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In the Light of Evolution: Volume II—Biodiversity and Extinction
The data paint a disturbing picture about current and projected ecological states for the world’s oceans.
In Chapter 2, David Wake and Vance Vredenburg describe a similarly gloomy scenario for the global status of amphibians. Of the approximately 6,300 extant species of frogs, salamanders, and caecilians, at least one-third are currently threatened with extinction, and many more are likely to become so in the near future. A dramatic worldwide decline in amphibian populations was first noticed in the late 1980s. Several ecological factors including habitat degradation and climatic changes probably are involved, but so too is an unanticipated, recently uncovered threat: an emerging virulent disease (chytridiomycosis) caused by a pathogenic fungus. The source of this fungus and its mode of spread are poorly understood, but the disease (perhaps in synergy with other ecological factors) has devastated amphibian populations in such distant sites as the Americas and tropical Australia. Whatever the proximate and ultimate causes of the ongoing amphibian extinctions, the trend is especially disturbing because amphibians otherwise have been quintessential evolutionary survivors that managed to persist across several earlier mass extinction events in the Earth’s history.
Biodiverse coral reefs are among the most threatened ecological systems on Earth. About 70% of coral reefs globally have either been degraded beyond recognition in recent years (20%), are in imminent danger of collapse (24%), or are under longer term threat of demise (26%) (Wilkinson, 2004). In Chapter 3, Marjorie Reaka and colleagues survey reef-dwelling stomatopods (a large group of marine crustaceans) as a model taxon to assess global hotspots of extant biodiversity, endemism, and extinction risk, the intent being to identify evolutionary sources and sinks of stomatopod diversity, infer driving mechanisms, and provide an additional focus for conservation and management efforts on coral reefs. Stomatopod species diversity (like that of several other reef-dwelling marine taxa) is highest in the Indo-Australian Archipelago, gradually declines eastward across the central Pacific, and shows a secondary peak of species richness in the southwestern Indian Ocean. From these and other data (related to body size, ecology, and spatial pattern of endemism), the authors explain how a “merry-go-round” evolutionary model might account for the differential dynamics of species origin and extinction in different ocean regions.
Extinctions in the ongoing biodiversity crisis apply not only to free-living organisms but also to their parasites. In Chapter 4, Andy Dobson and colleagues address the possible magnitude of this problem by reviewing estimates of the total number of parasitic species on Earth (with special reference to helminthes that parasitize vertebrate animals) and the fraction of extant biodiversity that is parasitic. The authors conclude that about 10–15% of parasitic helminthes (Trematoda, Cestoda, Acanthocephala,