FROM THE ACADEMY: COLLOQUIUM PERSPECTIVE In the light of evolution II: Biodiversity and extinction John C. Avise*†, Stephen P. Hubbell‡, and Francisco J. Ayala*† *Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92697; and ‡Department of Ecology and Evolutionary Biology, University of California, Los Angeles, CA 90095 he Earth’s biodiversity is a well- (from species-area curves and other evi- extend to many kinds of terrestrial, spring for scientific curiosity dence) to taxa that undoubtedly are aquatic, and marine organisms. The arti- about nature’s workings. It is disappearing even before they can be cles under this heading, and the next, also a source of joy and inspira- identified and studied. Nevertheless, illustrate some of the challenges of Ttion for inquisitive minds, from poets to they do reveal the general magnitude of quantifying the magnitude of extant philosophers, and provides life-support the ongoing extinction crisis. For many biodiversity and deciphering extinction services. According to Kellert (2), biodi- species that manage to avoid extirpation, rates and patterns in a representative versity affords humanity nine principal local and regional populations are being selection of diverse contemporary types of benefit: utilitarian (direct eco- decimated. biotas. nomic value of nature’s goods and ser- The modern extinction crisis is Oceans cover three-quarters of the vices), scientific (biological insights), prompting scientific efforts on many Earth’s surface, and their inhabitants aesthetic (inspiration from nature’s fronts. Systematists are striving to de- might seem at first thought to be some- beauty), humanistic (feelings deeply scribe biodiversity and reconstruct the what buffered (compared with terrestrial rooted in our inherent attachment to Tree of Life. Ecologists are mapping the and freshwater species) against anthro- other species), dominionistic (physical distributions of biodiversity and global pogenic disturbance. However, Jeremy and mental well-being promoted by hotspots that merit special conservation Jackson (6) compiles evidence from four some kinds of interactions with nature), attention. Paleontologists are placing the major marine realms—estuaries and moralistic (including spiritual uplifting), current crisis in temporal context with coastal areas, continental shelves, open naturalistic (curiosity-driven satisfaction regard to the Earth’s long geological ocean pelagic zone, and coral reefs— from the living world), symbolic (nature- history, and also to the recent history of that marine ecosystems are under ex- stimulated imagination, communication, human impacts on biodiversity across treme duress from the oft-synergistic and thought), and even negativistic timescales ranging from decades to mil- effects of habitat destruction, overfish- (fears and anxieties about nature, which lennia. Educators and concerned scien- ing, introduced species, warming and can actually enrich people’s life experi- tists are striving to alert government acidification, toxins, and nutrient runoff. ence). Whether or not this list properly leaders, policy makers, and the public to One common result has been the degra- characterizes nature’s benefits, the fact the biodiversity crisis. Conservation ef- dation of biodiverse marine ecosystems is that a world diminished in biodiversity forts (including those by many nongov- with complex food webs capped by an would be greatly impoverished. ernment organizations) are underway to abundance of top-echelon predators into Many scientists have argued that, as a slow the pace of biological extinctions. simplified biotic communities increas- consequence of human activities, the However, unless conservation achieve- ingly dominated by smaller animals, al- Earth has entered the sixth mass extinc- ments accelerate quickly, the outlook gae, and microbes. Among the many tion episode (and the only such event for biodiversity in and beyond the 21st ramifications have been the economic precipitated by a biotic agent) in its century remains grim. collapse of numerous marine fisheries 4-billion-year history (3, 4). The last cat- The goals of this Colloquium were to and massive degradation of coral reefs astrophic extinction, which occurred synthesize recent scientific information that formerly rivaled tropical rainforests Ϸ65 million years ago and was the coup- and ideas about the abundance and dis- in terms of spatial coverage and biotic de-grace for non-avian dinosaurs, marine tribution of biodiversity and to compare richness. The data paint a disturbing ammonites, and many other evolution- contemporary biodiversity and extinc- picture about current and projected eco- ary lineages, happened rather suddenly tion patterns