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Home , Coextinction, Ecosystem collapse, Extinction, Extinction debt, Extinction event, Extinction vortex

Copyrighted Material V.1 Causes and Consequences of Navjot S. Sodhi, Barry W. Brook, and Corey J. A. Bradshaw

OUTLINE . This refers to large-bodied (>44 kg) ani­ mals, commonly (but not exclusively) used to refer 1. Introduction to the large biota of the Pleistocene. 2. drivers minimum viable . 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 [490–443 1. INTRODUCTION million ago (mya)], (417–354 mya), (299–250 mya), (251–200 mya), and In the Americas, charismatic large-bodied (146–64 mya) periods. Most recently, actions espe­ (megafauna) such as saber-toothed cats (Smilodon cially over the past two centuries have precipitated a global spp.), (Mammuthus spp.), and giant ground extinction crisis or the ‘‘sixth great extinction wave’’ com­ ( jeffersonii) vanished following hu­ parable to the previous five. Increasing human man arrival some 11,000–13,000 years ago. Similar over the last 50,000 years or so have left measurable losses occurred in 45,000 years ago, and in negative footprints on . many oceanic islands within a few hundred years of the arrival of . Classic examples of the loss of is­ endemics include the (Raphus cucullatus) GLOSSARY from , (e.g., maximus) from Allee effects. These factors cause a reduction in the , and elephantbirds ( maximus) growth rate of small populations as they decline from . Megafaunal collapse during the late (e.g., via reduced survival or reproductive success). Pleistocene can largely be traced to a variety of negative . Extinction of one species triggers the loss human impacts, such as overharvesting, biological in­ of another species. vasions, and transformation. . 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 . 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 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 . 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 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 . 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 , , and mammal species, re­ responsible for an extinction by facilitating the estab­ spectively (by far the best-studied of all 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 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 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 . Despite substantial prediction error, Asian 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 is also an important driver of ex­ portant commonalities: (1) they caused a catastrophic tinctions among and tends to operate syn­ loss of ; (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 created to allow op­ time); (3) taxonomically, their impact was not random erations to penetrate into virgin 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 and , with cur­ extinction is likely to be most catastrophic in tropical rently extracted from tropical forests at approximately regions given the high 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 (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, (global warming) connected tion time (interval from birth to reproductive age) is a to increasing concentration of atmospheric carbon di­ function of body mass (), 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 , introduced predators and diseases, and habi­ , (e.g., the lion, leo), and Af­ tat destruction when early people first arrived in virgin rican elephants (Loxodonta africana), are particularly . 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 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 cow sive volcanic eruption, or even a rapid loss of large (Hydrodamalis gigas), an aquatic herbivorous mam­ areas of unique and because of defor­ mal that inhabited the Asian coast of the , estation. But ultimately, any phenomena that can cause is the quintessential example of the rapid demise of a Copyrighted Material

516 Conservation 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 , its skin for making boats, and its invasion catastrophe occurred in the world’s largest subcutaneous fat for use in oil lamps. freshwater — in tropical East . The and biological changes Celebrated for its amazing collection of over 600 precipitated by invasive species represent another endemic (i.e., formerly of the leading cause of . Of 170 extinct spe­ ) cichlid fishes ( Cichlidae), the cies for which causes have been identified reliably, Lake Victoria cichlid community is perhaps one of the invasive species contributed directly to the demise of most rapid, extensive, and recent vertebrate radiations 91 (54%). In particular, the rates of extinctions oc­ known. There is also a rich community of endemic curring on islands have been greatly elevated by the noncichlid fish that inhabit the Lake. In addition to the introduction of novel predators. Several ecological and threats posed to this unique biota by a rapid rise in -history attributes of island species, such as their fisheries exploitation, human density, deforestation, naturally constrained geographic range, small popula­ and agriculture during the past century, without doubt tion sizes, and particular traits (e.g., lack of flight in the most devastating effect was the introduction of the birds or lack of thorns in ) make