Badger cull didn’t kill enough badgers to be effective

You’re next. Chris Beckett, CC BY-NC-ND

Badger cull didn’t kill enough badgers to be effective

You’re next. Chris Beckett, CC BY-NC-ND

In the flurry of the holiday season, many people will have missed the government’s verdict on the 2014 badger culls, published on December 18. Farmers’ representatives have branded these recent culls “successful”, and environment secretary Liz Truss claims that they show how culling “can work to reduce disease”, confirming her plan to extend this controversial approach across western England.

Cattle farmers have suffered terribly as a result of bovine tuberculosis (TB). Many are desperate, and would welcome a cull of badgers, which research (including my own) has shown to be a source of infection for cattle. Sadly, a closer look at the evidence suggests that the 2014 culls bring little hope of succour.

Despite the environment secretary’s optimism, there is so far no evidence that these pilot culls have reduced disease. The government has commissioned research to estimate the impacts of pilot badger culling on cattle TB but no results have been published to date, nor are any benefits anticipated so soon after the start of the annual culls. Culled badgers have not even been tested for TB.

Since changes in cattle TB take so long to emerge, in the short-term the government measures culling success in terms of reduced badger numbers. This is an appropriate measure because, perversely, killing too few badgers increases cattle TB rather than reducing it.

The effects of badger culling

In a randomised controlled trial conducted in 1998-2007, cattle TB was consistently elevated where culling reduced indices of badger numbers by 10-35%. By contrast, nearby farms saw gradual reductions in cattle TB where large-scale culling reduced the same indices by 69-73%. To achieve similar benefits (and to avoid increasing cattle TB), the 2013-4 culls were intended to reduce badger numbers by at least 70%.

The first two culls, conducted in 2013, clearly failed to achieve this aim. Government scientists, overseen by an independent expert panel, estimated the reduction in numbers by identifying individual badgers from hair entangled in barbed wire traps. They estimated that between 37 and 51% of badgers were killed in the Somerset cull zone, with between 43 and 56% killed in Gloucestershire.

If only badgers could read. jayneandd, CC BY

For the second year of culling, the government discarded both independent oversight and the hair trapping method which had revealed the first year’s failures. Before the 2014 culls commenced, the government’s planned monitoring methods were so inadequate that I warned “any future claim that the 2014 culls have reduced badger numbers sufficiently to control TB will be completely baseless”.

Although ministers and farming representatives do indeed now claim success, the numbers tell a different story. There are no published estimates of the percent reductions achieved by the 2014 culls. Instead, claims of success are based on the number of badgers killed in Somerset, which reached the minimum target required by the culling licence (the Gloucestershire cull spectacularly failed to meet its target, killing just 274 badgers against a target of 615).

Yet the Somerset target was derived from the lower bound on the range of possible badger numbers, rather than from the best estimate. If the estimation method was accurate, there would be a 97.5% chance that the true population size was greater than this lower bound, and hence that the target was too low. Despite having met this target, statistically it is still far more likely than not that the 2014 Somerset culls failed to reduce badger numbers by 70% as planned.

Simple calculations provide further evidence of ineffective culling in Somerset. Government scientists estimate that, before any culling took place, the Somerset zone contained between 1,876 and 2,584 badgers. The total number of badgers killed (341 last year plus 955 in 2013) comprises just 69% of the lowest estimate. Taking into account the fact that births and immigration would have increased badger numbers between the two culls, the population cannot have been reduced by “at least 70%” if the government’s population estimates were correct.

Government documents describe Somerset’s low target as “precautionary”. But from the perspective of disease control – the justification for killing otherwise protected wildlife – it risked worsening cattle TB and was hence the opposite of precautionary. With separate maximum targets in place to avoid killing too many badgers, the only risk reduced by a low target was the risk of a cherished project being branded a failure.

Failing to reduce badger populations sufficiently risks exacerbating cattle TB, potentially making a bad situation worse. Farming leaders have managed to press forward with badger culling in the face of scientific consensus, legal challenge, public opinion and a groundswell of protest. In future they may look back on such victories as Pyrrhic: one more such victory might undo the farmers they strive to support.