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