island biotas predatory (Lates niloticus) in the 1950s. vulnerable to from invading species. For This voracious predator, which can grow to more than example, the introduction of the brown 2 m in length, was introduced from Albert and (Boiga irregularis) shortly after World War II wreaked Turkana (Uganda and Kenya, respectively) to com­ havoc on the biodiversity of the island of Guam in the pensate for depleting commercial fisheries in Lake South Pacific. In all likelihood, tree were di­ Victoria. Although the Nile perch population remained rectly responsible for the loss of 12 of 18 native bird relatively low for several decades after its introduc­ species, and they also reduced the populations of other tion, an eventual population explosion in the 1980s vertebrates such as flying ( mariannus), caused the devastating direct or indirect extinction of mainly because of the inability of the island’s native 200–400 cichlid species endemic to the Lake as well as species to recognize the novel predator as a threat. the extinction of several noncichlid fish species. Al­ Despite an annual expenditure of US$44.6 million for though many other threats likely contributed to the the management of this problem, tree snakes on Guam observed extinctions, including direct overexploitation are still not under control, largely because of their and from agriculture and deforestation ability to penetrate artificial snake barriers such as leading to a change in the algal plankton community, fences. there are few other contemporary examples of such a The Culex quinquefasciatus was inad­ rapid and massive extinction event involving a single vertently introduced to in 1826, and the group of closely related species. disease-causing parasite (Plasmodium relictum) it car­ Human-mediated climate change represents a po­ ries arrived soon after. Since then, avian (in tentially disastrous sleeping giant in terms of future conjunction with other threats) has been responsible biodiversity losses. Climate warming can affect species for the decline and extinction of some 60 species of in five principal ways: (1) alterations of species densi­ endemic forest birds on the Hawaiian Islands. Having ties (including altered community composition and evolved in the absence of the disease, Hawaiian bird structure); (2) range shifts, either poleward or upward species were generally unable to cope with the debili­ in elevation; (3) behavioral changes, such as the phe­ tating effects of the novel parasite. However, more nology (seasonal timing of life cycle events) of migra­ than 100 years after the establishment of the disease, tion, breeding, and flowering; (4) changes in mor­ some native thrushes (Myadestes spp.) are now show­ phology, such as body size; and (5) reduction in genetic ing resistance to the disease. Sadly, many of the re­ diversity that leads to depression. A related maining species, especially forest birds in the family threat for island and coastal biotas is the predicted loss Drepanididae, are still vulnerable and are now re­ of habitat via inundation by rising sea levels. Although stricted to altitudes where temperatures are below the large fluctuations in climate have occurred regularly thermal tolerance limits of the mosquito vector. Global throughout ’s history, the implications of an­ warming is predicted to increase the altitudinal distri­ thropogenic global warming for contemporary biodi­ bution of the mosquito, thus spelling doom for disease- versity are particularly pessimistic because of the rate susceptible birds as mosquito-free habitats disappear. of change and previous heavy modification of land­ The most feasible method of reducing transmission of scapes by humans. Good empirical evidence for some malaria is to reduce or eliminate vector mosquito of these effects is rare, and speculations abound, but Copyrighted Material

Species Extinctions 517 there are already many local or regional examples habitat loss. For example, even if net deforestation and model-based predictions that support the view rates can be reduced or even halted, the extinction debt that rapid climate change, acting in concert with other of remnant and secondary forest patches will see the drivers of species loss and habitat degradation, will be extinction of countless remaining species over this one of the most pressing conservation issues global interval. biodiversity faces over the coming centuries. One glimpse of a possible future crisis comes 3. EXTINCTION VULNERABILITY from the highland forests of Monterverde (Costa Rica), where 40% (20 of 50) of and toad species dis­ Certain life-history, behavioral, morphological, and appeared following synchronous population crashes in physiological characteristics appear to make some spe­ 1987, with most crashes linked to a rapid progressive cies more susceptible than others to the extinction warming and drying of the local climate. The locally drivers described above. In general, large-sized species endemic (Bufo periglenes) was one of the with a restricted distribution that demonstrate habitat high-profile casualties in this area. It has been sug­ specialization tend to be at greater of extinction gested that climate warming resulted in a retreat of the from human agency than others within their respective clouds and a drying of the mountain