Photo 3.1
The Golden Toad Bufo periglenes (Extinct) has become the flagship species of the amphibian decline phenomenon. This photo of males congregating at a breeding pool in the Monteverde Cloud Forest in Costa Rica gives an indication of its former abundance within its tiny range. It last bred in normal numbers in 1987. In 1988, only eight males and two females could be located. In 1989, a single male was found, this being the last record of the species.
Photo: © Michael and Patricia Fogden.
The global extinction of a species usually represents an end point in a long series of population extinctions. During this process of extinction unique evolutionary history is lost at every stage, but the death of the last individual of a species represents the permanent and irreversible loss of one of life's unique evolutionary and functional forms. Creating an inventory of recent extinctions helps to highlight the long list of unique species that have been lost forever. Understanding the extent of recent extinctions provides insight into historic extinction rates, which in turn can be compared to the rates over geological time to determine if current trends are normal or a cause for concern. An insight into the process of extinction can help us to identify species that are at risk of extinction and enable us to highlight taxonomic groups or species from specific regions that are or will be particularly prone to extinction. This section focuses on defining the extent of recent extinctions as well as identifying patterns of extinction. These patterns are discussed further in Section 6 where the dynamic process of extinction is addressed.
The IUCN Red List documents extinctions that have occurred on a global scale during historic times (for population extinctions see Box 3.1). A species qualifies for the IUCN Red List Category of Extinct (EX) when there is no reasonable doubt that the last individual has died (IUCN 2001, see Appendix 2a). At least 784 documented extinctions have occurred since 1500 AD, but this almost certainly represents a very small proportion of species that have become extinct during this time period. Many historic extinctions have either not been detected, or have taken place in taxonomic groups that have not yet been evaluated for the IUCN Red List.
Identifying the actual number of historic extinctions is difficult because only c. 1.9 million of the world's estimated 5 – 30 million species (see Erwin 1982; Hodkinson and Casson 1991; Novotny et al. 2002) have been described. Recent extinctions may be even more prevalent among undescribed species due to the sheer number and the fact that the discovery and description of species tends to be biased toward more broadly distributed and abundant taxa (see Collen et al. 2004).
Among the c. 1.9 million described species, only a few taxonomic groups have recently undergone thorough conservation assessments to determine whether or not all taxa are still extant (see Table 2.1). The Global Amphibian Assessment is an example of where a recent assessment of all species has resulted in the listing of an additional 29 Extinct species. Similar assessments of large and poorly known groups such as insects, spiders, crustaceans, plants, fungi or species from poorly studied regions will undoubtedly result in significant increases in the list.
While the focus of the IUCN Red List is on global extinctions, it is also important to consider all the population or local extinctions that occur on the path towards the final end point of a species. Significant biodiversity may be lost with the extinction of each population as they tend to carry unique genetic material and are often distinct in terms of morphology or behaviour. Population extinctions that are not followed by re-colonization also result in the loss of functional biodiversity, as the species no longer plays a functional role in the local ecosystem (such as decomposition or pollination). Such species are described as functionally or ecologically extinct. When species have been reduced to a fraction of their former range they become ecologically extinct on a global scale. Although the IUCN Red List captures some population extinctions during the documentation process, population extinctions are not specifically listed. This is because the quality and resolution of data required to assess populations simply does not exist for most species. However, as data quality improves, the documentation of population extinctions will play an increasingly important role in monitoring both biodiversity trends and ecosystem function.
Even where assessments have been conducted it can take years or decades to prove that a species is truly Extinct. The basic paradox of “documenting” extinctions is that absence of evidence is not necessarily evidence of absence (Stine and Wagner in press). IUCN has begun to highlight species that are believed to be Extinct, but have not yet been included in the list because appropriate surveys are still required to confirm that the last individual has died (see Box 3.2). Systematically flagging these Possibly Extinct species will help to provide a much clearer picture of the true extent of recent extinctions.
The Red List Programme is currently developing criteria for the identification of some Critically Endangered species as “Possibly Extinct” (see Appendix 2c for further details). For birds the tag of “Possibly Extinct” is applied to those species listed as Critically Endangered “which are, on the balance of evidence, likely to be Extinct, but for which there is a small chance that they may still be extant” (Butchart et al. in prep). For other taxa, the tag has been applied more generally to species that may possibly be Extinct. Listing species as Extinct when there is a chance that they are still extant can have significant conservation implications, because conservation funding is rarely targeted at species believed to be Extinct. It is therefore suitably precautionary to retain species in threatened categories if there is any reasonable possibility that they may still be extant. The “Possibly Extinct” tag has only been applied to amphibians and birds and a few representatives of other taxonomic groups and is therefore not indicative of the total number of CR (Possibly Extinct) species on the Red List. However, for amphibians and birds it provides a good indication of the number of extinctions that may be confirmed in the not too distant future. In total, 208 species have been identified as CR (Possibly Extinct), 122 of which are amphibians. Many of these amphibians have disappeared relatively rapidly and recently, including 18 species of harlequin toad in Central and South America from the genus Atelopus. Eighteen species of bird are CR (Possibly Extinct), including the Nukupu'u Hemignathus lucidus (last seen in Hawaii in 1996) and Spix's Macaw Cyanopsitta spixii (last seen in Brazil towards the end of 2000). Incomplete assessments of other groups such as mammals, reptiles, fishes, molluscs and plants account for the remaining 69 Possibly Extinct species.
