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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,



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