habitats, making taxa (e.g., Javan , Rhinoceros sondaicus), amphibians more susceptible to fungal and parasite especially to processes such as rapid habitat loss. outbreaks. Indeed, the pathogenic chytrid Ba­ Because of their high habitat specificity and/or low trachochytrium dendrobatidis, which grows on am­ population densities, may be more prone to phibian skin and increases mortality rates, has been extinction than common species. The size of a species’ implicated in the loss of harlequin ( spp.) range is also a major determinant of its extinction in Central and and reductions in proneness. Small ranges may make species more vul­ other amphibian populations elsewhere. It is hypoth­ nerable to stochastic perturbations, even if local abun­ esized that warm and dry conditions may stress am­ dance is high; for example, proportionally more phibians and make them more vulnerable to the fungal passerines (perching birds) with relatively small geo­ infection. graphic ranges in the Americas are at risk of extinction Irrespective of the reason for a population’s decline than their more widely distributed counterparts. Such from a large to , unusual (and trends are worrisome because those species with often random and detrimental) events assume promi­ shrinking ranges as a result of adverse human activities nence at low abundances. For instance, although become particularly vulnerable to other drivers such as among individuals is reduced at low den­ climate change. Habitat loss also reduces the patch sities and can induce a population rebound, a coun­ sizes necessary for species requiring large home ranges, tervailing phenomenon known as the ‘‘’’ can making them vulnerable to extinction from a loss of act to draw populations toward extinction by (for in­ subpopulation connectedness, reduced dispersal ca­ stance) disrupting behavioral patterns that depend on pacity, and the ensuing lower population viability. numbers (e.g., herd defense against predators) or by Larger-bodied vertebrates are considered to be more genetic threats such as . Small extinction-prone than smaller-bodied ones when the populations, dominated by chance events and Allee threatening process unfolds rapidly or intensely. In­ effects, are often considered to have dipped below their deed, threatened are an order of magnitude ‘‘minimum viable population’’ size. Thus, once a major heavier than nonthreatened ones. A common expla­ has occurred (from habitat loss, nation for this trend is that body size is inversely cor­ overexploitation, or in response to many other possible related with population size, making large-bodied an­ stressors), an ‘‘extinction vortex’’ of positive feed­ imals less abundant and more vulnerable to chronic back loops can doom species to extinction, even if the environmental perturbations (while being buffered original threats have been alleviated. Further, many against short-term environmental fluctuations). The species may take decades to perish following habitat extinction proneness of large-bodied animals to human degradation. Although some species may withstand activities is further enhanced because of other corre­ the initial shock of land clearing, factors such as the lated traits, such as their requirement of large area, lack of food , breeding sites, and dispersers greater food intake, high habitat specificity, and lower may make populations unviable, and they eventually reproductive rate. succumb to extinction. This phenomenon evokes the Large species can also be more vulnerable to human concept of ‘‘living-dead’’ species, or those ‘‘committed persecution such as hunting, whereas smaller species to extinction.’’ The eventual loss of such species is are generally more vulnerable to habitat loss. It is im­ referred to as the ‘‘extinction debt’’ caused by past portant, however, to be cautious when constructing Copyrighted Material

518 generalized rules regarding the role of body size in the penetrating smaller forest fragments. Absence of some extinction process. Because they have a slower repro­ insectivorous bird species from small fragments may ductive rate, larger are more vulnerable to not be related to food scarcity; rather, it may result overexploitation than smaller finches, despite fewer from their poorer dispersal abilities. The ability to numbers of the former being captured for the pet trade. disperse in birds and depends on morphological However, some smaller species (e.g., white-eyes, Zos­ characteristics such as wing loading, and physiological terops spp.) with small population sizes are also vul­ restrictions such as intolerance to when mov­ nerable to extinction because of heavy rates for ing within the nonforested matrix separat­ the pet trade, suggesting that only when the threatening ing fragments. As a result, poor dispersal ability may processes are approximately equivalent will the larger make certain species vulnerable to extinction because of two species being compared demonstrate a higher they cannot readily supplement sink habitats (habi­ risk of extinction. In addition to body size, other tats in which populations cannot replace themselves), morphological characteristics affect extinction prone­ supporting otherwise unviable subpopulations, or ness. For instance, large investment in secondary sexual colonize new areas. Because of