By Paul Rincon
Science reporter, BBC News
Culling badgers in order to control bovine tuberculosis (bTB) can cause a doubling in fox numbers, UK government scientists have found.
Badgers are blamed for the spread of TB among cattle herds
This could impact on livestock farming and conservation, the authors write in Biology Letters journal.
The researchers looked at effects on foxes during the badger culling trials in England between 1998 and 2006.
Their figures show that intensive culling of badgers resulted in roughly one extra fox per square kilometre.
Red foxes are of concern to farmers and conservationists alike because they prey on livestock, ground-nesting birds and brown hares. They are widely culled by farmers and gamekeepers.
Many farmers blame badgers for a sharp increase of bTB in their herds. But culling the animals remains a controversial option.
The Randomised Badger Culling Trial (RBCT) was set up to investigate how bTB spread between cattle, badgers and other wildlife. It also enabled scientists to assess the effects of badger culls on other species sharing the same ecosystem.
It was understood that red foxes might be affected because the foxes use badger setts as breeding dens and share a similar diet - suggesting the two species may compete for food.
The researchers from Defra's Central Science Laboratory in York and the University of Aberdeen established four separate study zones across five English counties.
Each of the four zones contained a "treatment" area (where badgers were culled) and a "control" area (where no culling took place). They then compared what happened in the treatment areas with the control areas.
Culling badgers removes a competitor of the red fox
Lead author Iain Trewby said the experiment had been designed to ensure any changes the researchers saw were due to the culling of badgers and not to other factors.
"What we saw was an increase of fox numbers in the culled areas," Mr Trewby, from the Central Science Laboratory, told BBC News.
He added: "Whether this increase that we've seen here is enough to impact on other species in the ecosystem, we can't say. But all these factors need to be taken into account when you're considering badger culls."
Mr Trewby said: "Obviously it is a contentious issue whether foxes have a significant impact on farming. They may have an impact, or there may be increased mortality, but that's something we can't comment on at the moment."
The National Farmers' Union (NFU) believes a cull is necessary to curb TB in cattle. About 2,500 cattle a year get bTB, and some 30,000 stock are killed every year because of the disease, according to the NFU.
In a statement, the NFU acknowledged that fox numbers would probably affect hare populations negatively. But it added: "The biggest beneficiary of proactive culling was the hedgehog, numbers of which doubled in the proactive culling zones."
"What the research confirms, however, is that badger removal has very few adverse impacts on other wildlife species and on balance - an important word in this debate - would bring significant benefits."
But Rosie Woodroffe, senior research fellow at the Zoological Society of London, commented: "I think it is another aspect of badger culling that needs to be taken into account in deciding on the modest benefits of culling badgers set against a number of costs."
The RBCT produced mixed evidence on the likely impact of culls. The Independent Scientific Group (ISG) report on the trial found badgers were a continuing source of infection for cattle, but also said culling them would have to be so extensive and co-ordinated it would be uneconomical.
Dr Woodroffe, who is a member of the ISG, said: "What we concluded was that the only way you could have even a modest benefit for control of cattle TB was by culling badgers on an extremely large geographic scale, over long periods of time in a highly co-ordinated way.
She added: "If you don't do it in that way, you actually make it worse. Badgers are social and highly territorial. This limits the spread of disease because infected badgers are mainly going to interact with their own group.
"When you cull the badgers, you break down that territoriality, so the badgers are ranging more widely and meeting more herds of cattle. But they are also more likely to be interacting with what used to be neighbouring social groups."
Nonetheless, Professor Sir David King, then the government's chief scientist, advised ministers to push ahead with a cull.
In November, Sir David told BBC News: "If we don't do this, we are actually leaving a disease to spread through the animals in the UK at increasing cost to the taxpayer and at a devastating cost to the farming community."
Foxes are also of concern because they would be principal vectors in the event of a rabies outbreak and a source of the parasitic worm Trichenella.