By Jonathan Reynolds, GWCT Head of Predation Control Studies

Having caused the existing system of General Licences to be dismantled, Wild Justice have published the arguments they made to Defra for implementing a revised licensing system. Wild Justice claim to have won their legal challenge of the old system, but I’m not sure it’s that clear yet. So let’s leave aside the legality of the General Licences as they were and concentrate instead on the problem of evidence for General Licences as they will be.

Wild Justice insist that the licensing authority, which has been Natural England since Defra delegated this role about ten years ago, must have a strong evidence base for authorising the lethal control of birds through General Licences. They believe that in most cases this is missing: there is no evidence, species by species, to say that magpies or jays or jackdaws have ever caused the decline of a native prey species. They don’t object to the licensing of lethal methods where these are justified, but there must be a clear reason and a proven benefit of lethal control. If the ‘problem’ is local and specific, they argue that individual licences are more appropriate.

So, essentially, they want to go back to the beginning and redesign the system from scratch. Unfortunately, we don’t have that luxury. We rarely do in real life, and when we do, things rarely work properly first time. Generally, we have to adapt what went before.

The now-dismantled General Licence system was put in place in 1995 to allow reasonable continuity of widespread pest and predator control practices, while also respecting the principles of the EU Birds Directive. It was a pragmatic solution that could be refined later, as and when better understanding came along. That’s essentially the dilemma: must we stop until evidence provides the qualification for starting, or do we carry on until there’s a reason to change what we do?

But why doesn’t the necessary evidence already exist? Perhaps because lethal control is a controversial topic, and only the highest grade of evidence will be persuasive enough to win the argument. Back in the 1980s, the Game Conservancy Trust (as it was then) was constantly being told that there was no sound evidence that predator control had any effect on population growth. Critics argued that it just created higher autumn densities and thus allowed more shooting, which served the interests of hunters but not conservation.

Well, as every GCSE pupil knows, if you want unambiguous evidence, you must set up a controlled experiment. At the necessary scale (several square miles at least), that’s an expensive and rarely repeated exercise. So when Dick Potts and Steve Tapper set up the Salisbury Plain Experiment (1985-91) they wisely chose to control the numbers of a suite of common predator species (fox, stoat, weasel, brown rat, grey squirrel, carrion crow, magpie, rook, jackdaw), as allowed by law.

The ‘suite’ was very deliberate. If only one significant predator species is removed, more nests and chicks are left for the remaining predators to find. The eventual outcome (number of birds fledging or reaching adulthood) may not be as bad as if you had done nothing, but it won’t fully measure the true impact of the one predator species you removed.

And after all, it’s the combined impact that matters to the prey species. So the top quality evidence that does exist (Salisbury Plain, Otterburn) shows how controlling a suite of predators benefits native wild gamebirds and waders. And it’s not surprising that we don’t have one of these massive experiments for every separate predator species and every prey species.

That’s a problem with knowledge. The better you know something, the less widely it applies. You put a heap of research effort into one set of circumstances, but it gives a poor basis for generalising. So high-grade experimental evidence should be accompanied by work that’s more widely representative (e.g. of different places, or gamekeepers, or years, or prey species) but can’t be done so thoroughly. Thus, we know from experiments that a predator control package including corvid control can turn local decline into local increase for both gamebirds and other ground-nesting birds.

We know from survey work on a broader scale that all these birds are more abundant on upland estates with predator control, amounting to a nationally significant effect. We also know there are circumstances where lethal predator control is less effective. This can be because culled predators are rapidly replaced through immigration, or because culling effort was inadequate, or because there wasn’t a predation problem in the first place.

To satisfy Wild Justice, there must be clear evidence that a particular predator species has caused the decline of a particular prey species. I think we can say, with confidence, that’s never going to happen, at least not with unarguable, high-grade evidence. By definition, a decline is only evident with hindsight, and you can’t do experiments with history. The best we can hope for are suggestive correlations, perhaps boosted with intensive field studies of declining species and computer models simulating alternative histories. That would get us to roughly where we were with grey partridges in 1985 after decades of work by the Game Conservancy Trust and its forerunners.

But in reality, there are always multiple factors involved in a species decline, and you can’t say that any one of them is the cause. Predation may often contribute to prey declines but is unlikely ever to be the sole cause. What we do know is that predator control is often transformative as a remedial measure. There are many examples where habitats have been lovingly restored but the response of the target species is underwhelming.

You worry that some unconsidered element of the habitat is missing. But add effective predator control and suddenly productivity booms. So clearly, at that late stage of decline (at least), predation is holding things back, and predator control is a way of getting them going again. If the motivation for doing that is hunting, so be it: that helps deliver conservation results across large swathes of land that are working landscapes rather than nature reserves.

I have spent my career as a scientist teasing out the complex interactions of man, predator and prey. I know how hard-won any evidence is, and how good it must be to carry the day. Wild Justice know this too. They are exactly the people who will test evidence to its limits before accepting it. Their suggestions of the starting evidence needed for General Licences sound philosophically principled, but they are flying a kite way up into the blue sky of unrealism. I hope Defra realises this.