The Forgotten Megafauna
Transcrição
The Forgotten Megafauna
PERSPECTIVES ECOLOGY The Forgotten Megafauna An expanded megafauna concept elucidates how extinctions of the largest vertebrates in any ecosystem have similar effects. Dennis M. Hansen1 and Mauro Galetti1,2 arge terrestrial vertebrates— Gomphothere Giant ground sloth called megafauna—play key Elephant bird Giant tortoise roles in ecosystem dynamics by feeding on plants and by maintaining habitat heterogeneity (1). A global wave of megafauna extinctions occurred 50,000 to 10,000 years ago, when many large continental mammals were lost (2–5). Classical definitions of megafauna are based on such contiICA M ER TH A U nental mammals and are variO S ously given as animals larger than 44 kg (6) or above 1000 kg (7). Here, we argue that the megafauna concept should be extended beyond an AR MADAGASC absolute animal size to be context-dependent. In any given ecosystem, the largest vertebrates IUS MAURIT have ecosystem impacts that are similar on a relative scale to those of the largest vertebrates Scaling the megafauna. The magnitude of loss of frugivorous megafauna is currently most dramatic on in another ecosystem: One ecosystem’s meso- islands, as illustrated by the smaller drawn sizes of the giant ground sloth and the gomphothere from South fauna is another ecosystem’s megafauna. America, compared with the elephant bird in Madagascar and the giant tortoise of Mauritius. However, many An ecosystem function that clearly illus- continental regions are poised to catch up. trates this argument is animal-mediated seed dispersal. Here, the link between animal body To illustrate our point, we have examined in relative terms, led to a greater megafaunal mass and ecosystem function is straight- tropical and subtropical faunas from three downsizing than the extinction of even the forward: The larger the fruit-eating animal kinds of ecosystems: continental, continen- largest gomphotheres in South America (frugivore), the larger the fruits it can tal islands, and oceanic islands. For each (see the figure). consume. Thus, extinction-mediated “ecolog- fauna, we compared the body masses of the Moreover, in the relatively species-poor ical shrinkage”—the loss of species inter- largest extant frugivorous vertebrate— and simple island ecosystems, cascading actions—in community-level seed dispersal mammal, bird, or reptile—to the largest that effects of megafaunal loss may manifest roughly scales with frugivore body mass. has gone extinct since the late Pleistocene. themselves faster and with more devastating Scientists have argued that in continental On continents, the body masses of extant results than in more complex continental Central and South America, the extinction of frugivores are an order of magnitude lower ecosystems. For example, lost megafaunal the classic mammalian megafauna—such as than those of extinct frugivores; in contrast, seed-dispersal interactions on islands will not giant ground sloths and gomphotheres— in some continental and oceanic islands, be compensated for by surviving frugivores, caused disruption of seed dispersal for large body masses of extant animals are two or because they are too small. In continental fruits (4, 5). However, on islands, the extinc- even three orders of magnitude lower than ecosystems, there is a higher functional tion of large birds and reptiles in the past two those of their extinct predecessors (see the redundancy, with medium- and even smallor three millennia has led to similar disrup- supporting online material). sized species capable of filling at least part of tions (8, 9). In both locations the demoFor instance, the largest frugivores in the megafaunal niche (12). graphic and genetic consequences of large- South America were gomphoteres (7580 Anthropogenic impacts are set to cause vertebrate extinctions for plants are likely kg), whereas the largest living frugivores are further extinctions among large vertebrates, similar—for example, disruption of long- the tapirs (300 kg). On the continental island with dramatic consequences for ecosystem distance gene