Causes and Consequences of Species Extinctions Navjot S

Copyrighted Material V.1 Causes and Consequences of Species Extinctions Navjot S. Sodhi, Barry W. Brook, and Corey J. A. Bradshaw OUTLINE megafauna. This refers to large-bodied (>44 kg) ani mals, commonly (but not exclusively) used to refer 1. Introduction to the large mammal biota of the Pleistocene. 2. Extinction drivers minimum viable population. This is the number of in 3. Extinction vulnerability dividuals in a population required to have a speci 4. Consequences of extinctions fied probability of persistence over a given period of 5. Conclusions time. The five largest mass die-offs in which 50–95% of species were eliminated occurred during the Ordovician [490–443 1. INTRODUCTION million years ago (mya)], Devonian (417–354 mya), Permian (299–250 mya), Triassic (251–200 mya), and Cretaceous In the Americas, charismatic large-bodied animals (146–64 mya) periods. Most recently, human actions espe (megafauna) such as saber-toothed cats (Smilodon cially over the past two centuries have precipitated a global spp.), mammoths (Mammuthus spp.), and giant ground extinction crisis or the ‘‘sixth great extinction wave’’ com sloths (Megalonyx jeffersonii) vanished following hu parable to the previous five. Increasing human populations man arrival some 11,000–13,000 years ago. Similar over the last 50,000 years or so have left measurable losses occurred in Australia 45,000 years ago, and in negative footprints on biodiversity. many oceanic islands within a few hundred years of the arrival of humans. Classic examples of the loss of is land endemics include the dodo (Raphus cucullatus) GLOSSARY from Mauritius, moas (e.g., Dinornis maximus) from Allee effects. These factors cause a reduction in the New Zealand, and elephantbirds (Aepyornis maximus) growth rate of small populations as they decline from Madagascar. Megafaunal collapse during the late (e.g., via reduced survival or reproductive success). Pleistocene can largely be traced to a variety of negative coextinction. Extinction of one species triggers the loss human impacts, such as overharvesting, biological in of another species. vasions, and habitat transformation. extinction debt. This refers to the extinction of species The rate and extent of human-mediated extinctions or populations long after habitat alteration. are debated, but there is general agreement that ex extinction vortex. As populations decline, an insidious tinction rates have soared over the past few hundred mutual reinforcement occurs among biotic and years, largely as a result of accelerated habitat de abiotic processes driving population size downward struction following European colonialism and the sub to extinction. sequent global expansion of the human population extirpation. This refers to extinction of a population during the twentieth century. Humans are implicated rather than of an entire species. directly or indirectly in the 100- to 10,000-fold in invasive species. These are nonindigenous species in crease in the ‘‘natural’’ or ‘‘background’’ extinction troduced to areas outside of their natural range that rate that normally occurs as a consequence of gradual have become established and have spread. environmental change, newly established competitive Copyrighted Material Species Extinctions 515 interactions (by evolution or invasion), and occasional mortality rates to exceed reproductive replacement chance calamities such as fire, storms, or disease. The over a sustained period can cause a species to become current and future extinction rates are estimated using extinct. Such forces may act independently or syner a variety of measures such as species–area models and gistically, and it may be difficult to identify a single changes in the World Conservation Union’s (IUCN) cause of a particular species extinction event. For in threat categories over time. Based on the global as stance, habitat loss may cause some extinctions directly sessment of all known species, some 31, 12, and 20% by removing all individuals, but it can also be indirectly of known amphibian, bird, and mammal species, re responsible for an extinction by facilitating the estab spectively (by far the best-studied of all animal groups), lishment of an invasive species or disease agent, im are currently listed by the IUCN as under threat. proving access to human hunters, or altering biophys Just how many species are being lost each year is ical conditions. As a result, any process that causes a also hotly debated. Various estimates range from a few population to dwindle may ultimately predispose that thousand to more than 100,000 species being ex population to extinction. tinguished every year, most without ever having been Evidence to date suggests that deforestation is cur scientifically described. The