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V.1 Causes and Consequences of Species Extinctions Navjot S. Sodhi, Barry W. Brook, and Corey J. A. Bradshaw OUTLINE 1. Introduction 2. Extinction drivers 3. Extinction vulnerability 4. Consequences of extinctions 5. Conclusions The five largest mass die-offs in which 50–95% of species were eliminated occurred during the Ordovician [490–443 million years ago (mya)], Devonian (417–354 mya), Permian (299–250 mya), Triassic (251–200 mya), and Cretaceous (146–64 mya) periods. Most recently, human actions especially over the past two centuries have precipitated a global extinction crisis or the ‘‘sixth great extinction wave’’ comparable to the previous five. Increasing human populations over the last 50,000 years or so have left measurable negative footprints on biodiversity. GLOSSARY Allee effects. These factors cause a reduction in the growth rate of small populations as they decline (e.g., via reduced survival or reproductive success). coextinction. Extinction of one species triggers the loss of another species. extinction debt. This refers to the extinction of species or populations long after habitat alteration. extinction vortex. As populations decline, an insidious mutual reinforcement occurs among biotic and abiotic processes driving population size downward to extinction. extirpation. This refers to extinction of a population rather than of an entire species. invasive species. These are nonindigenous species introduced to areas outside of their natural range that have become established and have spread. megafauna. This refers to large-bodied (>44 kg) animals , commonly (but not exclusively) used to refer to the large mammal biota of the Pleistocene. minimum viable population. This is the number of individuals in a population required to have a speci- fied probability of persistence over a given period of time. 1. INTRODUCTION In the Americas, charismatic large-bodied animals (megafauna) such as saber-toothed cats (Smilodon spp.), mammoths (Mammuthus spp.), and giant ground sloths (Megalonyx jeffersonii) vanished following human arrival some 11,000–13,000 years ago. Similar losses occurred in Australia 45,000 years ago, and in many oceanic islands within a few hundred years of the arrival of humans. Classic examples of the loss of island endemics include the dodo (Raphus cucullatus) from Mauritius, moas (e.g., Dinornis maximus) from New Zealand, and elephantbirds (Aepyornis maximus) from Madagascar. Megafaunal collapse during the late Pleistocene can largely be traced to a variety of negative human impacts, such as overharvesting, biological invasions , and habitat transformation. The rate and extent of human-mediated extinctions are debated, but there is general agreement that extinction rates have soared over the past few hundred years, largely as a result of accelerated habitat destruction following European colonialism and the subsequent global expansion of the human population during the twentieth century. Humans are implicated directly or indirectly in the 100- to 10,000-fold increase in the ‘‘natural’’ or ‘‘background’’ extinction rate that normally occurs as a consequence of gradual environmental change, newly established competitive interactions (by evolution or invasion), and occasional chance calamities such as fire, storms, or disease. The current and future extinction rates are estimated using a variety of measures such as species–area models and changes in the World Conservation Union’s (IUCN) threat categories over time. Based on the global assessment of all known species, some 31, 12, and 20% of known amphibian, bird, and mammal species, respectively (by far the best-studied of all animal groups), are currently listed by the IUCN as under threat. Just how many species are being lost each year is also hotly debated. Various estimates range from a few thousand to more than 100,000 species being extinguished every year, most without ever having been scientifically described. The large uncertainty comes mainly through the application of various species–area relationships that vary substantially among communities and habitats. Despite substantial prediction error, it is nevertheless certain that human actions are causing the structure and function of natural systems to unravel . The past five great extinctions shared some important commonalities: (1) they caused a catastrophic loss of global biodiversity; (2) they unfolded rapidly (at least in the context of evolutionary and geological time); (3) taxonomically, their impact was not random (that is, whole groups of related species were lost while other related groups remained largely unaffected); and (4) the survivors were often not previously dominant evolutionary groups. All four of these features are relevant to the current biodiversity crisis. This sixth great extinction is likely to be most catastrophic in tropical regions given the high species diversity there (more than two-thirds of all species) and...


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