The total number of extinctions listed by IUCN has increased from 766 in 2000 (Hilton-Taylor 2000) to 784 in 2004 (Table 3.1; Appendix 3i). However, because the documentation of the number of extinctions remains very incomplete, this increase does not provide much information on the rate at which extinctions are occurring, or the number of extinctions between 2000 and 2004. In fact all but one of the extinctions that have been added to the IUCN Red List in 2004 (the exception being the St. Helena Olive Nesiota elliptica, see Box 3.3) probably occurred before 2000. However, the new additions do highlight groups that have recently been investigated and have experienced significant numbers of extinctions. The taxonomic group with the largest increase in the number of documented extinctions is the amphibians with 29 additions. In the case of amphibians the increase of documented extinctions reflects both high rates of decline over the past 50 years (Houlahan et al. 2000; Alford et al. 2001) and a greater focus on the conservation status of this taxonomic group as a result of the Global Amphibian Assessment (see Appendix 1). Further information on amphibian extinctions is provided in Box 3.4. The 13 additional plant listings in the IUCN Red List (Appendix 3i) are primarily the result of recent work on the Hawaiian, Ascension and Soqotran islands. This represents just the beginning of a long documentation process of extensive island plant extinctions. A recent assessment of the Hawaiian flora alone regarded 82 species as presumed or possibly extinct (Wagner et al. 1999). Regardless of recent additions, we know that 85 plant extinctions is a gross underrepresentation, as many of the 380 species listed as Extinct in the 1997 IUCN Red List of Threatened Plants (Walter and Gillett 1998) have not yet been added to the 2004 IUCN Red List. This is because new standards have been applied in the documentation process and many of the 1997 Extinct plant species are still under review (see Appendix 1). The addition of Bennett's Seaweed (Vanvoorstia bennettiana) represents an entirely new kingdom in the Red List (see Box 3.5).
Table 3.1
The numbers of Extinct (EX) and Extinct in the Wild (EW) species by taxonomic group in 2004
The St. Helena Olive Nesiota elliptica (Rhamnaceae) was a small tree (up to 4 (perhaps as high as 7) m tall) that grew on the highest parts of the island's eastern central ridge. It became very rare in the 19th century, probably as a consequence of habitat loss, and by 1875 only 12 to 15 trees were recorded as growing on the northern side of Diana's Peak. It had been thought to have become Extinct until a single tree was discovered in August 1977. The tree was found to suffer from numerous systemic fungal infections, which may have been exacerbated by damage sustained during attempts to conserve it. It was found dead on 11 October 1994 and the species became Extinct in the Wild.
The St. Helena Olive continued to survive in cultivation, but ensuring its survival proved difficult, as cuttings were difficult to root (of hundreds attempted, success was only achieved with one). The species very rarely set good seed as it was 99% self-incompatible, and it was susceptible to fungal infections. The last seedling surviving showed signs of ill health due to fungal infections and in 2003 deteriorated extremely quickly following a dry winter. In December 2003, despite extensive efforts to rejuvenate the species, we witnessed the extinction of the St. Helena Olive.
Photo 3.2
The St. Helena Olive Nesiota elliptica.
Photo: ©Rebecca Cairns-Wicks
Based on information provided by Rebecca Cairns-Wicks, IUCN/SSC South Atlantic Islands Plant Specialist Group
Until recently, there has been little focus on amphibian extinctions. Only 34 amphibian species are recorded as having become Extinct, 20 of these being endemics to Sri Lanka, most of which disappeared over 100 years ago. It is likely that there have been many undetected amphibian extinctions over the last two centuries, and the concentration in Sri Lanka, although real, is also a reflection of the detailed taxonomic studies of frogs that have taken place there. Nine of the 34 amphibian extinctions have taken place since about 1980, these being the species listed in Table 3.4, plus two others from northeastern Australia, the Southern Gastric Brooding Frog Rheobatrachus silus and the Southern Day Frog Taudactylus diurnus. Eight of these nine recent extinctions were sudden disappearances in suitable habitats, and are probably the result of the fungal disease, chytridiomycosis, probably operating in conjunction with climate change (Laurance et al. 1996; Berger et al. 1998; Ron et al. 2003; Burrowes et al. 2004).