flow or changes in species of Madagascar, the role of largest frugivore dynamics (11). If all currently threatened vercomposition (10, 11). Yet, by the classic def- has passed from the elephant bird (450 kg) to tebrate frugivores were to go extinct, the relainitions, large insular vertebrates would not the radiated tortoise (10 kg). On Mauritius, tive ecological shrinkage in many continental be considered megafauna. giant tortoises weighing up to 100 kg were ecosystems would equal that of islands. For the largest native frugivores; today, the title instance, if all threatened frugivores in South goes to a fruitbat weighing only 0.54 kg. America were to go extinct, the largest 1Department of Biology, 371 Serra Mall, Stanford UniThus, the loss of the island giants—tortoises, remaining frugivore would be the howler 2 versity, Stanford, CA 94305, USA. UNESP-São Paulo State lizards, and flightless birds such as the dodo monkey, weighing 9 kg—a factor of 700 less University, Department of Ecology, Rio Claro, São Paulo, that were once found on many islands—has, than the giant ground sloth. Even some islands Brazil 13506-900. E-mail: [email protected] 42 3 APRIL 2009 VOL 324 SCIENCE Published by AAAS www.sciencemag.org CREDIT: MEGAFAUNA ARTWORK BY CARL BUELL Downloaded from www.sciencemag.org on May 10, 2009 L PERSPECTIVES References and Notes 1. R. Dirzo, A. Miranda, in Plant-Animal Interactions: Evolutionary Ecology in Tropical and Temperate Regions, P. W. Price, T. M. Lewinshon, G. W. Fernandes, W. W. Benson, Eds. (Wiley, New York, 1991), pp. 273–287. 2. P. L. Koch, A. D. Barnosky, Ann. Rev. Ecol. Evol. Syst. 37, 215 (2006). 3. A. D. Barnosky et al., Science 306, 70 (2004). 4. D. H. Janzen, P. S. Martin, Science 215, 19 (1982). 5. P. R. Guimaraes Jr., et al., PLoS ONE 3, e1745 (2008). 6. P. S. Martin, R. G. Klein, Quaternary Extinctions: A Prehistoric Revolution (Univ. of Arizona Press, Tucson, 1995). 7. R. N. Owen-Smith, Megaherbivores: The Influence of Very Large Body Size on Ecology (Cambridge Univ. Press, New York, 1988). 8. H. J. Meehan et al., J. Biogeogr. 29, 695 (2002). 9. D. M. Hansen et al., PLoS ONE 3, e2111 (2008). COMPUTER SCIENCE 10. R. G. Colevatti et al., Mol. Ecol. 12, 105 (2003). 11. S. J. Wright et al., Biotropica 39, 289 (2007). 12. W. Hallwachs, in Frugivores and Seed Dispersal, A. Estrada, T. H. Fleming, Eds. (Kluwer, Boston, 1986), pp. 285–304. 13. A. R. Wallace, The Geographical Distribution of Animals (Harper, New York, 1876). 14. Supported by the Velux Foundation (D.M.H.), Fundação de Amparo à Pesquisa do Estado de São Paulo (M.G.), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (M.G.). Supporting Online Material www.sciencemag.org/cgi/content/full/324/5923/42/DC1 Fig. S1 Table S1 References 10.1126/science.1172393 Downloaded from www.sciencemag.org on May 10, 2009 stand to suffer further losses; in Mauritius, the largest nonthreatened native frugivore is the gray white-eye, a bird weighing a mere 0.009 kg (see the supporting online material). An extended megafauna concept has the potential to promote synergy between otherwise disparate research and conservation foci, and to facilitate broader syntheses of ecosystem-level effects of extinctions of the largest vertebrates and the resulting ecological shrinkage. It is high time to more fully understand and ameliorate the recent and ongoing losses of all “the hugest, and fiercest, and strangest forms” (13). Computers with intelligence can design and run experiments, but learning from the results to generate subsequent experiments requires even more intelligence. Automating Science David Waltz1 and Bruce G. Buchanan2 he idea of automating asmotion; today, computer-controlled pects of scientific activity data collection is commonplace dates back to the roots of and necessary for both expericomputer science, if not to Francis mental and observational science. Bacon. Some of the earliest proAutomating many activities begrams automated the processes of yond data collection offers even creating ballistic tables, cracking more benefits. cyphers, collecting laboratory In the near term, a