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A classic case of meso-predator release hurting another species

My Times column on the hedgehog decline, and the effect of badgers:

Hedgehogs, subjects of the Times Christmas Appeal, are to get their own summit, the Environment Secretary Liz Truss said last week. Hedgehogs really are in trouble. Their numbers have plunged, their range has shrunk and they have disappeared from large parts of the countryside altogether. The population has probably at least halved during this century and may now be 3% of what it was in the 1950s.

Yet when asked why this has happened conservation organisations nearly always talk of habitat loss and urban development. This makes no sense because hedgehogs now survive mostly in suburbs, not rural areas. The one thing the pressure groups hate mentioning is badgers. Yet the scientific evidence that an increase in Mr Brock may well be the chief cause of Mrs Tiggywinkle’s demise is – as I have been discovering by reading the scientific literature – overwhelming.

The evidence comes in many forms. There’s the direct evidence – the hollowed-out prickle-bearing skins left behind after a badger eats a hedgehog. Only badgers can open a hedgehog after it rolls up in defence with prickles on the outside. Foxes and dogs cannot. Badger predation is often found to be the main cause of hedgehog death in studies in this country.

There’s the timing evidence. The decline in hedgehogs coincided with the rise in badgers since the latter’s protection began in the 1970s and was strengthened in the 1990s. Anecdotally, there are clear-cut cases to show this is likely to be cause and effect, not coincidence. Kew Gardens had a thriving hedgehog population until the mid 1980s when badgers first arrived there. Today, says the Royal Botanic Gardens’ website, “Kew is teeming with badgers”. There are now more than 20 badger setts and hedgehogs are hardly ever seen, and if they are don’t last long. Habitat change and development cannot be the cause in Kew, because there has not been any.

There’s the spatial evidence. Throughout England hedgehogs (great worm eaters) have vanished from worm-rich pastureland, where badgers thrive, and cling on only in suburban gardens and amenity grassland, where badgers so far have not penetrated much. Even here, as one study put it: “the probability of hedgehog occurrence in suburban habitats declined towards zero in areas of high badger density.” As a general rule, wherever badgers are present at a density higher than about one main sett every four square kilometres, hedgehogs are absent. In most of rural England, sett density is two or three times higher.

There’s the behavioural evidence. Back in the 1990s, ecologists discovered that the smell of badgers alone is enough to deter hedgehogs from entering an area. And hedgehogs released into areas of excellent habitat high-tailed it out of there if they smelt badger. They do not wait to be eaten.

There’s the international evidence. Sweden has also blamed its decline in hedgehogs on rising badger numbers. In the Netherlands, scientists reported this year that hedgehogs are now confined mostly to urban areas, despite roads and development, because of the spread of badgers in rural areas.

There’s the experimental evidence, which is the clincher. The randomized badger control trials, carried out between 1998 and 2006 in 30 different parts of the country, provided an opportunity to see what happened to hedgehog numbers if badger numbers were reduced.

In pasture land, where hedgehogs are mostly extinct, there was of course little effect. But in “amenity grassland” – parks, lawns, gardens, playing fields etc – there was a dramatic explosion in hedgehog numbers in those areas where badgers were culled: hedgehog density almost trebled from an average of 0.9 per hectare to 2.4 per hectare in just five years. By contrast in the control areas where no badgers were culled, the hedgehog density remained unchanged at 0.3 per hectare over the same period. 

So we know one way to increase hedgehog numbers: reduce badger numbers. No other approach has been demonstrated to have such an effect. Given that badgers are highly effective predators of bumble bees (those who try to study bumble bees complain that most of the nests they find get dug out and eaten by badgers) as well as of ground nesting birds, and that they carry bovine tuberculosis, there is little doubt that lower badger densities would be good for the ecology of the British countryside as well as for farming.

After all, even the Royal Society for the Protection of Birds accepts that that foxes and crows need to be controlled on its reserves because of the impact they have on wildlife, so why not badgers? The key point is that there is nothing natural about the density at which badgers and foxes live in Britain today. In a natural system wolves, bears and lynx would control their numbers to the benefit of hedgehogs, bumble bees and birds.

This general pattern is known as the “mesopredator-release problem” and elsewhere in the world, wildlife managers are much less squeamish about confronting it. If top (“apex”) predators are missing, then middle-ranking (“meso”) predators thrive with devastating results for species on which they prey. For example, in southern California where coyotes are absent, foxes, skunks and domestic cats are more numerous and many bird species suffer badly. In the north Pacific, a decline in sea otters (caused by killer whales) unleashed a population explosion of sea urchins resulting in overgrazing of kelp to the detriment of many fish. The return of wolves to Yellowstone park benefited many plants, insects, rodents and birds by suppressing numbers of elk (not that elk can be described as predators). 

The hedgehog’s plight is only going to get worse. As social memories of persecution fade, badgers are moving into suburbs, the last refuges of hedgehogs. Providing hedgehogs with ideal homes in our gardens, and love tunnels under garden fences, may slow their decline, but it won’t stop them going extinct from much of the country altogether. Unless we are prepared to unleash brown bears, lynx and wolves into the English countryside to control badgers and help hedgehogs, which seems unlikely, then we are under an obligation to do the apex predator job ourselves, and control badgers for the sake of birds, bumble bees and hedgehogs. For sentimental reasons we may decide not to do so, but then we will have the decline of the hedgehog on our conscience.








By Matt Ridley | Tagged:  rational-optimist  the-times