However, these figures are probably a very large under-estimate of the level of amphibian extinctions since 1980. A total of 122 amphibian species are listed as Critically Endangered (Possibly Extinct), and 113 of these could have disappeared since 1980. Most of these took place in Central and South America, in particular from southern Mexico south to Ecuador, with others recorded from Puerto Rico, Hispaniola, Jamaica, Venezuela, and southern Brazil. Other possible extinctions have been noted in Australia, Indonesia, China, Kenya, and Tanzania. Most of the disappearances happened very suddenly, and it seems increasingly likely that chytridiomycosis, linked to climate change, is the main cause (see Section 6.5). Proving extinction beyond reasonable doubt is often very difficult. A few species that were thought to be Extinct were subsequently rediscovered in remnant populations. For example, Atelopus cruciger was not seen in its native Venezuela after 1986, until a tiny population was found in 2003 (Manzanilla and La Marca 2004). Thus, the true number of amphibian extinctions since 1980 is somewhere between nine and 122 species. These dramatic amphibian declines appear to be spreading, with recent reports from Dominica (Magin 2003), Spain (Bosch et al. 2001) and New Zealand (Bell et al. 2004).
The current catastrophic wave of amphibian extinctions is taking out major evolutionary lineages. Already, one entire family, the Gastric-brooding Frogs from Australia (Rheobatrachidae), has been lost, and another, the Darwin's Frogs from Chile and Argentina (Rhinodermatidae) is at severe risk, as are the primitive New Zealand Frogs (Leiopelmatidae). Among the larger families, the toads (Bufonidae) have been hit particularly hard, most notably the beautiful harlequin toads (Atelopus spp.). Of 77 Atelopus species, three are Extinct (two since 1980), and 18 are Possibly Extinct (all since 1980). Amphibian extinctions are happening so rapidly, and so few scientists are monitoring them that it is hard to gain a clear, current picture of their status. But the indications are that this is the most serious wave of all extinctions currently taking place.
Photo 3.3
The Southern Gastric-brooding Frog Rheobatrachus silus (Extinct) from northeastern Australia is one of only two members of the family Rheobatrachidae, both of which are now extinct. The name of this species comes from its breeding behaviour: the females brood the larvae in their stomachs, and they give birth to froglets through the mouth.
Photo: © Michael J. Tyler
Photo 3.4
Atelopus chiriquiensis (Critically Endangered), a species of harlequin toad, has, like other members of its genus, undergone a catastrophic decline, probably due to the fungal disease chytridiomycosis. The species occurred in the lower montane zone of Costa Rica and western Panama, but it is now believed to have disappeared from Costa Rica (last record in 1996), and might also have gone from Panama (last record in the late 1990s).
Photo: © Michael and Patricia Fogden.
Based on information provided by Bruce Young and the contributors to the IUCN Global Amphibian Assessment
One addition to the Red List that deserves mention is a species of red algae, Bennett's Seaweed (Vanvoorstia bennettiana). It was only ever known from two sites in Australia: Spectacle Island in Parramatta River (New South Wales) in 1855; and the seabed between Point Piper and Shark Island in Port Jackson (Sydney Harbour) in 1886. No specimens have been recorded in the intervening 117 years despite numerous surveys and it has therefore been declared Extinct. This species is a member of the kingdom Protista (or Protoctista) which is a diverse assemblage that are united based on the lack of characteristics expressed in members of other kingdoms. They are defined as eukaryotic organisms that are distinct from plants, animals and fungi. It is unfortunate that the first, and so far only, representative of an entire kingdom enters the Red List as Extinct. However, this addition represents an important first attempt at documenting the extinction of smaller life forms that are fundamental to the survival of life on this planet.
Based on information provided by Alan J.K. Millar
Photo 3.5
Bennett's Seaweed Vanvoorstia bennettiana.
Photo: © Alan J.K. Miller
Although the numbers of recent extinctions are stable or increasing for most taxonomic groups, both mammals and insects show a decline in the number of recorded extinctions in the 2004 Red List (Appendix 3i). This is primarily because of taxonomic revision and the removal of erroneous listings (species previously listed that went extinct before 1500 AD) rather than the rediscovery of species declared as Extinct. Only one mammal, the Bavarian Pine Vole Microtus bavaricus, has been rediscovered over the past four years. It had not been seen since 1962, but a small population was found on the German-Austrian border in 2000. Other recent rediscoveries include: the New Zealand Storm Petrel Oceanites maorianus, a seabird rediscovered in 2003; Miller Lake Lamprey Lampetra minima, a fish endemic to a small area in Oregon, United States, and thought to have become Extinct in 1958, but its continued existence was confirmed after thorough surveys in the late 1990s;Gulella thomasseti, an endemic snail from the Seychelles rediscovered in August 2002; the Fabulous Green Sphinx Moth Tinostoma smaragditis from Hawaii, rediscovered in 1997 but only recently brought to the attention of the IUCN Red List office; the Pitt Island Longhorn Beetle Xylotoles costatus, rediscovered on South East Island in the Chatham Islands group; the Lord Howe Island Stick Insect Dryococelus australis, rediscovered in 2001 on Balls Pyramid, a rocky outcrop 23km from Lord Howe Island (Australia); and Pittosporum tanianum, a tree endemic to New Caledonia, rediscovered in May 2002 but only known from three remaining individuals.