useful metadata, etc., by carrying out a set of phor is to consider computers as instructions from start to finish. intelligent assistants. Some assisStarting with DENDRAL in the tants gather data and attend to 1960s (1), artificial intelligence such tasks as noise filtering, data programs such as Prospector (2), smoothing, outlier rejection, and Bacon (3), and Fahrenheit (4) data storage. Other assistants are automated some of the planning, specialists at statistical analysis, Semiautomated. Scientists at Stanford’s Instrumentation Research Laboratory analysis, and discovery portions (circa 1970) linked a gas chromatograph and high-resolution mass spectrome- still others at bench work. This of the scientific enterprise. How- ter to computers to automate studies of biological fluids, meteorites, and other metaphor has driven many reever, most of these programs were materials. Stanford’s DENDRAL Project experimented with automated interpreta- search projects over the past sevstill designed to run a calculation tion of the data and experiment planning to specify nuclear magnetic resonance eral decades and has led to many to completion, produce an answer, or infrared data that would resolve ambiguities in the mass spectral data. of the most successful applicaand then stop. They did not fully tions of computers. “close the loop” in the sense of examining the systems, derived from visual observation of An early articulation of this metaphor results of their actions, deciding what to try such systems. As these reports show, it is pos- is Joshua Lederberg’s effort at Stanford next, potentially cycling forever. sible for one computer program to step University School of Medicine to develop an Two reports on pages 85 and 81 of this through the activities needed to conduct a con- automated biomedical laboratory to examine issue push the boundaries of automatic scien- tinuously looping procedure that starts with a the soil of Mars for traces of life, as part of tific experimentation and discovery. King question, carries out experiments to answer the 1975 Viking mission deployed by the et al. (5) describe a robotic system for running the question, evaluates the results, and refor- U.S. National Aeronautics and Space Adbiological experiments, evaluating their mulates new questions. ministration. The robot assistant Lederberg results, and deciding what experiments to try The main goals of automation in science designed, with engineer Elliott Levinthal, next. Schmidt and Lipson (6) describe their have been to increase productivity by increas- consisted of a conveyor belt that scooped up work on discovering compact equations that ing efficiency (e.g., with rapid throughput), to samples of Martian soil and deposited them characterize complex nonlinear dynamical improve quality (e.g., by reducing error), and within a computer-controlled mass spectromto cope with scale, allowing scientific treat- eter. Each soil sample was bombarded with 1Center for Computational Learning Systems, Columbia ment of topics that were previously impossi- electrons, producing a fragmentation pattern University, New York, NY 10115, USA. 2Computer Science ble to address. Tycho Brahe spent a lifetime that sorted the charged particles (ions) accordDepartment, University of Pittsburgh, Pittsburgh, PA recording observations that allowed Johannes ing to their mass. This pattern was transmitted 15260, USA. E-mail: [email protected]; waltz@ ccls.columbia.edu Kepler to formulate Kepler’s laws of planetary to Earth, where scientists could analyze it for CREDIT: ROBERT K. LINDSAY T www.sciencemag.org SCIENCE VOL 324 Published by AAAS 3 APRIL 2009 43 www.sciencemag.org/cgi/content/full/324/5923/42/DC1 Supporting Online Material for The Forgotten Megafauna Dennis M. Hansen* and Mauro Galetti *To whom correspondence should be addressed. E-mail: [email protected] Published 3 April 2009, Science 324, 42 (2009) DOI: 10.1126/science.1172393 This PDF file includes: Fig. S1 Table S1 References Supplementary online material Figure S1. Current and projected ecological shrinkage in seed dispersal interactions on