Amphibian Decline Problem Requires Plan of Scientific Attack
The rapid, global decline of amphibians is the sort of problem that seems insurmountable, too big and too complex to think about solving. But according to ASU biologists James Collins and Elizabeth Davidson, there might just be a way to do it..
James Collins is heading a multimillion dollar, multinational project looking at the problem, and Elizabeth Davidson is one of his 23 collaborators. They have been working since 1998 to track down the pathogens behind the alarming amphibian deaths, and the list of suspects is intimidating. Habitat destruction and climate change, important but often vague and tangled issues, are probably involved, and two new diseases also seem to be playing a part.
Collins, Davidson, and the rest have been investigating these diseases, the first a type of virus called an iridovirus, and the second a common fungus called a chytrid fungus. Iridoviruses usually affect insects and fish but have been found to cause periodic salamander die-offs from Saskatchewan to Arizona. Scientists have known about chytrids for a long time -- there are roughly 80 species worldwide -- but until recently they have only been known as feeders of algae and other plant material. Now a chytrid fungus appears to be responsible for amphibian extinctions in Central and South America and Australia.
An obvious goal of the researchers is to halt these deadly diseases, especially the chytrid, but Collins and his collaborators realize that nailing the amphibian decline culprit is only part of the job. Equally important is gathering basic information of how a pathogen interacts with a host population. Amphibian decline is only one example -- if an extreme one -- of a general trend towards a global loss of biodiversity, and the research team wants to create a plan of scientific attack that can address many of these sorts of issues. Building a foundation of knowledge is a key step.
For this reason, the virus, despite not causing extinction, has remained
interesting to Collins and Davidson. They think that by studying the
host-pathogen relationship of the virus system, they can perhaps understand
how it differs from that of the chytrid system. Ideally, this would lead
to a deeper understanding of host-pathogen relationships in general as
well as providing clues to curbing the chytrid's deadliness. "There is
a huge value in having a host-pathogen system in which the host isn't
killing the pathogen and the pathogen isn't killing the host," says Collins, "and
in being able to study that."
The iridovirus is particularly interesting because it has the ability to decimate a population without permanently extinguishing it. Collins and Davidson, in their trips to the salamander's ponds in Arizona's San Rafael Valley, would often arrive to a gruesome scene. Nearly all the salamanders in a population would be dead and floating in the pond. Months later the two scientists would return to find a relatively healthy population, with none of the salamanders showing any signs of the disease but, ominously, none of them showing immunity to the disease either. This phenomenon remains mysterious but also important to understanding the dynamics of the system.
Another key aspect of any host-pathogen system is the ability of the pathogen to spread from one population to another. The Arizona salamanders live in cattle tanks, watering holes that are dozens of miles apart. The little slimy creatures cannot make the trek across the desert from one tank to another, and it is unclear how the virus manages to do it.
Danna Schock, a graduate student in Davidson's lab, noticed that the disease seemed to crop up in areas along the migratory paths of some birds. This sparked an interest in birds as a means of transporting the disease, but the theory remains unsubstantiated.
Other speculation includes the idea that scientists themselves might be behind the virus's spread. "Unfortunately, [the virus] could have been hitching a ride right here...." says Davidson, pointing at her feet. Now all researchers sterilize their boots and equipment after trips to the tanks.
Perhaps the most exciting possibility is that the virus could be attacking populations through infected water dogs, salamanders used as bait by fisherman. If this turns out to be the case, scientists could explore restricting the water dog trade to prevent the spread of the disease.
Of course, these issues transcend the bounds of the traditional scientific disciplines, and according to Collins collaboration will be the secret to resolving them. "There are certain classes of questions like these big interdisciplinary questions that require teams of investigators to go after them.... There are terrific advantages in being able to pick up a phone and talk to a molecular biologist or a geneticist when you need to."
With this collaboration, and with an understanding of host-pathogen interaction, Collins thinks global amphibian decline can be broken down into something more manageable. It's hard,' says Collins, "but having said that it can be done, and we're doing it."
James Hathaway, 480-965-6375 or Hathaway@asu.edu