Photo 3.6
The New Zealand Storm Petrel Oceanites maorianus (Critically Endangered) was assumed to be Extinct following the lack of records since specimens were collected in the 1800s. However, it was rediscovered in 2003, with two separate observations of birds identified as this species, followed by further sightings in 2004.
Photo: © Tony Palliser / BirdLife International.
Photo 3.7
The Fabulous Green Sphinx Moth Tinostoma smaragditis (Endangered) was listed as Extinct in 1996, but in February 1998 a single male was attracted to a light trap on the island of Kauai, Hawaii. Since 1998, further individuals have been trapped, but the species is threatened due to the impacts of invasive species on its habitat.
Photo: © Mandy Heddle
In addition to Extinct species, IUCN also records species that are Extinct in the Wild (EW). This includes species that are now only found in captivity, cultivation or as naturalized populations (IUCN 2001, see Appendix 2a). Extinct in the Wild species are in many respects Extinct, as they no longer play a functional role in their ecosystems. Also, because successful re-introductions are rare (see Box 3.6), it cannot be assumed that most of these species will be restored to the wild.
The number of EW species has increased from 50 in 2000 to 60 in 2004 (Table 3.1; Appendix 3i). The growth in the number of EW species is easier to document because these species are usually well monitored and conservationists are usually involved in keeping the species alive in captivity or cultivation. However, proving that a species is EW can take years, as it requires confirmation that the last wild individual has died. Three species appear to have genuinely moved from Critically Endangered to Extinct in the Wild since 2000, all of them from the Hawaiian Islands. These include two plants, the ‘Oha Wai Clermontia peleana and Haha Cyanea pinnatifida, and one bird, the Hawaiian Crow Corvus hawaiiensis. The Po'ouli Melamposops phaeosoma, a bird also from the Hawaiian island of Maui, looks set to become the next addition to this list. Efforts are underway to take the last three known individuals into captivity as they are failing to breed in the wild.
Re-introduction of Extinct in the Wild species to their original habitat can only be successful if sufficient habitat remains to support the re-introduced populations, and if the factors which caused the initial extinction in the wild have been addressed (see IUCN 1995). The Taiwanese endemic Rhododendron kanehirai, for example, cannot be re-introduced to its native habitat because this was entirely destroyed by the construction of a dam at its only known site (Lu and Pan 1997). Conversely, although sufficient habitat remains on Guam to allow the re-introduction of the Guam Rail Gallirallus owstoni into its native habitat, it will always require protection due to the continued presence of the predatory Brown Tree Snake Boiga irrelgularis. However, not all re-introduction programmes are doomed to failure. The recent re-introduction of the Scimitar-horned Oryx Oryx dammah to Tunisia has so far run smoothly (Mallon and Kingswood 1999), and once two generations of the re-introduced animals have bred successfully this species will be downlisted. Other promising re-introductions include the Black-footed Ferret Mustela nigripes (see Box 8.8), Red Wolf Canis rufus, California Condor Gymnogyps californianus, Mallorcan Midwife Toad Alytes muletensis (see Box 8.7) and the Redwood Trochetiopsis erythroxylon on St. Helena (see Box 8.6).
Photo 3.8
The last two known wild individuals of the Hawaiian Crow Corvus hawaiiensis (Extinct in the Wild) disappeared in 2002. Some individuals remain in captive breeding facilities and a reintroduction plan is being developed.
Photo: © Jack Jeffrey Photography.
The evolution of new species and the extinction of others is a natural process. This is evidenced by the fact that species present today only represent between two and four per cent of all species that have ever lived (May et al. 1995). Over geological time there has been a net excess of speciation over extinction that has resulted in the diversity of life that we experience today. However, the high number of recent extinctions suggests that the world might now be facing a rapid net loss of biodiversity. This can be tested by comparing recent extinction rates to average extinction rates over geological time.
