers.
Project duration: 7 months
In response to potential changes in government policy towards the management of the badger population in England, Defra identified a requirement for data on the current status of the population. Such baseline data on badger numbers is a vital prerequisite to assessing any change in population status following the implementation of any new management policy.
Based on the location of farms identified by the Veterinary Laboratory Agency (VLA) as having recent recurrent bovine tuberculosis (bTB) herd breakdowns, four study regions located in south-west England were delineated: Cornwall, Devon, Gloucestershire and Herefordshire/Worcestershire/Staffordshire (henceforth Herefordshire).
Sampling was based on a series of randomly selected survey areas in each region. In each survey area, line transects were established across approximately 20 randomly selected fields. The orientation of each transect line was also selected randomly. Distance sampling involving spotlights counts along these transects at night was used to estimate population densities of badgers in each of the four regions in open habitats, particularly pastures. The technique also allowed the simultaneous collection of data on fox, hare and deer. Sampling along each transect was conducted twice during an overall 11-week survey period (13 March to 18 May).
Using distance analysis, the mean densities of badgers foraging in open pasture were estimated to be: Cornwall 4.5 badgers per square kilometre (km-2) (95% confidence limits: 3.2 – 6.5); Devon 4.1 badgers km-2 (3.0 – 5.6); Gloucestershire 4.4 badgers km-2 (3.2 – 6.1); Herefordshire 3.9 badgers km-2 (2.9 – 5.4). The relative densities of badgers in this study do not represent absolute regional abundances. Badgers only spend a proportion of their time in open, surveyable habitat, so absolute abundance will be underestimated. The derived values represent relative population density estimates in open pasture and can be applied to the assessment of potential changes in density over time in the same region.
Estimates for the mean densities of other target species in open pasture were: fox - Cornwall 2.6km-2 (95% confidence limits: 1.9 – 3.6); Devon 1.9km-2 (1.3 – 2.9); Gloucester 2.4km-2 (1.8 – 3.3); Herefordshire 2.0km-2 (1.4 – 2.7); hare - Cornwall 0.4km-2 (0.2 – 0.8); Devon 1.8km-2 (1.0 – 3.2); Gloucester 2.6km-2 (1.7 – 3.8); Herefordshire 3.9km-2 (2.7 – 5.8). Deer were recorded on pasture in all regions. Numbers (and encounter rates*) were: Cornwall 23 (0.13 deer per kilometre (km-1)); Devon 149 (0.77km-1); Gloucester 130 (0.79km-1); Herefordshire 19 (0.11km-1). Roe deer were the species encountered most frequently.
This project represents a significant contribution to the existing knowledge of regional badger population densities. It is both the geographically most extensive and regionally intensive distance sampling study, using walked lamped transects, that has been undertaken on badgers and other British mammals to date. The project has produced baseline estimates of spring badger densities in open pasture against which any potential future changes can be assessed.
Note:
* For deer sampling, the data reported are encounter rates (deer sighted per kilometre travelled - km-1) rather than population densities (deer per square kilometre - km-2). The monitoring of deer numbers was incidental to that of badgers, fox and hare, since many deer prefer habitat such as copses and plantations and would have been unavailable for detection from field transects. Therefore, the majority of the individuals in any survey area were unlikely to be detected.
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Page last modified:
January 28, 2008