continents, continental islands, and oceanic islands, illustrated by declines in frugivore body mass. Arrows span declines from the largest recently extinct frugivore to the largest extant frugivore (red arrows), as well as maximum projected future decline if all currently threatened frugivores went extinct (IUCN status of VU, EN or CR; yellow arrows). Regions: Africa [AF], South America [SA], Australia [AU], Southeast Asia [AS]; Cuba [CU], Madagascar [MA], New Caledonia [NC]; Tonga [TO], Mauritius [MR], Fiji [FI]. Table S1. Largest extinct and extant frugivores in selected continents, continental islands and oceanic islands. Body mass in kg. IUCN status, from least concern to critically endangered: LC = Least Concern, NT = Near Threatened, VU = Vulnerable, EN = Endangered, CR = Critically Endangered. Region Largest extinct Largest extant Body mass Largest extant non-threatened Body IUCN mass status (1) Body mass IUCN status (1) Africa Pleistocene elephant Elephas iolensis (2) 6500 African bush elephant Loxodonta africana (2) 3940 NT African bush elephant Loxodonta africana (2) 3940 NT South America Gomphothere Stegomastodon superbus (2) 7580 Baird’s tapir Tapirus bairdii (2) 300 EN Red howler monkey Alouatta seniculus (2) 9 LC Australia Diprotodon Diprotodon australis (3) 2700 Southern cassowary 85 Casuarius casuarius (8) VU Emu Dromaius novaehollandiae (8) 60 LC SE Asia Stegodon Stegodon sp. (2) 10000 Asian elephant Elephas maximus (2) 3000 VU Tibetan macaque Macaca thibetana (2) 45 NT Cuba Cuban ground sloth Megalocnus rodens (4) 200 Cuban rock iguana Cyclura nubila (9) 4.1 VU Prehensile-tailed hutia Mysateles prehensilis (11) 1.9 NT Madagascar Elephant bird Aepyornis maximus (5) 450 Radiated tortoise 10 Astrochelys radiata (10) VU Brown lemur Eulemur fulvus (2) 5.8 LC New Caledonia Giant horned tortoise Meiolania sp. (6) 500 Pacific flying fox Pteropus tonganus (2) 0.6 LC Pacific flying fox Pteropus tonganus (2) 0.6 LC Tonga Giant iguana Brachylophus gibbonsi (7) 3.3 Pacific flying fox Pteropus tonganus (2) 0.6 LC Pacific flying fox Pteropus tonganus (2) 0.6 LC Mauritius Mauritian saddle-backed giant tortoise Cylindraspis triserrata (6) 100 Greater Mascarene flying fox Pteropus niger (2) 0.54 EN Mauritius grey white-eye Zosterops mauritianus (8) 0.009 LC Fiji Giant horned tortoise Meiolania sp. (6) 300 Pacific flying fox Pteropus tonganus (2) 0.6 LC Pacific flying fox Pteropus tonganus (2) 0.6 LC Acknowledgements: Thanks to the following for providing information relating to Table S1: Richard Corlett, Anthony Cheke, Trevor Worthy, Christopher Johnson, Pedro Jordano. References: 1. IUCN Redlist online: http://www.iucnredlist.org (accessed February 01, 2009). 2. Smith, F. A. et al. Ecological Archives E084-094, (2003) and personal communications (see above). 3. C. Johnson, Australia’s Mammal Extinctions: A 50,000 year history. Cambridge University Press (2006) 4. D. W. Steadman et al., Proc. Natl. Acad. Sci. U.S.A. 102, 11763 (2005). 5. D. Amadon, Condor 49, 159 (1947). 6. No published body mass estimates exist for these species; we estimated the body masses based on known carapace length/body mass relations for the giant Aldabran tortoise, Aldabrachelys gigantea, from M. J. Coe, D. Bourn, I. R. Swingland, Phil. Trans. R. Soc. Lond. B. 286, 163 (1979). 7. No published body mass estimate exists for this species; we estimated the body mass using the SVL reported in G. K. Pregill, D. W. Steadman, J. Herpetol. 38, 15 (2004), and the relationship between SVL and body mass for Iguana iguana reported in R. W. Van Devender, in Iguanas of the World, G. M. Burghardt, A. S. Rand, Eds. (Noyes Publications, Park Ridge, 1982), pp. 162–183. 8. J. B. Dunning, Ed., CRC Handbook of Avian Body Mass, 2nd edition (CRC Press, Cleveland, 2007). 9. K. Beovides–Casas, C. A. Mancina, Animal Conservation and Biodiversity 29, 1 (2006). 10. M. Pedrono, The Tortoises and Turtles of Madagascar (Natural History Publications, Borneo, 2008). 11. M. Hutchins, D. Kleiman, V. Geist, M. McDade, Eds. Grzimek's Animal Life Encyclopedia, Second Edition (Thomson-Gale, Farmington Hills, 2003).