The fossil record appears to be punctuated by five major mass extinctions (Jablonski 1986), the most recent of which occurred 65 million years ago. However, the majority of extinctions have been spread relatively evenly over geological time (Raup 1986), enabling estimates of the average length of species' lifetimes through the fossil record to be made. Studies of the marine fossil record indicate that individual species persist from one million to ten million years (May et al. 1995). These data probably underestimate background extinction rates, because they are necessarily largely derived from taxa that are abundant and widespread in the fossil record. Using a conservative estimate of five million as the total number of species on the planet, we would therefore expect anywhere between five extinctions per year to roughly one extinction ever two years (for all five million species on the planet). Bird, mammal and amphibian extinctions over the past 100 years total to roughly 100 species which in itself would appear similar to background extinction rates, but these groups represent only 1% of described species. Over the same time period, one would therefore assume that 100 times this number of species (i.e., 10,000 species) were lost over the past 100 years (assuming that the susceptibility to extinction of birds, mammals and amphibians is similar to species as a whole, which of course, is not known). Given the uncertainty over the total number of species on the planet it is more meaningful to convert these data into a relative extinction rate, measured as the number of extinctions per million species per year (E/MSY) (Pimm et al. 1995). A background extinction rate of 0.1–1 E/MSY corresponds to average fossil species lifetimes.
Measuring recent extinction rates is difficult, not only because our knowledge of biodiversity is limited, but also because even for the best studied taxa there is a time-lag between the decline towards extinction and actual loss of species. In the case of extinctions caused by habitat loss, in particular, it may take thousands of years before a restricted remnant population is finally driven to extinction (Diamond 1972). With this in mind it is possible to use recent documented extinctions to make a very conservative estimate of current extinction rates, though this is limited because only a few taxonomic groups have been reasonably well analysed for extinctions. Recent extinctions have been best studied for birds, mammals and amphibians. With a total of approximately 21,000 described species of birds, mammals and amphibians (see Table 2.1), the E/MSY for these groups is 48 to 476 times greater than the background extinction rate. If Possibly Extinct species are included in this analysis, the total number of extinctions and possible extinctions over the past 100 years for these groups is 215 species, which results in an E/MSY that is 102 to 1,024 times higher than background rates.
Photo 3.9
Only a single individual of Wood's Cycad Encephalartos woodii (Extinct in the Wild) was ever found in Kwa-Zulu-Natal, South Africa. Its extinction may have been a natural event, although the final end of the wild population may have been hastened by over-exploitation for medicinal purposes by local people. There is no likelihood of ever reintroducing the species back into the wild as there are only male plants in existence, and the risk of theft would be too great.
Photo: © John S. Donaldson.
A range of other techniques has been used to estimate contemporary extinction rates more generally, involving the measurement of a range of proximate (e.g., habitat destruction) and ultimate (e.g., human energy consumption) drivers of extinction. These studies give rise to estimates of E/MSY that are 1,000 to 11,000 times higher than background rates (Pimm and Brooks 1997), generally higher than the very conservative estimate for birds, mammals and amphibians given here. However, because the IUCN Red List is very conservative in recording species as Extinct, or even Possibly Extinct, and because many extinctions have probably been missed due to limited survey effort for most taxonomic groups, the result presented here is believed to underestimate extinction rates very significantly. Whatever the exact rate of species loss, it is clear that contemporary extinction rates are vastly higher than those typical over the planet's history, and are probably much too fast to be balanced by speciation. It is therefore likely that the world is experiencing a net loss of species, perhaps for the first time in millions of years.
Recorded extinctions are not randomly distributed across taxonomic groups. However, sampling biases confound the extent to which some groups are more or less susceptible to extinction than others. For example, molluscs (291 Extinct species) and birds (129 species) are the groups with the most recorded recent extinctions, but their comparatively high levels of extinction reflect the interests of early collectors and observers, and do not necessarily indicate that these groups are unusually extinction prone. The relatively low number of Extinct species reported in species-rich groups such as insects (59 extinctions), crustaceans (7 extinctions) and other invertebrates (2 extinctions) is most certainly the consequence of their being poorly studied, and does not indicate that they are more resilient in the face of threats. What is certain is that all groups that have been relatively well assessed, such as birds, mammals, amphibians, trees, and molluscs, have all experienced high numbers of extinctions.
Although it is not possible to compare the extent to which different major taxonomic groups vary in their susceptibility to extinction, it is possible to make comparisons within groups that have been completely or almost completely assessed. Analyses of bird and mammal taxa have found extinctions to be ‘selective’, with clustering in certain genera and families (Russell et al. 1998). Similar patterns have now also been identified in amphibians (see Box 3.4). In addition, birds and mammals in species-poor genera tend to have higher probabilities of extinction than those in more species-rich genera (Bennett and Owens 1997; Purvis et al. 2000b and c), which in turn increases the likelihood that entire genera will go extinct. This, coupled with the tendency for extinction-prone taxa to be evolutionarily “old” (Gaston and Blackburn 1997), indicates that extinction is non-random and that we are losing a disproportionate number of evolutionally unique species.
