Bovine TB: Research project summary
Project WM0311: Establishment of baseline population densities for the monitoring of badgers and other selected species.
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.
A preliminary study (Defra Project WM0310), using night-time lamped distance sampling surveys, was conducted in spring 2006 to provide baseline data on badger densities to allow any subsequent changes in densities to be assessed. This project (WM0311) repeated the survey work in autumn 2006. The same areas were re-surveyed, and wherever possible, the same fields as in the spring study. The study also surveyed additional, previously un-surveyed, areas. Sampling was conducted twice during a 13-week period (1 September to 30 November). In addition, during summer (June - August), WM0311 conducted night-time surveys for hedgehogs in a sample of the areas in which spring badger surveys had been conducted.
Using distance analysis, the mean densities of badgers foraging in open pasture were estimated to be: Cornwall 2.9 badgers per square kilometre (km-2) (95% confidence limits: 2.1–4.0); Devon 4.3 km-2 (3.2–5.7); Gloucestershire 3.3 km-2 (2.4–4.6); Herefordshire 1.5 km-2 (1.0–2.4).
Estimates for mean densities of other target species in open pasture were: fox - Cornwall 3.6 fox km-2 (95% confidence limits: 2.8–4.5); Devon 4.0 km-2 (2.9–5.6); Gloucester 3.1 km-2 (2.4–3.8); Herefordshire 1.5 km-2 (1.0–2.2); hare - Cornwall 0.4 hare km-2 (0.2–1.2); Devon 2.7 km-2 (1.7–4.3); Gloucester 0.9 km-2 (0.5–1.7); Herefordshire 2.6 km-2 (1.8–3.9). Deer were recorded on pasture in all four regions. Numbers (and overall encounter rates*) were: Cornwall 34 (0.14 deer per kilometre (km-1)); Devon 183 (0.73 km-1); Gloucester 151 (0.66 km-1); Herefordshire 15 (0.06 km-1). Roe deer were the species encountered most frequently.
Hedgehog surveys were conducted in a sub-sample of the survey areas that had been used in the spring badger surveys. Surveys were designed to compare the occurrence and density of hedgehogs on amenity grassland and open pasture. Individual sample sites comprised an amenity grassland, located in or around a village, paired with a neighbouring (>500m but <1500m away) randomly selected pasture field. Individual survey areas contained between 1 and 4 amenity:pasture pairs.
Each amenity:pasture pair was surveyed for hedgehogs on two separate nights during June-August, using night-time lamped whole plot searches. The density of hedgehogs on an individual field was based on the maximum number of hedgehogs recorded during any one of the two surveys.
The occurrence of hedgehogs (percentage of fields with hedgehogs present) on pasture was markedly lower than on amenity grassland. In total, hedgehogs were recorded on 3 (2%) of 125 pasture fields, and on 32 (26%) of 125 amenity grasslands. Regionally, the percentage of amenity grasslands in which hedgehogs were recorded varied between 19% and 30%. Hedgehogs were not recorded at all on pasture in two regions (Devon and Gloucestershire).
Overall, the density of hedgehogs per field varied from 0 to 4.88 per ha on amenity grassland (mean = 0.47 ±0.09) and from 0 to 2.68 per ha (mean = 0.04 ±0.02) on pasture. The mean density of hedgehogs was significantly greater in amenity grassland than in pasture in all four regions. For amenity grassland, there was no overall correlation between mean hedgehog density within a survey area and badger encounter rate (from distance sampling). Relatively high densities of hedgehogs, however, occurred almost exclusively in areas with relatively low badger encounter rates.
The project has produced baseline estimates of autumn badger densities in open pasture against which any potential future changes can be assessed. In addition the project has provided regional estimates of hedgehog densities where none previously existed, and that represent estimates against which any future density 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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January 28, 2008