poor dispersal abil­ characteristics may render highly dimorphic species ity, patchy distributions, and generally low popula­ less adaptable in a changing environment or more at­ tion densities, the of species in tractive to specimen or pet-trade collectors. fragmented landscapes may be difficult to maintain, When an environment is altered abruptly or sys­ with the resulting inbreeding depression further re­ tematically at a rate above normal background change, ducing population size toward extinction. However, or beyond the capacity of via natural se­ clear and quantitative demonstrations of the role of lection, specialist species with narrow ecological niches life-history traits in the extinction process of biotas are often bear the brunt of progressively unfavorable still rare. conditions such habitat loss and degradation. For in­ stance, highly specialized forest-dependent taxa are 4. CONSEQUENCES OF EXTINCTIONS acutely vulnerable to extinction following deforesta­ tion and forest fragmentation. Possible mechanisms The extinction of certain species such as large preda­ include reductions in breeding and feeding sites, in­ tors and may have more devastating eco­ creased predation, elevated and nutrient logical consequences than the extinction of others. loss, dispersal limitation, enhanced , and Ironically, avian vulnerability to predation is often other stressors. Conversely, non-forest-dependent spe­ exacerbated when certain large predatory species be­ cies or those that prefer open habitats are often better come rarer in tropical communities. For example, al­ able to persist in disturbed landscapes and may even be though large cats such as jaguars (Panthera onca)do favored by having fewer competitors or expanded not prey on small birds directly, they exert a limiting ranges following deforestation. It is important to be force on smaller predators such as medium-sized and aware that in relatively stable systems, evolution en­ small mammals (), which become more genders the of taxa that occupy all available abundant with the former species’ decline. The cor­ niches so both specialist and generalist species can co­ ollary is that abundant mesopredators inflict an above- exist. As a result, the rapid pace of habitat and climate average predation rate on the eggs and nestlings of change renders specialization a modern ‘‘curse’’ in small birds. Although this ‘‘-release’’ evolutionary terms. hypothesis has been applied largely to mammals (e.g., specialization is one mechanism that can Australian dingoes, Canis lupus, suppressing foxes and compromise a species’ ability to persist in altered cats; coyotes in California controlling cat ), habitats. Many studies have shown that frugivorous the loss of large predatory birds such as the harpy and insectivorous birds are more extinction-prone than (Harpia harpyja) may have similar ecosystem effects. other avian feeding guilds, with the lack of year-round Similar mesopredator release has been demonstrated access to fruiting plants in fragmented forests being the for the first time in the marine environment, where the culprit for the former. A number of hypotheses have overexploitation of large pelagic sharks resulted in an been proposed to explain the disappearance of insec­ increase in rays and skates that eventually suppressed tivorous birds from deforested or fragmented areas. commercially important scallop populations. Likewise, First, deforestation may impoverish the does the disappearance of a competitor result in the and reduce selected microhabitats (e.g., niche expansion and higher densities of subordinate dead leaves). Second, may be poor dis­ species? This phenomenon has been observed between persers and have near-ground nesting habits, the latter unrelated taxa—the extinction of insectivorous birds trait making them more vulnerable to nest predators from scrub forests of West Indian islands correlated Copyrighted Material

Species Extinctions 519 with the subsequent higher of competing forest bees were worth US$60,000 to a 1100-ha farm. Anolis lizards. A forest patch as small as 20 ha located near farms Conservation biologists have traditionally focused can increase coffee yield and thus bring large eco­ on the study of the independent declines, extirpations, nomic benefits to the farmers. Such findings illustrate or extinctions of individual species while paying rela­ the imperative of preserving native forests near agro­ tively less attention to the possible cascading effects of forestry systems to facilitate the travel by forest- species coextinctions (e.g., hosts and their parasites). dependent pollinating insects. However, it is likely that many coextinctions between interdependent taxa have occurred, but most have gone unnoticed in these relatively understudied systems. For 5. CONCLUSIONS example, an extinct feather (Columbicola ex­ Although extinctions are a normal part of evolution, tinctus) was discovered in 1937, 23 years after likely human modifications to the planet in the last few coextinction with its host (Ectopistes centuries, and perhaps even millennia, have greatly migratorius). Ecological processes disrupted by ex­ accelerated the rate at which extinctions occur. Habitat tinction or species decline may also lead to cascading loss remains the main driver of extinctions, but it