The majority of documented extinctions have been of terrestrial species (582), followed by freshwater (226), and marine (15) (a few of these species are classified in more than one of these systems). While the extinction record of terrestrial species is incomplete, the documentation of freshwater and marine extinctions is virtually non-existent. Freshwater extinctions have been best documented in the United States, where 105 such species are known to have been lost (17 fishes, 2 amphibians, 78 molluscs, 8 insects and 2 crustaceans). Whether or not this is representative of other regions is unknown (see Box 3.7). Until recently, there were very few documented extinctions of marine species, and this was generally interpreted as these taxa being more resilient, rather than extinctions taking place unnoticed. However, a consensus is now emerging that there is no a priori reason to consider marine species to be necessarily less susceptible to extinction than terrestrial species (see Box 3.8).
The majority of recorded species extinctions since 1500 AD have occurred on islands. A total of 72% of recorded extinctions in five animal groups (mammals, birds, amphibians, reptiles, and molluscs) was of island species. Furthermore, for each individual taxonomic group the percentage of recorded extinctions occurring on islands was greater than that occurring on continents. In total, 62% of mammals, 88% of birds, 54% of amphibians, 86% of reptiles, and 68% of molluscs were island species. Nevertheless, there are major differences in the extinction patterns between the five taxonomic groups mapped in Figure 3.1. Bird extinctions are overwhelmingly biased towards oceanic islands (including New Zealand), whereas the largest concentration of mammal extinctions is in Australia. Documented amphibian extinctions are focused on Sri Lanka, but this might be an artefact of under-recording extinctions elsewhere. Mollusc extinctions are concentrated in North American river systems (as indicated in Box 3.7).
The southeastern United States is a world centre of diversity for freshwater species and accounts for most of the diversity summarized in Box 2.5. But this area also stands out in another way as well: species that depend on riverine habitats are, as a whole, faring the worst of any groups of North American organisms (Chaplin et al. 2000). Molluscs in particular have been seriously impacted. NatureServe data identifies 39 species of freshwater mussels and snails are presumed extinct and another 76 species are possibly extinct, having not been seen in many years despite searches (Master et al. 2000).
The leading cause for these freshwater mollusc extinctions is thought to be habitat destruction and alteration due primarily to dam construction, which has turned most large free-flowing rivers in the United States into a series of impoundments. However, point and nonpoint pollution, invasive alien species (e.g., zebra mussels), and altered hydrologic regimes have also impacted mollusc populations and continue to threaten many species (Richter et al. 1997; Master et al. 1998). These problems are not restricted to the United States, and are likely having significant impact on freshwater faunas worldwide.
Based on information provided by Larry Master, NatureServe
Like marine mammals, birds and turtles, marine fishes and invertebrates are coming under increasing pressure from human activities and face the threat of extinction, as indicated by severe and widespread declines (Casey and Myers 1998; Davis et al. 1998; Baum et al. 2003; Myers and Worm 2003; Sadovy and Cheung 2003; Hutchings and Reynolds 2004) and fisheries management failures (Hutchings and Myers 1994), as well as emerging science that is revealing the inherent vulnerability to overexploitation and extinction of many species and species groups (Hoenig and Gruber 1990; Grimes and Turner 1999; Huntsman et al. 1999; Reynolds et al. 2001; Dulvy et al. 2003) and the complexity of species' and ecosystems' responses to severe depletions (e.g., Dayton et al. 1995; Jackson et al. 2001; Hutchings and Reynolds 2004).
As a phenomenon, extinction in the sea has been a contentious issue. One reason for this is the fact that there are many fewer examples of recent extinctions in the marine environment than on land or in fresh waters, which has led to the belief that marine extinctions have been and are uncommon. Yet the work of several authors in recent years (Carlton et al. 1999; Dulvy et al. 2003) suggests and provides evidence that marine extinctions are occurring but are simply not being detected in the same way that they are in other environments. Another factor in the debate is the widespread and persistent perception that marine species are more resilient to extinction because they are – or are presumed to be – highly fecund, wide-ranging, and/or fastgrowing, and, thus, should be capable of withstanding high levels of exploitation, and of recovering rapidly from low numbers. However, a growing body of scientific evidence indicates that marine species are characterized by the same attributes that account for vulnerability in terrestrial and freshwater species (see section 6.10.3). Many marine species are long-lived and are late to reach sexual maturity, which puts them at an inherent disadvantage when subjected to exploitation (Camhi et al. 1998; Coleman et al. 1999; Musick et al. 2000a; Parker et al. 2000). Many marine species are not as widespread as is commonly believed but are restricted by the heterogenous nature of the marine environment and/or limited dispersal capabilities to small, in some instances very small, ranges (Hawkins et al. 2000; Smith et al. 2002). Further, high fecundity has been shown not to be associated with higher rates of reproduction (Denney et al. 2002) or to ensure against overexploitation (Sadovy 2001). Finally, there is increasing evidence that marine populations do not recover from severe depletion even when fishing ceases (Hutchings 2000; Hutchings and Reynolds 2004).
