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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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Why do diseases cause species decline?

My latest Mind and Matter column in the Wall Street Journal is about the role of disease in species conservation:

Some beekeepers, worried by the collapse of their bee colonies in recent years, are pointing a finger this month at a class of insecticide (neo-nicotinoids) that they think is responsible for lowering the insects' resistance to disease. They may be right, but I'm cautious. History shows that, again and again, blaming chemicals for the decline of a species has prematurely exonerated the real culprit, which is often disease alone.

RIDLEY

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The role of parasites in causing species to decline is often overlooked. Native European red squirrels, for example, have long been retreating in Britain at the hands of the American gray squirrel, which menagerie-owning aristocrats introduced in the 19th century. For years it was thought to be the competition for food that prevented the squirrels' co-existence, but now scientists place most of the blame on a parapox virus that causes a mild illness to the grays but kills the reds.

As this case shows, blaming a pathogen does not exculpate people. A new disease usually runs rampant because human beings have introduced it inadvertently-or, in the case of the rabbit disease myxomatosis, deliberately. A virus of native South American rabbits, "myxy" (as it is called) killed 90% of European rabbits when deliberately released in Australia and Europe in the 1950s. Resistance has since grown, but slowly.

A particular concern for conservationists at present is the decline of frogs and toads throughout much of the world. When this was first noticed in the late 1980s, it coincided with concern for the ozone layer, and so increasing ultraviolet light was blamed for causing mutations in the eggs of frogs.

I never found this convincing, because the decline in frog populations was happening in temperate and tropical latitudes, where no change in ozone-layer thickness was even discernible (it still is not). Habitat loss and pollution were then mentioned, too, and later global warming, though plural causes of species decline are rarely convincing. When a fungal chytrid disease called Bd was also identified as a possible culprit, it was said to be a side effect of pollution or radiation, which supposedly were lowering immune resistance.

It is now clear that the main cause of the decline of amphibians is the chytrid fungus alone, unaided by pollution or radiation. Once again, it was our fault for introducing it into habitats where species were susceptible, especially the forests of central America and Australia but also in the American West. Most African frogs are unaffected by the fungus, which suggests that international traffic in African clawed toads as experimental laboratory animals may be to blame. Carried by us, they might be the carriers of the fungus. But eastern North American and Asian frogs are also partly immune, so perhaps it came from there.

Worryingly, there is now some evidence that scientists themselves may have inadvertently made the problem worse by studying it and carrying the fungus to pristine habitats on their boots and equipment. The same possibility may yet be mooted for a similar collapse in bat populations in North America. Many bat species have seen sharp declines in numbers, especially the little brown bat. Once again, the cause turns out not to be habitat changes, pesticides or climate change, but a fungal disease called white-nose syndrome. Nobody knows where it originated, but probably in the Old World. Could scientists have spread it from cave to cave?

I notice a surprising reluctance among conservationists to embrace germ theories of decline, just as there was a long reluctance to embrace germ theories of disease in medicine. There still is, as shown by the shocking story of how in the 1980s two Australians identified a bacterial disease as the cause of stomach ulcers and fought to win over their colleagues against stubborn resistance. Too often germs are allowed to be supporting actors in stories, when they should be the leads.