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In the Light of Evolution: Volume II—Biodiversity and Extinction
Part I
CONTEMPORARY PATTERNS AND PROCESSES IN ANIMALS
There is no doubt that humans are the root cause of most ecosystem stresses and biotic extinctions in the modern world. Negative human pressures on biodiversity occur via pollution, introductions of alien species, overexploitation, landscape transformations, and other factors. Like the asteroid impact 65 million years ago, human impacts extend to many kinds of terrestrial, aquatic, and marine organisms. The chapters in Part I illustrate some of the challenges of quantifying the magnitude of extant biodiversity and deciphering extinction rates and patterns in a diverse but representative selection of contemporary animal taxa.
Oceans cover three-quarters of the Earth’s surface, and their inhabitants might seem at first thought to be somewhat buffered (compared to terrestrial and freshwater species) against anthropogenic disturbance. However, in Chapter 1, Jeremy Jackson compiles evidence from four major marine realms—estuaries and coastal areas, continental shelves, open ocean pelagic zone, and coral reefs—that marine ecosystems are under extreme duress from the oft-synergistic effects of habitat destruction, overfishing, introduced species, warming and acidification, toxins, and nutrient runoff. One common result has been the degradation of biodiverse marine ecosystems with complex food webs capped by an abundance of top-echelon predators into simplified biotic communities increasingly dominated by smaller animals, algae, and microbes. Among the many ramifications have been the economic collapse of numerous marine fisheries and massive degradation of coral reefs that formerly rivaled tropical rainforests in terms of spatial coverage and biotic richness.
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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,
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In the Light of Evolution: Volume II—Biodiversity and Extinction
Nematoda) are at risk of extinction by virtue of being dependent on threatened or endangered species of vertebrate host. They also conclude that parasite species diversity does not map linearly onto host species diversity, and that approximately three-quarters of all links in food webs involve a parasitic species. These findings provide a sobering reminder that the current extinction pulse is affecting many kinds of organisms (not just the conspicuous megafauna), and that extinction processes could therefore have many unforeseen ramifications for ecosystem operations.
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In the Light of Evolution: Volume II—Biodiversity and Extinction
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