Dulvy et al. (2003) document 133 local and global extinctions in marine species (21 of which are global), including 3 global and 62 local fish extinctions. Exploitation is the most important factor in causing extinction in marine species. In addition, while it may once have been true that commercial extinction would precede biological extinction, such that fishing would cease once it became unprofitable, the multi-species nature of most current fisheries and the high value of others (Sadovy and Cheung 2003; Sadovy et al. 2003) result in the persistence of fishing effort that can lead to biological extinction. The currently available evidence indicates that extinction in the sea is, and will become, a much larger problem than is currently recognized.
Based on information provided by Elodie Hudson and Amie Bräutigam
A detailed examination of bird extinctions since 1500 AD indicates that the geographical distribution of extinctions may be changing over time. Although more than 80% of birds are found on continents, all extinctions prior to 1800 occurred on islands. This pattern has started to change in recent years with more extinctions occurring on continents (Figure 3.2).
Photo 3.10
The last confirmed report of the Thylacine Thylacinus cynocephalus (Extinct) in Tasmania was in 1930, and the last captive animal died in 1936. The Thylacine was driven to extinction primarily by direct persecution, but habitat loss, competition with domestic dogs and disease all played a role.
Photo: © Zoological Society of London.
Figure 3.1
The distribution of Extinct and Extinct in the Wild amphibians, birds, mammals, molluscs and reptiles.
Figure 3.2
The number of bird extinctions that have occurred on islands and continents since 1500 AD.
While there is no doubt that island species have been particularly prone to extinction, it is important to note that significant numbers of extinctions might have taken place in certain continental tropical regions where there has been very limited survey effort. Such a bias in sampling possibly explains the concentration of mollusc extinctions in North America. Thus Figure 3.1 highlights regions where large numbers of extinctions are known to have taken place, but it does not provide much insight into regions where even basic knowledge of the recent history of extinctions is lacking.
Humans have played a significant role in the extinction of species prior to historic times (see Box 3.9) but the true extent of such anthropogenic impacts during the Holocene (the last 11,000 years) remains unclear. However, after 1500 AD it is clear that humans are responsible for most recorded extinctions.
The strong correlation between the arrival of humans and a rapid increase in extinction has been demonstrated in regions such as Australia (Roberts et al. 2002), the Americas (Alroy 2001) and Madagascar (Goodman and Patterson 1997), but nowhere is the pattern more evident than in the human colonization of oceanic islands and subsequent extinction of birds (Milberg and Tyrberg 1993; Pimm et al. 1994; Steadman 1995; Steadman et al. 1999). On the tropical Pacific Islands, Steadman (1995) and Steadman et al. (1995) have estimated that more than 2,000 species of birds became extinct during the period of prehistoric human colonization (most of which were flightless rails). If this estimate is correct, then about one-fifth of all birds extant during the early Holocene (about 11,000 years ago) are now extinct (Milberg and Tyrberg 1993).
Figure 3.3 Causes of extinction for birds.
The exact causes of most extinctions are poorly documented, but invasive alien species, habitat loss, and over-exploitation have all been major factors (see Section 6). Even when species are relatively well studied it is often difficult to identify the main cause of extinction as most species are threatened by more than one process and these often interact in unpredictable ways. Furthermore, the threat process that causes a species to become susceptible to extinction (such as habitat loss) may be very different to the final process that drives it to extinction (such as a hurricane).
Relatively good data exist for birds indicating that the impacts of invasive alien species, over-exploitation by humans, and habitat destruction and degradation have been the major causes of extinctions (see Figure 3.3), with invasive species being associated with the extinction of at least 65 species. Predation by introduced dogs, pigs and mongooses, and habitat destruction by sheep, rabbits and goats have been implicated in the extinctions of some of these species. However, it is predation by introduced rats and cats, and diseases caused by introduced pathogens that have been the most deadly overall, contributing to the extinction of some 30, 20 and 10 species respectively.
While little has been documented about most historic extinctions, much more information is available on species that have been lost over the past few decades. This section focuses on extinctions over the past 20 years to provide greater insight into patterns of recent extinction and to highlight those species that have most recently disappeared.
At least 27 species are recorded as having become Extinct or Extinct in the Wild during the last 20 years (1984–2004) (Tables 3.2 and 3.3). Inherent in identifying very recent extinctions is the problem of extinctions not being included because they are not yet confirmed. For example, eight species of birds are thought to have become Extinct or Extinct in the Wild over the past 20 years, but they are not included, as further research is needed prove the last individual has died (Box 3.2).
