Double whammy for the disappearing frogs

New species of red-eyed toad
Image caption New frog and toad species are being discovered - others may be wiped out before being identified

There's a strange twist this week to the long-running story of the disappearing frogs.

As regular readers of these pages will know, frogs and their amphibian cousins, the salamanders and caecilians, are more threatened than any other group of animals, with more than a third of assessed species on the danger list.

With some species, it's easy to find a single culprit for the decline. With others, as I've noted before, it's a bit of this and a bit of that - the attack of the killer everything.

Usually, the various threats are symbiotic; pollution reduces resistance to disease, for example, while loss of habitat caused by expansion of the human footprint also brings invasive species.

Image caption The study reveals something rarely seen before

This week's unusual twist comes in the shape of a paper in Proceedings of the National Academy of Sciences (PNAS), which suggests a very different relationship between two of the major amphibian threats: loss of habitat, and the fungal disease chytridiomycosis.

Gui Becker and Kelly Zamudio from Cornell University in the US analysed statistics on amphibian decline in Brazil, Costa Rica and Australia, and found that chytrid appears to do more damage in pristine forests than in lands that have been cleared or otherwise modified by human hands.

Why this should be the case isn't entirely clear.

One possible link is temperature; where forests have been cleared, daytime temperatures will be higher (at least in the regions studied), and the Batrachochytrium dendrobatidis (Bd) fungus doesn't like such hot conditions.

An alternative idea is that the greater diversity of amphibian species in virgin forest helps the fungus to spread.

Each species will breed, for example, or disperse, at a different point in the year; so the more species there are, the more frequent these events will be.

And these activities are likely to carry the fungus from one place to another.

The finding has been greeted with some caution, and Nature News carries a good discussion of arguments over what might lie behind it.

The Cornell team now plans to do some laboratory experiments to see whether species richness does encourage chytrid to spread.

Image caption The scarlet frog of Venezuela may have been a victim of chytridiomycosis

In the meantime, if the finding is correct, what does it mean?

One interpretation is that there is now "no hiding place", in Gui Becker's words - with amphibians damned either by loss of their home, or by the visit of a lethal fungus that prefers to knock at unopened doors.

Comments by Karen Lips, one of the world authorities on chytrid, on the Nature post amplify the point.

"This is now a Bd world," she says.

With the fungus active on every continent except Antarctica, the point is well made.

What the new work doesn't do is point a way forward for conservation. Encouraging the destruction of habitat in order to hinder the spread of a disease would hardly be a rational strategy.

But it does confirm that just preserving tracts of intact forest and wetland isn't going to be enough to save all the extant species of amphibian.

Doing that is likely to need something that can tackle chytrid in the wild - which, as yet, does not exist.