Are We in the Midst of the Sixth Mass Extinction A View from the World of Amphibians

DAVID B. WAKE* and VANCE T. VREDENBURG*+

Many scientists argue that we are either entering or in the midst of the sixth great mass extinction. Intense human pressure, both direct and indirect, is having profound effects on natural environments. The amphibians—frogs, salamanders, and caecilians— may be the only major group currently at risk globally. A detailed worldwide assessment and subsequent updates show that one-third or more of the 6,300 species are threatened with extinction. This trend is likely to accelerate because most amphibians occur in the tropics and have small geographic ranges that make them susceptible to extinction. The increasing pressure from habitat destruction and climate change is likely to have major impacts on narrowly adapted and distributed species. We show that salamanders on tropical mountains are particularly at risk. A new and significant threat to amphibians is a virulent, emerging infectious disease, chytridiomycosis, which appears to be globally distributed, and its effects may be exacerbated by global warming. This disease, which is caused by a fungal pathogen and implicated in serious declines and extinctions of >200 species of amphibians, poses the greatest threat to biodiversity of any known disease. Our data for frogs in the Sierra Nevada of California show that the

*Museum of Vertebrate Zoology and Department of Integrative Biology, University of California, Berkeley, CA 94720-3160; and ^Department of Biology, San Francisco State University, San Francisco, CA 94132-1722.

28 / David B. Wake and Vance T. Vredenburg fungus is having a devastating impact on native species, already weakened by the effects of pollution and introduced predators. A general message from amphibians is that we may have little time to stave off a potential mass extinction.

Biodiversity is a term that refers to life on Earth in all aspects of its diversity, interactions among living organisms, and, importantly, the fates of these organisms. Scientists from many fields have raised warnings of burgeoning threats to species and habitats. Evidence of such threats (e.g., human population growth, habitat conversion, global warming and its consequences, impacts of exotic species, new pathogens, etc.) suggests that a wave of extinction is either upon us or is poised to have a profound impact.

The title of this chapter is an appropriate question at this stage of the development of biodiversity science. We examine the topic at two levels. We begin with a general overview of past mass extinctions to determine where we now stand in a relative sense. Our specific focus, however, is a taxon, the Class Amphibia. Amphibians have been studied intensively since biologists first became aware that we are witnessing a period of their severe global decline. Ironically, awareness of this phenomenon occurred at the same time the word ''biodiversity'' came into general use, in 1989.

FIVE MASS EXTINCTIONS

It is generally thought that there have been five great mass extinctions during the history of life on this planet (Jablonski, 1995; Erwin, 2001). [The first two may not qualify because new analyses show that the magnitude of the extinctions in these events was not significantly higher than in several other events (Alroy, Chapter 11, this volume).] In each of the five events, there was a profound loss of biodiversity during a relatively short period.

The oldest mass extinction occurred at the Ordovician-Silurian boundary («439 Mya). Approximately 25% of the families and nearly 60% of the genera of marine organisms were lost (Jablonski, 1995; Erwin, 2001). Contributing factors were great fluctuations in sea level, which resulted from extensive glaciations, followed by a period of great global warming. Terrestrial vertebrates had not yet evolved.

The next great extinction was in the Late Devonian («364 Mya), when 22% of marine families and 57% of marine genera, including nearly all jawless fishes, disappeared (Jablonski, 1995; Erwin, 2001). Global cooling after bolide impacts may have been responsible because warm water taxa were most strongly affected. Amphibians, the first terrestrial vertebrates, evolved in the Late Devonian, and they survived this extinction event (Clack, 2002).

The Permian-Triassic extinction («251 Mya) was by far the worst of the five mass extinctions; 95% of all species (marine as well as terrestrial) were lost, including 53% of marine families, 84% of marine genera, and 70% of land plants, insects, and vertebrates (Jablonski, 1995; Erwin, 2001). Causes are debated, but the leading candidate is flood volcanism emanating from the Siberian Traps, which led to profound climate change. Volcanism may have been initiated by a bolide impact, which led to loss of oxygen in the sea. The atmosphere at that time was severely hypoxic, which likely acted synergistically with other factors (Huey and Ward, 2005). Most terrestrial vertebrates perished, but among the few that survived were early representatives of the three orders of amphibians that survive to this day (Marjanovic and Laurin, 2007; Cannatella et al., in press).

The End Triassic extinction («199-214 Mya) was associated with the opening of the Atlantic Ocean by sea floor spreading related to massive lava floods that caused significant global warming. Marine organisms were most strongly affected (22% of marine families and 53% of marine genera were lost) (Jablonski, 1995; Erwin, 2001), but terrestrial organisms also experienced much extinction. Again, representatives of the three living orders of amphibians survived.

The most recent mass extinction was at the Cretaceous-Tertiary boundary («65 Mya); 16% of families, 47% of genera of marine organisms, and 18% of vertebrate families were lost. Most notable was the disappearance of nonavian dinosaurs. Causes continue to be debated. Leading candidates include diverse climatic changes (e.g., temperature increases in deep seas) resulting from volcanic floods in India (Deccan Traps) and consequences of a giant asteroid impact in the Gulf of Mexico (Jablonski, 1995; Erwin, 2001). Not only did all three orders of amphibians again escape extinction, but many, if not all, families and even a number of extant amphibian genera survived (Vieites et al., 2007).

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