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Wednesday, November 13

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Tackling novel threats to biodiversity

We do not usually think of cancers as being infectious because cancers themselves are generally not pathogens. As in all of biology, however, there are exceptions, and one such exception is the case of an actually pathogenic, infectious cancer that is causing tremendous conservation threats to the Tasmanian devil (Sarcophilus harrisii) in Australia.

Those of you who have followed some of my work will know that I have spent decades tracking and studying the amphibian chytrid fungi. The two known species have been emerging infectious diseases in amphibians around the globe, with catastrophic consequences in many populations and species. Here is a link to our recent global summary of that phenomenon. Tasmanian devils currently occur only on the island of Tasmania, off the southeastern coast of Australia. They occurred on the mainland until about 3,000 years ago; the causes for the mainland extinction still are contested and surely are multiple, including the arrival of humans and dingoes (a wild dog of unclear origins, likely associated with humans). But the devils remained on insular Tasmania and thrived until European settlers there persecuted them as pests, greatly reducing their numbers.

In the mid-1990s, Tasmanian devils were found with facial tumors, a condition quickly earning the name Tasmanian Facial Tumor Disease (TFTD). Further research, nicely summarized here, revealed that this was an example of the poorly known, and exceedingly uncommon, transmissible cancers. TFTD and the very few other known transmissible cancers are caused by the cancer itself. Interestingly, the only other transmissible cancer in a vertebrate is in our favorite domestic dogs. In any case, there is no treatment for TFTD. So, we are left in a situation like the amphibians facing chytrid fungi in that we can document the disease and its toll, but we cannot control it or treat it in any effective way.

While we do not have Tasmanian devils in the animal population here at Zoo Atlanta, the solutions at hand, and that are being pursued now, involve zoos and the establishment of survival-assurance colonies of cancer-free Tasmanian devils that are safe from the spreading disease as long as they cannot come into contact with wild individuals. The term “biosecurity” has never seemed so important! The end-game on that strategy may well be to simply let the cancer run its course across Tasmania, eliminating all wild devils, and then re-introducing progeny from the cancer-free survival assurance colonies. I can’t imagine the helplessness felt by our colleagues to be sidelined and watch the wild populations suffer and disappear. Other initiatives being considered included re-establishing populations of devils on the mainland which, of course, is cancer-free because no Tasmanian devils live there. To many conservationists, the introduction of a species outside of its current geographic distribution is to launch yet another invasive species. Australia has a history of challenges arising from invasive species, from cane toads to cats to rabbits. But if the devils were naturally there “only” 3,000 years ago, then would such a program represent establishment of an invasive, or would it be a conservation-reintroduction to re-found a vanquished population? You can be certain that this fuzzy gray-zone will be the source of considerable controversy. The Tasmanian devil is in real conservation trouble, the primary threat is unmanageable, and the evident solutions are not simple and likely to be controversial.  

In my lectures on conservation, I make a clear distinction between the glib-sounding “familiar” threats to biodiversity such as poaching or habitat loss and the more intractable “novel” threats, such as diseases. From the purely practical perspective, the familiar threats are conceptually simple to control—humans simply stop killing the species or draining/cutting the habitat. The biological aspect of the conservation problem is simple to resolve. The real challenges, of course, lie in the realms of society, economics and ethics. The novel threats are the scarier issues because, as we know all too well from human epidemics, pathogens can be very difficult to control in human populations and are even more so in wildlife. The path to mitigating amphibian chytrid fungi still eludes us, and the populations that persist or are recovering from decimation appear to be doing so as a result of natural evolutionary events, i.e., natural selection.

Our colleagues in Australia are fighting hard to save the Tasmanian devil from what certainly appears to be relatively imminent extinction, and in doing so they are being creative and innovative. They are carefully considering options such as re-populating the mainland that, only recently, would have been considered unacceptable. Kudos to our colleagues and let’s hope that we can keep a disease from eliminating this species.
Joe Mendelson, PhD
Director of Research

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