may and catastrophic coextinctions. Frugivorous animals act synergistically with other drivers such as over­ and fruiting plants on which they depend have a key harvesting and , and, in the future, climate interaction linking and dispersal change. Large-bodied species, rare species, and habitat with animal nutrition. Thus, the two interdependent specialists are particularly prone to extinction as a re­ taxa are placed in jeopardy by habitat degradation. sult of rapid human modifications of the planet. Ex­ Many produce large, lipid-rich fruits adapted for tinctions can disrupt vital ecological processes such as animal dispersal, so the demise of avian frugivores may pollination and seed dispersal, leading to cascading have serious consequences for forest regeneration, even losses, , and a higher extinction rate if the initial drivers of habitat loss and degradation are overall. annulled. Essential ecosystem functions provided by forest are also highly susceptible when species FURTHER READING are lost after habitat loss and degradation. Acting as Brook, Barry W., Navjot S. Sodhi, and Peter K. L. Ng. 2003. in Southeast Asian , figs Catastrophic extinctions follow deforestation in Singa­ rely on tiny (1–2 mm) species-specific wasps for their pore. 424: 420–423. This is one of few pollination. Some fig wasps may have limited dispersal reporting broad-scale extinctions driven by tropical de­ ability, suggesting that forest can reduce forestation. wasp densities and, by proxy, the figs that they polli­ Clavero, Miguel, and Emili Garcia-Berthou. 2005. Invasive nate. Similarly, dung beetles are essential components species are a leading cause of animal extinctions. Trends in of ecosystem function because they contribute heavily and Evolution 20: 110. The article highlights that to nutrient- processes, seed dispersal, and the invasive species represent one of the primary threats to reduction of disease risk associated with dung accu­ biodiversity. mulation. In Venezuela, heavier dung beetles were Dirzo, Rudolfo, and Peter J. Raven. 2003. Global state of biodiversity and loss. Annual Review of Environment and more extinction-prone than lighter species on artifi­ Resources 28: 137–167. The article constitutes a major cially created forested islands, which predicts particu­ review of the state of the modern global biodiversity and larly dire ecosystem functional loss given the former its associated losses. group’s greater capacity to dispose of dung. Fagan, William F., and E. E. Holmes. 2006. Quantifying the Almost all flowering plants in tropical rainforests extinction vortex. Ecology Letters 9: 51–60. This is the are pollinated by animals, and an estimated one-third only study yet to quantify the final phases of extinction in of the human diet in tropical countries is derived from vertebrates for which date of extinction was known. insect-pollinated plants. Therefore, a decline of forest- IUCN Red List of . Download from http: dwelling pollinators impedes plant reproduction not //www.iucnredlist.org. This presents an up-to-date clas­ only in forests but also in neighboring agricultural sification of and reasons for a listed species’ . areas visited by these species. Lowland coffee (Coffea Koh, Lian P., Robert R. Dunn, Navjot S. Sodhi, Robert canephora) is an important tropical cash crop, and it K. Colwell, Heather C. Procter, and Vince S. Smith. depends on bees for cross-pollination. A study in Costa 2004. Species co-extinctions and the biodiversity crisis. Rica found that forest bees increased coffee yield by 305: 1632–1634. This models how loss of a spe­ 20% in fields within 1 km of the forest edge. Between cies could indirectly result in the extinction of dependent 2000 and 2003, the pollination services provided by species. Copyrighted Material

520 Conservation Biology

Pimm, Stuart L., and Peter Raven. 2000. Extinction by to coffee production. Proceedings of the National Acad­ numbers. Nature 403: 843–845. The article summarizes emy of Sciences U.S.A. 34: 12579–12582. This work the likely extent of biodiversity losses as a result of human shows how the loss of ecosystem services can affect pol­ activities. lination of commercial crops. Pounds, J. Alan, Martin R. Bustamante, Luis A. Coloma, Rosser, Alison M., and Sue A. Manika. 2002. Over- Jamie A. Consuegra, Michael P. L. Fogden, Pru N. Foster, exploitation and species extinctions. Conservation Biol­ Enrique La Marca, Karen L. Masters, Andres Merino- ogy 16: 584–586. This work provides a quantitative Viteri, Robert Puschendorf, Santiago R. Ron, G. Arturo overview of the extent of threat faced by birds and Sanchez-Azofeifa, Christopher J. Still, and Bruce E. mammals from direct exploitation by people. Young. 2006. Widespread amphibian extinctions from Sekercioglu, Cagan H., Gretchen C. Daily, and Paul R. Ehr­ epidemic disease driven by global warming. Nature 439: lich. 2004. Ecosystem consequences of bird declines. 161–167. The article provides evidence on the role of Proceedings of the National Academy of Sciences U.S.A. climate change in recent amphibian extinctions. 101: 18042–18047. This article provides a framework for Ricketts, Taylor H., Gretchen C. Daily, Paul R. Ehrlich, and assessing the loss of ecosystem functions caused by avian C. D. Michener. 2004. Economic value of tropical forest declines.