with those in the distant after a large asteroid slammed into the and near evolutionary past as well as logical states for the world’s oceans. planet. Today, most of the biotic holo- with those plausible in the near-term David Wake and Vance Vredenburg caust is due—directly or indirectly—to future. Articles from the Colloquium (7) describe a similarly gloomy scenario local, regional, and global environmen- address biodiversity and extinction in for the global status of amphibians. Of tal impacts from a burgeoning human four contexts: Contemporary Patterns population. The first phase of the cur- and Processes in Animals; Contemporary Ϸ This paper serves as an introduction to this PNAS supple- rent extinction episode started 50,000– Patterns and Processes in Plants and ment, which resulted from the Arthur M. Sackler Collo- 100,000 years ago, when modern Microbes; Trends and Processes in the quium of the National Academy of Sciences, ‘‘In the Light of humans began dispersing around the Paleontological Past; and Prospects for Evolution II: Biodiversity and Extinction,’’ held December planet. The second phase started 10,000 the Future. 6–8, 2007, at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, years ago with further population in- CA. It is the second in a series of colloquia under the general creases and land-use changes associated Contemporary Patterns and Processes title ‘‘In the Light of Evolution’’ (see Box 1). The complete with the invention of agriculture. A in Animals program and audio files of most presentations are avail- third phase of environmental alteration There is no doubt that humans are the able on the NAS web site at www.nasonline.org/ Sackler࿝biodiversity. Papers from the first colloquium in the and biodiversity loss was ushered in by root cause of most ecosystem stresses series, titled ‘‘In the Light of Evolution I: Adaptation and the industrial revolution. E. O. Wilson and biotic extinctions in the modern Complex Design,’’ appeared in ref. 1. (5) estimated that the Earth is currently world. Negative human pressures on Author contributions: J.C.A., S.P.H., and F.J.A. wrote the losing Ϸ0.25% of its remaining species biodiversity occur via pollution, intro- paper. per year (such that at least 12,000 spe- ductions of alien species, overexploita- The authors declare no conflict of interest. cies may be going extinct annually). tion, landscape transformations, and †To whom correspondence may be addressed. E-mail: Such estimates are educated guesses be- other factors. Like the asteroid impact [email protected] or [email protected]. cause they represent extrapolations 65 million years ago, human impacts © 2008 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0802504105 PNAS ͉ August 12, 2008 ͉ vol. 105 ͉ suppl. 1 ͉ 11453–11457 Downloaded by guest on September 30, 2021 the Ϸ6,300 extant species of frogs, extant biodiversity that is parasitic. The and Steven Gaines (11) examine histori- salamanders, and caecilians, at least authors conclude that Ϸ10–15% of par- cal records from islands around the one-third are currently threatened with asitic helminthes (Trematoda, Cestoda, world to ask whether native plant spe- extinction, and many more are likely to Acanthocephala, and Nematoda) are at cies likewise often have gone extinct become so in the near future. A dra- risk of extinction by virtue of being de- when exotic plants were introduced and matic worldwide decline in amphibian pendent on threatened or endangered became naturalized. The answer seems populations was first noticed in the late species of vertebrate host. They also to be a clear no, at least yet. One possi- 1980s. Several ecological factors includ- conclude that parasite species diversity bility is that native plant species on is- ing habitat degradation and climatic does not map linearly onto host species lands are accumulating an extinction changes probably are involved, but so diversity and that approximately three- debt that will be paid in future species too is an unanticipated, recently uncov- quarters of all links in food webs involve losses; alternatively, the number of na- ered threat: an emerging virulent dis- a parasitic species. These findings pro- tive plus exotic plants on islands may ease (chytridiomycosis) caused by a vide a sobering reminder that the cur- reach a stable equilibrium or saturation pathogenic fungus. The source of this rent extinction pulse is affecting many point that is much higher than the en- fungus and its mode of spread are kinds of organisms (not just the conspic- demics alone had been able to achieve. poorly understood, but the disease (per- uous megafauna) and that extinction The authors examine the evidence per- haps in synergy with other