Dr Rosie Woodroffe and colleagues from the Independent Scientific Group on Cattle TB and the Central Science Laboratory found that, where badger densities had been reduced by culling, territorial organisation was disrupted and badgers travelled more widely. Where culling was widespread, badger densities appear low enough that, despite this increased ranging, cattle herds encounter few badgers.
However, localised culling - similar to that carried out as government policy for more than 20 years - increases movement but reduces density only slightly, so that cattle herds potentially encounter more badgers than in areas without culling. These effects also help to explain new findings on the impact of badger culling on cattle TB to be published on 14 December by the journal Nature.
According to Woodroffe: "We found that badgers in and around areas subject to culling range more widely than those in undisturbed populations, potentially increasing their contact rates both with cattle and other badgers. These results help to explain why badger culling appears to have failed to control cattle TB in the past, and should be taken into account in determining what role, if any, badger culling should play in future control strategies."
Woodroffe and her team used bait marking to assess the impact of culling on badger behaviour and spatial organisation. The study was based on 13 of the 30 trial areas enrolled in the Randomised Badger Culling Trial, including Devon, Cornwall, Herefordshire, Staffordshire, Derbyshire and Somerset. By placing bait (peanuts mixed with treacle) containing colour markers (small indigestible plastic beads) at main badger setts for 12 days, and mapping faeces containing the colour markers, the researchers were able to work out how far badgers ranged in culled compared with non-culled areas.
Badger home ranges were consistently larger in culling areas. Moreover, in areas not subjected to culling, home range sizes increased with proximity to the culling area boundary. Patterns of overlap between home ranges were also influenced by culling.
As well as providing a plausible biological mechanism explaining the apparently greater incidence of cattle TB in some areas, it also underscores the importance of ecological research in evidence-based policy making. "These ecological insights are of crucial importance in evaluating badger culling as a TB control measure. Ecological data will also be important in determining whether other management actions, such as badger vaccination or improved cattle controls, might be more effective," Woodroffe says.
A related paper by the same authors will be published online by Nature on 14 December 2005.