Table 3.2
Species recorded as having become Extinct over the last 20 years (1984 – 2004)
Table 3.3
Species recorded as having become Extinct in the Wild in the last 20 years (1984 – 2004)
Twelve of the post-1983 extinctions were of flowering plants, six were of birds, eight were of amphibians and one was of a mammal. All of these species were terrestrial, although all of the amphibians were also freshwaterdependant. In total, fourteen recently Extinct species were from islands, while thirteen were continental species. This pattern is very different from that of extinctions over the past 500 years, during which time documented extinctions have always been much greater on islands than on continents. The greatest concentration of very recent documented extinctions has taken place on the Hawaiian archipelago, which has seen the recent demise of five plant species and three bird species. These extinctions, in addition to an extinction in the wild on Guam, result in Oceania having more recorded extinctions over the last 20 years than any other biogeographic realm.
Most of species that have become Extinct or Extinct in the Wild over the last 20 years had restricted ranges. Of the 27 recently Extinct species, approximately 85% had a restricted range in historic times. The Atitlán Grebe Podilymbus gigas, for example, was restricted to Lake Atitlán in Guatemala, while the St. Helena Olive Nesiota elliptica was found only at high elevations of St. Helena Island's eastern central ridge. However, several somewhat more widespread species have been lost over the last 20 years. The Jambato Toad Atelopus ignescens, for example, was originally widespread throughout the humid forests of Ecuador's northern Andes at elevations of 2,800–4,200m. Surveys in the 1960s, 70s and 80s found this species to be abundant, but population sizes began to decline rapidly in approximately 1984 (Ron et al. 2003). There have been no records since 1988, and the species is considered to be Extinct, apparently the result of infection with the chytrid fungus, probably linked to extreme droughts.
Several of the recently EX and EW species experienced extremely rapid declines. The Northern Gastric Brooding Frog Rheobatrachus vitellinus, for example, underwent a very rapid population reduction in 1984–5, seemingly the result of infection by the chytrid fungus, and perhaps forest fires that spread throughout its habitat. Despite always having had a restricted range (it was found only in undisturbed forest from 400–1,000m above sea level in the Eungella National Park), this species was considered common across its range until January 1985 (McDonald et al. 2001), but there have been no records since May of that year. The Guam Rail Gallirallus owstoni experienced a similarly dramatic decline following the accidental introduction of the Brown Tree Snake Boiga irrelgularis in the 1940s (Haig et al. 1990, Wiles et al. 2003). Previously widespread, this species became Extinct in the Wild in 1987 following a collapse in its population size from 2,000 individuals in 1981 to 100 in 1983 (BirdLife International 2004a).
The most commonly recorded threat to the species that have been lost over the last 20 years is habitat loss, which had a severe impact on thirteen of these species. In addition, other major threats have included the introduction and invasion of non-native species (impacting nine species), and disease (in particular the fungal amphibian disease chytridiomycosis (see Box 3.4) which impacted seven species, and avian pathogens, spread by introduced mosquitoes, which impacted three species) (see Tables 3.2 and 3.3). It is noteworthy that over-exploitation has not played a significant role in these recent extinctions, and also that diseases have had a relatively larger impact than they did over the last 500 years.
Photo 3.11
The Atitlàn Grebe Podilymbus gigas (Extinct) was endemic to Lake Atitlán, Guatemala. The extinction of this species in 1986 was the result of a number of independent factors that combined, drastically reduced the population to levels where it was no longer viable.
Photo: © BirdLife International.
The number of documented extinctions (844 species since 1500 AD) grossly underrepresents the number of extinctions that have taken place in historic times, due to very incomplete and uneven sampling, both geographically and taxonomically.
An additional 208 species could already be Extinct, but further information is required to confirm this.
Data from the IUCN Red List indicate a current extinction rate that is at least two, and probably three, orders of magnitude higher than the background rate typical over the planet's geological history.
Very little is known about marine and freshwater extinctions, but preliminary evidence from North America indicates a very high level of extinctions in freshwater habitats.
Although information is still very limited, there is growing evidence that marine species are less resilient to extinction in the face of threats than was once thought.
Although the island species have experienced the greatest number of extinctions in historic times, continental extinctions are becoming more frequent, and account for almost 50% of the extinctions confirmed over the last 20 years.
Humans have been the main cause of extinction since 1500 AD with invasive alien species, habitat loss, and over-exploitation being the main causal factors.
While habitat loss and invasive alien species have been major drivers of extinction over the last 20 years, over-exploitation appears to have had little impact in causing non-marine extinctions over this time period, but disease appears to be a growing threat.
Photo 3.12
The Bottle Palm Hyophorbe lagenicaulis (Critically Endangered) endemic to Round Island, Mauritius, almost went extinct. Numbers declined to just seven individuals because of the lack of regeneration due to the impacts of introduced goats and rabbits. An effective eradication programme resulted in the removal of all the goats and rabbits by the late 1980s. An active seed-planting programme is now helping the recovery of this species, and it is widely cultivated worldwide.
Photo: © Wendy A. Strahm.