What I make of this is obvious: the distribution of the surviving birds is disease-controlled. I also think that the populations or species that could not survive at high altitude¿the birds that have gone extinct¿became extinct because there was nowhere for them to go. Everywhere they went they were greeted by buzzing mosquitoes who were carrying the disease, and the birds dropped out. I think that is a pretty convincing case. And all of the leading list-makers¿like IUCN, Nature Conservancy, U.S. Fish and Wildlife¿when they attribute cause of extinction to those particular birds use the word disease, as opposed to habitat clearance, as opposed to introduced species, persecution, all of these other things that are believed to cause endangerment and extinction. So at least in some quarters it is accepted that those bird extinctions are due to disease per se.
Image: CLARE FLEMMING
The other great example is golden toad extinction in Panama and apparently huge declines in certain frog species in places like Queensland [Australia], all due to a fungal infection, chytridiomycosis. Up until the mid '90s in Panama the census numbers were consistently high. Then something happened to the golden toad. Within the space of a year or two the numbers of sightings and sound recordings and so on dropped from the average level established over many decades to zero, essentially overnight. For the past five years there have been no sightings of that particular frog in Panama and Costa Rica, which were the places where it lived. It looks like the populations have just sunk to nothing.
Peter Daszak, a parasitologist at the University of Georgia, and several other investigators got interested in this problem because they were given some autopsy specimens of golden toads to examine. What they found was that a particular chytrid was consistently present in autopsy specimens from areas in which the populations were known to have sunk to zero in the manner that I described. And what they found on further investigation was that the specific cause of death seems to have been that this fungus, which is an epidermal infection, caused local thickening of the skin¿especially over an area known as the drink patch down in the pelvis, which is what these frogs use for osmoregulation, for balancing out their water. With any problem with that area of skin, the frog in effect suffocates or drowns. Interestingly, tadpoles had no epidermal infection, but they did have the chytrid present in their mouthparts. Presumably, as they metamorphosed, the chytrid infection became general on the epidermis and they died out as well. So what you¿ve got, it seems to me, is the worst of all possible cases, which is a universal infection¿that every individual either has it or could potentially have it. It¿s something that is obviously very easily passed on or distributed within the environment. There would seem to be no escape for the populations that suffered from the chytrid infection: it wasn¿t like the chytrid would pass through the adult group and the next generation would be okay.
A similar chytrid¿perhaps even the same chytrid¿has appeared in Queensland, Australia. It is not known that it has been responsible for any complete extinctions, but it has certainly been responsible for massive depressions in population in the frog groups that it affects. It has also turned up in the southern part of South America. Why this particular chytrid at this particular time? Who knows? Perhaps it¿s because of people. Because you can get anywhere on the planet¿s surface nowadays within about 48 hours, the chances for pathogen pollution¿in other words, bringing pathogens from one area to another where they might be able to take off¿is incredibly enhanced over what it was even a few decades ago. From that point of view it¿s perhaps not even all that remarkable that we¿d start to see diseases that are essentially pan-global. Not because they¿re getting distributed better by winds or currents, or whatever, but by people moving around.