Amphibians are under attack from a changing environment
The final event happened away from prying eyes. The exact circumstances will always remain uncertain, but somewhere around 1989, humans saw the last golden toad in the Costa Rican cloud forest. Hidden in the leaves of the forest, the final survivor died, its body shredded by a skin fungus. A fungus most likely introduced from Africa by humans as they traversed the globe, carrying with them the seeds of the golden toad’s demise.
The golden toad is not alone. An estimated 168 amphibian species have gone out of existence in the past several decades, and at least 2,469 species (43% of the known amphibian species) have populations in decline. When considering ecosystems, change is often not as significant as the rate of change.
Geologic history is replete with examples of species that went extinct. There is a natural background rate of species extinction. When that rate is grossly exceeded, we refer to it as a “mass extinction.” Amphibians don’t preserve particularly well in the fossil record, so their background extinction rate is fuzzy. Nevertheless, studies using what data is available, indicate the current amphibian extinction rate is currently as high as 200 times the background rate.
Extinction and species loss
Most people reference five historical mass extinctions. I place the number at eight since I include oxygenation of the earth’s original oceans as the first primordial extinction event. This event wiped out the existing chemosynthetic organisms who were first-comers to this thing we call life. Oxygen was poison to them.
The great oxygen catastrophe was the second mass extinction event. Cyanobacteria (blue-green algae) were so successful as a species, the oxygen they produced changed the atmospheric composition and sent earth into a deep freeze, almost wiping them out along with other species. Following the first two events were the five well-known extinctions: the Ordovician/Silurian event, the Devonian Oxygen crisis, the “Great Dying” in the Permian, an end Triassic extinction event, and the K-T extinction that took out the dinosaurs.
The accounting presented above leaves us with event number eight. The eighth extinction event is one we are getting to see in a very closeup and personal way. It is an extinction event triggered by a single species, a species that has achieved unbridled dominance over the biosphere, Homo sapiens. The Anthropocene is the age of humans. The rapidity with which Homo sapiens have ascended to dominance is unparalleled in geological history. The change from being a small part of the animal kingdom to total dominance has been over thousands of years, not the tens of millions of years that generally accommodate evolution. When change occurs rapidly, evolution has a hard time keeping up.
But it is not all gloom and doom. During 2019, on a cool August evening in northern Ecuador’s cloud forests, researchers spotted a Mindo harlequin toad in the forest. The excitement that evening among the conservation biologist ran high because this was the first sighting in 30 years. Before this sighting, most scientists believed this species was extinct. It was thus considered one of the many amphibian species fallen prey to the fungal disease, chytrid.
During the past three decades, chytrid wreaked disaster on amphibian populations around the world. The chytrid fungus (Batrachochytrium dendrobatidis) is a pathogen that attacks the skin tissue. However, it doesn’t necessarily kill the infected amphibian. But it does lead to a gradual degradation of the skin tissue, lethargy, weight loss, and potentially death. Biologists don’t fully understand the actual physiology of chytrid-induced death. But disruption of the skin’s ability for fluid transport and gas exchange is a suspected cause.
The origins of the current chytrid devastation are also not well understood. One hypothesis is the fungus was local to some African ecosystems but has since been globally transported to other environments. When introduced into a new environment, chytrid is a novel pathogen, and the amphibians lack defenses to fight it. Therefore, the disease spreads rapidly and decimates populations of susceptible species. A second hypothesis believes the pathogen is naturally present globally, but climate change and habitat destruction have weakened some amphibian species’ ability to fight it off.
A ray of hope
The return of the Mindo harlequin toad from the brink of extinction offers the possibility that this species has developed a resistance to the chytrid fungus. Development of resistance to this fungus occurs in a few species, including rocket frogs, harlequin frogs, and Sierra Nevada yellow-legged frogs.
This example of a small bit of evolution-in-action provides hope that some of the affected amphibian species will return from the edge of extinction and thrive again in their native ecosystems. The rapid loss of amphibian species is a blow to many ecosystems. But perhaps the tides are turning in favor of the species that remain.
‘Extinct’ toad rediscovery offers hope amid amphibian apocalypse (By Jason Bittel; National Geographic)
Chytrid Fungus (Source: UC Riverside Center for Invasive Species Research)
Amphibian Decline or Extinction? Current Declines Dwarf Background Extinction Rate, MALCOLM L. MCCALLUM, Journal of Herpetology, Vol. 41, №3, pp. 483–491, 2007Copyright 2007 Society for the Study of Amphibians and Reptiles
Feature Image: Cloud Forest (Modified by ArcheanWeb) – Original Credit: By Peter Angritt – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79931324