Lessons of the Lost
Amphibians are all but gone, bequeathing us lessons that must not be squandered
Dead on Arrival
It is a bittersweet experience to formally name a new species of animal, knowing that it is probably already extinct. Even more unsettling is the saga of Rabbs’ fringe-limbed treefrog (Ecnomiohyla rabborum), a spectacular new species that my colleagues and I discovered in central Panama in 2005. The publication formally naming it appeared in 2008 in the Journal of Herpetology, but the last individual in the wild was noted in 2007. At this writing, there are two individuals safe in captivity; both of them are males. The species is evidently extinct, even if those remnant males are yet unaware. Have you seen the iconic film clip of the last thylacine—the Tasmanian tiger—in its zoo enclosure? There is now a companion clip on YouTube of the second-to-last Rabbs' treefrog.
The crisis of global amphibian extinctions is profound, it is changing ecological systems in ways that we barely understand, and it is teaching us painful lessons about research and conservation. Amid all of the horrible environmental insults we inflict upon the planet’s biodiversity, Rabbs’ fringe-limbed treefrog did not disappear in the wild because of overharvest for food, pets, or science, and we cannot lay the blame on familiar threats such as deforestation, climate change or environmental pollution. The culprit was emerging infectious disease. Amphibian chytridiomycosis is capable of directly eradicating otherwise large and stable populations and directly causing extinction of species that are otherwise unthreatened.
Amphibian chytridiomycosis is a disease caused by the recently discovered chytrid fungus Batrachochytrium dendrobatidis. In the thallus phase of its life cycle, when fungi colonize to form vegetative masses, this chytrid forms a vase-shaped structure that embeds in the superficial epidermis of amphibians, allowing the fungus to harvest energy from the keratin in the host’s skin.* It then produces motile flagellate zoospores that disperse aquatically. Ironically, the zoospores resemble microscopic tadpoles. Chytrid fungi are generally decomposers or parasites of invertebrates or plants. This is the first taxon in that large group known to be parasitic on vertebrates.**
Batrachochytrium dendrobatidis likely is capable of infecting every species of amphibian on Earth (nearly 7,000 species), representing the broadest host-range of any known pathogen. Host responses vary from complete tolerance to rapid death, with death resulting from cardiac arrest stemming from osmotic imbalance. (Among its roles, keratin is a water-proofing protein). Tolerant host species, such as American bullfrogs (Lithobates catesbeianus), can act as disease vectors as they disperse naturally or are transported globally by humans. Tolerant species in any ecosystem may also act as reservoirs, ensuring that the pathogen remains on site and thereby reducing the probability that vanquished amphibian populations may recover, recolonize or be reintroduced. At some sites, such as the upland areas of Central America, the Andes, eastern Australia or western North America, the arrival of the pathogen into a new area promptly eliminates some amphibian populations, severely reduces others (with little evident recovery even after decades), yet leaves a subset of species infected but otherwise unaffected. At lower elevations, or at other sites such as upland areas of eastern North America, the disease is present but seems to have little effect on local species. Considerable research has been done in an effort to elucidate the synergistic influences that underlie these variable effects among sites and species, but the general answers remain frustratingly unclear. Batrachochytrium dendrobatidis is behaving as an emerging infectious disease and an invasive species on every continent (except perhaps Asia, and of course Antarctica, where no amphibians currently exist). Its native distribution (and naturally coevolved host species) remains unknown.
Herpetologists and wildlife biologists began observing inexplicable disappearances of amphibians around the globe in the mid-1970s and especially by the mid-1980s but were at a complete loss to explain them. Finally, in the late 1990s, an insightful team of pathologists at the U.S. National Zoo, led by Don Nichols, collaborated with one of the few chytrid fungus scholars in the world, Joyce Longcore, and identified this quite unusual new genus and species. Conservationists and disease ecologists were unprepared for the reality of a pathogen capable of directly and rapidly—mere months!—causing the elimination of a population or an entire species that was otherwise robust. Classical host-pathogen theory held that such dramatic consequences to the host population or species were only realized when the host population was already drastically reduced in size or otherwise compromised. The concept of a lightning extinction was foreign to researchers and conservationists, and we argued vehemently about it throughout the 1990s at symposia worldwide. In retrospect, the scenario of a spreading pathogen is parsimonious and clear, but in the midst of the massacre we were entangled in logical quagmires along these lines: “The disappearances cannot be the result of disease; diseases are not capable of such.” Not to mention the fact that the smoking gun, the pathogen itself, was not described until 1999. While we were debating the issue, a terrible lesson was playing out for us around the world as an unknown disease decimated amphibian populations. This lesson has led to an overdue focus on other neglected amphibian diseases, such as those caused by ranaviruses. I highly recommend the recent and quite readable book Extinction in Our Times: Global Amphibian Decline by James P. Collins, Martha L. Crump and Thomas E. Lovejoy III, which offers a complete history of the phenomenon and the desperate plight of the global stakeholders.
*Due to an editing error, this sentence has been corrected in the
online version of the essay. The original sentence began, "In the thallus or mycelium phase of its life cycle. . . ." 10/13/2011
**Due to an editing error, this sentence has been corrected in the online version of the essay. The original sentence read "Chytrid fungi are generally decomposers, and this is the first taxon in that large group known to be parasitic." 10/13/2011