large uncertainty comes rently, and is projected to continue to be, the prime mainly through the application of various species–area direct and indirect cause of reported extirpations. For relationships that vary substantially among communi example, it is predicted that up to 21% of Southeast ties and habitats. Despite substantial prediction error, Asian forest species will be lost by 2100 because of past it is nevertheless certain that human actions are causing and ongoing deforestation. Similar projections exist for the structure and function of natural systems to un biotas in other regions. ravel. The past five great extinctions shared some im Overexploitation is also an important driver of ex portant commonalities: (1) they caused a catastrophic tinctions among vertebrates and tends to operate syn loss of global biodiversity; (2) they unfolded rapidly (at ergistically with other drivers such as habitat loss. For least in the context of evolutionary and geological example, roads and trails created to allow logging op time); (3) taxonomically, their impact was not random erations to penetrate into virgin forests make previ (that is, whole groups of related species were lost while ously remote areas more accessible to human hunters, other related groups remained largely unaffected); and who can, in turn, cause the decline and eventual ex (4) the survivors were often not previously dominant tirpation of forest species. It is estimated that overex evolutionary groups. All four of these features are rel ploitation is a major threat to at least one-third of evant to the current biodiversity crisis. This sixth great threatened birds and amphibians, with wildlife cur extinction is likely to be most catastrophic in tropical rently extracted from tropical forests at approximately regions given the high species diversity there (more six times the sustainable rate. In other words, the than two-thirds of all species) and the large, expanding quantity, and most likely the diversity, of human prey— human populations that threaten most species there as both fisheries and ‘‘bush’’ (wild) meat—are rapidly well. diminishing. The major ‘‘systematic drivers’’ of modern species Megafauna—those species weighing in the tens to loss are changes in land use (habitat loss degradation hundreds of kilograms—are among the most vulnera and fragmentation), overexploitation, invasive species, ble to overexploitation. In general, a species’ genera disease, climate change (global warming) connected tion time (interval from birth to reproductive age) is a to increasing concentration of atmospheric carbon di function of body mass (allometry), so larger, longer- oxide, and increases in nitrogen deposition. Mechan lived, and slower-reproducing animal populations are isms for prehistoric (caused by humans >200 years generally unable to compensate for high rates of har ago) extinctions are likely to have been similar: over- vesting. Because slow-breeding large animals, such as hunting, introduced predators and diseases, and habi apes, carnivores (e.g., the lion, Panthera leo), and Af tat destruction when early people first arrived in virgin rican elephants (Loxodonta africana), are particularly landscapes. vulnerable to hunting, the potential for population recovery in these animals over short time scales is low. As an example supporting this generality, there is evi 2. EXTINCTION DRIVERS dence that 12 large vertebrate species have been ex Some events can instantly eliminate all individuals of tirpated from Vietnam, primarily because of excessive a particular species, such as an asteroid strike, a mas hunting, within the past 40 years. The Steller’s sea cow sive volcanic eruption, or even a rapid loss of large (Hydrodamalis gigas), an aquatic herbivorous mam areas of unique and critical habitat because of defor mal that inhabited the Asian coast of the Bering Sea, estation. But ultimately, any phenomena that can cause is the quintessential example of the rapid demise of a Copyrighted Material 516 Conservation Biology species as a result of overexploitation. Discovered in populations through chemical treatments and the elim 1741, it became extinct by 1768 because of overhunt ination of larval habitats. ing by sailors, seal hunters, and fur traders. This species Perhaps one of the most infamous examples of an was hunted for food, its skin for making boats, and its invasion catastrophe occurred in the world’s largest subcutaneous fat for use in oil lamps. freshwater lake—Lake Victoria in tropical East Africa. The ecosystem and biological community changes Celebrated for its amazing collection of over 600 precipitated by invasive species represent another endemic haplochromine (i.e., formerly of the genus leading cause of biodiversity

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